Human Impacts on Sea Fisheries and Aquaculture PDF

CHAPTER 8 Human impacts 1: sea fisheries and aquaculture Sea fisheries and aquaculture are immense topics and are the subjects of a wide variety of books and scientific papers. This chapter aims to provide the basic background information needed to understand the impact our fishing and aquaculture...

CHAPTER 8 Human impacts 1: sea fisheries and aquaculture Sea fisheries and aquaculture are immense topics and are the subjects of a wide variety of books and scientific papers. This chapter aims to provide the basic background information needed to understand the impact our fishing and aquaculture activities are having on fish stocks and the ocean ecosystem itself. Fishing is an essential part of the increasingly difficult endeavour to feed the world population. It not only provides food but also employment and an income for millions of people worldwide. Fisheries and aquaculture have a significant and growing role in providing food, nutrition and employment (FAO, 2020a,b,c). However, with advanced technology it is now possi- ble to fish almost anywhere and at depths that were previously considered impossible. The variety of species utilised has consequently also increased, not only for human consumption but also for the production of animal feed. This is reflected in fish mar- kets around the world where an increasing variety of rare and unusual species can be found, such as Frilled Sharks (Chlamydoselachus anguineus), caught as bycatch from deep bottom trawls. Species targeted by fisheries include everything from seaweeds and invertebrates, to fish of all types, as well as marine mammals. However certain species and certain areas are particularly important on a world scale. Fishery landing statistics and other data are maintained and updated each year by the FAO (Food and Agriculture Organisation) (http://www.fao.org). From this it can be seen that five pelagic species regularly account for around 15% of global annual landings: Peruvian Anchovy (Engraulis ringens), Alaskan Pollock (Theragra chalcogramma), Chilean Jack Mackerel (Trachurus murphyi), Atlantic Herring (Clupea harengus) and Chub Mackerel (Scomber japonicus). The most productive fishing areas are over continental shelves and in regions of upwelling, where primary production is high and therefore the food web can ultimately support a higher biomass of fish. Even with the refinements of modern science and technology, fishing remains an essentially primitive method of obtaining food. In the exploitation of the fish stocks of the sea, humans still behave mainly as nomadic hunters or trappers of natural populations of animals living in the wild state. This is in stark contrast to the situation on land, where almost all food is produced by farming of one sort or another. However, the balance between fishing and aquaculture is changing and in a world context and including fresh- water, aquaculture is now (2020) approaching 50% of world fish production. Elements of Marine Ecology r 2022 Elsevier Ltd. DOI: https://doi.org/10.1016/B978-0-08-102826-1.00006-5 All rights reserved. 389 390 Elements of Marine Ecology Inappropriate fishing methods and over-fishing threaten not only the target spe- cies but also entire ecosystems, through, for example, habitat destruction and removal of top predators and keystone species. Targeting specific species and avoiding bycatch is also difficult, particularly on a commercial scale. However, fisheries that discard bycatch or produce waste in the form of offal can sometimes benefit marine species such as seabirds. In some areas around Great Britain, Northern Gannets (Morus bassanus) populations have increased or remained stable, whilst other seabirds in the same regions have declined. This may be partly due to the impressive diving abilities of this species, enabling them to take a greater pro- portion of sinking bycatch. This also explains why fish that live well below the maximum diving range of these birds have nevertheless been found in their sto- machs (see also Box 8.1). Fisheries comprise two major groups, demersal and pelagic. Demersal fishing takes place on the sea floor for species which live on and close to the bottom, for example cod, flatfish, rays and deepwater Orange Roughy (Hoplostethus atlanticus). Pelagic fish- eries target shoals of fish within the water column mostly caught near the surface, for example herring. Fishing methods include both active (towed) techniques such as trawls and seines and passive (static) techniques such as traps, drift nets and long lines BOX 8.1 The changing fortunes of Fulmars. The Northern Fulmar (Fulmarus glacialis) is a common seabird in the North Atlantic and one of the commonest around northern Britain. However, this was not always the case and until the second half of the 19th century there was only one breeding colony in the British Isles, on the remote island of St Kilda, plus scattered colonies in Iceland. Subsequently, their breeding range slowly started to expand. This expansion really took off in the 20th century and fulmars spread rapidly round the British Isles, across NW Europe and eventually to north- ern areas of the western Atlantic. Fulmars are inveterate scavengers and readily follow fish- ing boats in the hope of an easy meal. This food source is the probable driver behind this impressive expansion. Initially whaling and then as that declined, the rise of commercial fishing provided them with increasing amounts of offal, discards and bycatch. Since the beginning of the 21st century, the number of breeding pairs in the UK and Europe has been steadily declining and again this could be linked to a reduction in discards as a result of fleet reductions and changing legislations. This, however, is unlikely to be the whole story either of their increase or the current decrease. It is not clear what proportion of a fulmar’s diet is sourced from fishing vessels and this may vary within their current north-south range. In the North Sea, natural sources of food, particularly sand eels and zooplankton are changing and declining due to climate change. Accidental catches in longline fisheries (Section 8.3.3) may also be a contributing factor. However, this example does serve to illustrate the close links between our fishing activities and completely unrelated, nontarget species.  Human impacts 1: sea fisheries and aquaculture 391 with baited hooks. The main types of gear used in particular fisheries and some of the problems associated with their use are described in Section 8.3. Fishing vessels vary in size from small-scale local boats operated by one or two crew to industrial so-called supertrawlers and factory ships. Many of these can stay at sea for days or weeks at a time as they are able to process their catch on board, freezing or preserving it. The largest are upwards of 140 m long and can carry a processed catch of up to 7000 tonnes. Further information on these topics can be found in Sainsbury (1996) and Moore and Jennings (2000). 8.1 Background biology of food fishes 8.1.1 Reproductive strategies and life cycles In terms of fisheries biology and management, it is important to know the life his- tory parameters and reproductive strategy of target species. Whilst there are infinite variations on a scale between different fish species, there are two main strategies, sometimes described as r and K, respectively. In simple terms, an r-strategist produces large numbers of offspring but puts little if any effort into helping their survival. A K-strategist produces a small number of offspring but puts considerable resources into their fitness to survive. Species inhabiting different environments will exhibit different life histories, ultimately determined by natural selection. The theory of r and K selection suggests that if mortality factors are variable or unpredictable (or both) then these factors will not have much selective bearing on population size or phenotype. Individual competitive fitness is of relatively low importance and so the best strategy is to produce lots of offspring without investing huge resources in them. In an environment where mortality factors are stable or predictable (or both) then this will result in strong selection for individual fitness. Mortality factors will also have variable effects on different phenotypes. Producing offspring well-fitted for competitive survival is then the best strategy and since this requires more parental input, then fewer offspring will result. However, there will always be a sliding scale between these two strategies (Adams, 1980). In very general terms, the majority of ray-finned fishes (Actinopterygii) lay large numbers of small eggs which are broadcast into the water and fertilised externally, that is they are r-strategists. This particularly applies to shoaling, pelagic species, including the five species mentioned in the introduction to this chapter, which contribute heavily to world landings of fish. However, there are over 34,000 known fish species, found in almost every aquatic habitat and a corresponding variety of reproductive strategies. Amongst pelagic marine species, the number of eggs produced can be immense. An Atlantic Cod (Gadus morhua), for example can lay 9 million eggs in a season. The majority of these do not survive as they are eaten by planktivorous fish, holoplankton and seabed filter-feeding invertebrates. Cod and many other inshore, 392 Elements of Marine Ecology pelagic, shoal-forming fish improve their reproductive chances by gathering together and spawning seasonally in specific and traditional breeding areas. Many species drift in the open ocean during their egg and larval stages but are carried or swim into sheltered nursery areas for their juvenile stages. Knowledge of spawning seasons, grounds and nurseries is obviously important in the management of fisheries, allowing closed seasons and areas to be implemen- ted. The spawning grounds of the main target species around the British Isles have been well-documented (see Section 8.4.3). A classic example of why such informa- tion is important is the Piked Dogfish (Squalus acanthias). This was once perhaps the world’s most abundant and important commercial shark species (Ebert et al., 2013) but is now listed as ‘Vulnerable’ globally on the IUCN Red List and as Critically Endangered in the NE Atlantic. It is intrinsically at risk of over- exploitation by fisheries because of its life history
it matures late, has few off- spring and a long generation time (30
40 years). However, in addition to this, it segregates by sex and maturity, with large mature and pregnant females aggregating together. Targeting such aggregations (whether intentionally or unknowingly) adds to the vulnerability of this species. Some coral and rocky reef fish species are also particularly vulnerable in this respect. Nassau Grouper (Epinephelus striatus) put all their reproductive efforts into gathering in large aggregations at specific sites for just a few days around the full moon in December and January. Targeting such aggregations can have dramatic negative effects on populations of this long-lived, slow-growing species. Australian Giant Cuttlefish (Sepia apama) have similar spec- tacular mass gatherings. As a group, cartilaginous fishes (Elasmobranchii) can be considered as K-strategists. Sharks, rays and chimaeras all produce relatively few young at an advanced stage of development, greatly enhancing their fitness for survival. Around 60% give birth to live young and the rest lay individual large eggs onto the seabed, encased in protective cases. All of them mate and have internal fertilization, a simi- lar strategy to mammals, but do not exhibit parental care. Many small benthic and bentho-pelagic, ray-finned fishes invest considerable resources in producing a few large eggs and then protecting them until (and sometimes beyond) hatching. Perhaps the best-known examples are seahorses (Syngnathiformes) in which the female lays her eggs into a brood pouch on the male’s abdomen. The eggs remain there in safety until they hatch. Other things being equal, the differences between reproductive strategies just described, explains why stocks of some species can recover quickly when the mortality pressure from over-fishing is removed, whilst others take many years or never fully recover. For fisheries management and conservation purposes, it is also important to know specific life history and population details for each exploited species, not just their general reproductive strategy. For example see Box 8.2.  Human impacts 1: sea fisheries and aquaculture 393 BOX 8.2 Orange Roughy. The Orange Roughy (Hoplostethus atlanticus) is a deepwater fish found in rocky areas over steep continental slopes and around seamounts down to around 2000 m. Otolith ring counts (see Section 8.4.4) have shown that it can live for 250 years. It grows extremely slowly and does not spawn until 20
30 years old. Even then individuals may not spawn every year and produce relatively low numbers of eggs, in the thousands rather than hundreds of thousands or millions. Large shoals gather together to spawn. All these life-history factors mean that this is a prime example of a species that does not recover well from over-exploitation. Little of this was known in the 1970s when commercial fishing for this species started, as a result of advances in deep-sea trawling technology and declining stocks of other more easily fished species. Intense fishing over the following three decades in New Zealand, Australia, Namibia, Scotland and other areas quickly led to a ‘boom and bust’ situation. Worldwide catches rose to over 90,000 tonnes between the start of the fishery and 1990 but then declined sharply over the next two decades to around 10,000 tons with stocks fished down to an estimated 10% of the starting stock. Since then the New Zealand industry has achieved a remarkable recovery through a combination of developing new, species-specific methods of stock assessment, leading to better modelling of populations and implementing consequent con- siderable reductions in catch limits. Overall catch is therefore far lower, but sustainable fish- ing limits mean a long-term future for the fishery, with some stocks now attaining MSC certification (see Section 7.5.4). 8.1.2 Example life histories and biology Atlantic cod (Gadus morhua) Cod has always been and remains one of the most important fishery species in the North Atlantic. This iconic species has been the subject of several books detailing its biology, ecology, life history and fisheries, of which the most recent (at the time of writing) is Rose (2019). In 2018 the world catch of Atlantic cod was 1.2 million tonnes, representing many millions of individual fish. The species was once so abun- dant that it was considered impossible to overfish it. Historic crashes of cod stocks, particularly the Grand Banks stock in Newfoundland, Canada disproved that theory. A complete moratorium on fishing that stock imposed in the early 1990s has still (2020) not led to significant recovery. Cod are mainly caught by bottom trawling and it is thought that sustained intensive trawling over decades has caused fundamental changes in the ecosystem, making recovery difficult if not impossible. Today (2020) there are small signs of recovery, but stocks remain at only about 10% of what they were in the 1960s. Currently, some Atlantic Cod stocks are well managed and fished sustainably, but many others remain in a poor state. Only five of 31 Atlantic Cod fish- eries identified in the Marine Conservation Society’s online Good Fish Guide (http:// www.mcsuk.org/goodfishguide) are rated within the first two of their five categories 394 Elements of Marine Ecology of sustainability (most sustainably caught and sustainably caught). The guide provides detailed stock, management and method of capture information for 140 fishery species, both wild and farmed. Distribution: Atlantic cod (Fig. 8.1) is a cold temperate (boreal) species with an extensive range throughout the Arctic and the northern part of the North Atlantic. It is found largely over the continental shelf and slopes to a depth of about 600 m, though most occur between about 150 and 200 m. Although the isotherms do not set firm limits to its distribution, cod is most abundant in seas within the temperature range 0 C
10 C. It is found along the western Canadian coast southwards to North Carolina in the USA, around Greenland and Iceland and from the Barents Sea south around the European coasts to the Bay of Biscay. In the northern part of the Pacific, a closely related form, G. macrocephalus, occurs over a wide area. Although tagging experiments (see Section 8.4.2) have revealed extensive migra- tions by individual cod throughout the North Atlantic and into the North Sea and Barents Sea, there does not appear to be any large-scale movement of populations between different areas. There are therefore a number of distinctive stocks (see Section 8.1.4) particularly the Arcto-Norwegian, North Sea, Faroe, Iceland, East and West Greenland, Newfoundland and Labrador stocks. The fish migrate between spawning and feeding areas and may move to find optimal temperatures, but all this is mainly within each stock’s normal geographical limits. Early tagging experiments in Figure 8.1 Atlantic cod (G. morhua). Identification features include three dorsal and two anal fins; light coloured lateral line; overhanging upper jaw; long chin barbel; mottled colour. From Shutterstock, Podolnaya Elena.  Human impacts 1: sea fisheries and aquaculture 395 the North Sea showed a maximum distance travelled from the release point, of about 200 miles. Occasionally much longer migrations have been reported. For example, a tagging experiment in the central North Sea resulted in two fish being recaptured off the Faroe Islands and one from Newfoundland (Macer and Easey, 1988). Life history: During winter, mature cod move towards particular areas for spawning in late winter and spring (see Ellis et al., 2012 for map of areas around the British Isles). In the northern part of their range there is a general tendency to move south- wards in winter prior to spawning, returning northwards for feeding in summer. Cod are sensitive to water temperature, which is reflected in the timing of life-history events such as spawning as well as size
age relationships and other life-history para- meters. In general terms, cod spawn in winter and early spring and in the North Sea stock, the peak is usually March to April in water temperatures of 4 C
6 C. Spawning usually occurs near the seabed and the number of eggs shed (fecundity) var- ies with size and age, between about 2.5 million and 7 million, exceptionally 9 mil- lion. The fertilized eggs are buoyant and gradually float to the surface. After spawning the shoals disperse. Time to hatching depends on the water temperature but is typi- cally 2
3 weeks. At the time of hatching the larva is about 4 mm long, the mouth has not yet formed and the animal is at first entirely dependent for food on the ven- trally attached yolk-sac beneath which it floats upside down. About a week later, the yolk-sac has become completely resorbed and the mouth has perforated and the young fish begin to feed for themselves in the surface waters. At this early stage the nauplius larvae of copepods are a major part of their food. The planktonic phase lasts for about 10 weeks, by the end of which time the young cod has grown to about 2 cm in length and increased in weight about 40 times. Throughout this period copepods remain the chief food. In the North Sea, Calanus, Paracalanus, Pseudocalanus and Temora are important foods for the cod fry, which in their turn are preyed upon by carnivorous zooplankton, particularly ctenophores and chaetognaths. At the end of the planktonic phase the cod fry disappear from the surface layers and go down to the seabed. In the North Sea there are nursery areas to the southeast of the Dogger Bank and around the Fisher Banks. The fish are not easy to find at this stage because they are quite small and occur mainly in areas where the sea bottom is rocky, making it difficult to operate nets. Young fish sometimes occur in rock pools on the shore. At this stage they are often an orange-brown with a distinctly chequered pattern, in contrast to the more usual adult sandy brown. The change from planktonic to demersal life involves a change of diet. The young demersal cod feed at first on small benthic crustaceans such as amphipods, isopods and small crabs. As the fish increase in size, they take larger and faster-moving prey. Shoals of adult cod actively hunt, chasing pelagic prey through the middle depths. They feed mainly on other fish such as sand eels, whiting and haddock and also squid. A variety 396 Elements of Marine Ecology of benthic annelids, crustaceans and molluscs are also included in the diet, when the fish are feeding on the bottom. The growth rate of cod varies in different areas. In the North Sea they have reached approximately 8 cm in length at the end of their first 6 months, 14
18 cm by the end of the first year and 25
35 cm by the end of the second year. Further north the growth rate is slower. Off the Norwegian coast, cod only attain 8 cm by about a year, reaching 30
35 cm by the end of the third year. The fish begin to be taken in trawl nets once they exceed about 25 cm in length. In the North Sea, cod reach maturity when about 50 cm long at 3
4 years of age. Given the chance, they can grow to a considerable size, sometimes reaching about 1.5 m in length and weighing 30 kg or more but fishing has greatly reduced their average size. European plaice (Pleuronectes platessa) Flatfish are a group of demersal fish adapted for life on the seabed, mostly in sediment areas. Different species form the basis for important commercial fisheries around the world, including European Plaice in the NE Atlantic. The life history of this species serves here as a typical example. Distribution: Plaice (Fig. 8.2) are commercially important flatfish occurring in the northeast Atlantic area. They range from the Barents Sea and Iceland, south to south Spain and the western Mediterranean, but are commonest throughout the North Sea, English Channel and Irish Sea. Plaice are usually found on sandy bottoms on the Figure 8.2 European Plaice, P. platessa. Identification features: right-eyed, distinct orange or red spots, row of bony knobs on top of the head. From Shutterstock, Vladimir Wrangel.  Human impacts 1: sea fisheries and aquaculture 397 shallow continental shelf in depths of less than 80 m. They occasionally extend down to 120 m or more and can be found on mud and gravel as well as sand. Plaice are caught mainly by trawl or seine and are taken in shallow water all around the British Isles, though a large part of the commercial catch comes from the southern North Sea. They are fished for all year round, but the quality varies and is rather poor around spawning time. Life history: Although not great long-distance swimmers, adult plaice from the main Irish Sea, English Channel and North Sea populations, make regular migrations between known feeding grounds and spawning areas. Spawning occurs right round the British Isles but the main well-defined areas are in the southern North Sea and the Irish Sea and here large numbers congregate (see Ellis et al. 2012 for map of areas around the British Isles). A large part of the southern North Sea plaice population migrates southward during the early winter months towards the tongue of slightly warmer and more saline water which flows into the North Sea through the Strait of Dover. Here, spawning occurs between January and March. After spawning, the spent fish return northwards to their main feeding grounds in the central North Sea. In more northerly latitudes, plaice spawn a little later in the year, for example in the Barents Sea in April and around Iceland between March and June. There is no special uniformity of depth, temperature or salinity in the spawning areas, but they are never far from the coast and always in areas where water movements will carry eggs and lar- vae towards sandy coasts for nursery grounds. Sexual maturity depends on size rather than age. In the North Sea most females mature and first lay eggs when between 30 and 40 cm in length and 4
5 years of age and males mature at 20
30 cm when most are 4 years old. A female plaice spawns between 10,000 and 600,000 eggs in a season. They are laid close to the seabed, but are buoyant and are mostly fertilized in mid-water as they float up to the surface, espe- cially at night. The eggs increase slightly in density as the embryos develop, so that by the time the larvae hatch they are drifting in near-surface waters. They hatch after about 2
3 weeks into a symmetrical larva, 6
8 mm in length with a ventral yolk sac that is resorbed within about 8 days. After that the larvae feed on flagellates and small diatoms. As they grow, they take larger plankton including diatoms, molluscan larvae, and larvaceans, a particularly important component of the diet. Normal symmetry and a planktonic mode of life continue until about 4
6 weeks after hatching, sometimes longer. At this stage, when the larva reaches about 10 mm long, its metamorphosis begins and within the next 2 weeks it becomes gradually con- verted into a ‘flatfish.’ The body flattens laterally and the fish acquires a new swim- ming position with its left side downwards. The skull is progressively transformed by the movement of the left eye to a new position dorsal and slightly anterior to the right eye on what now becomes the uppermost side of the body. The swim bladder, which is present in the planktonic larva, is gradually lost. Colour disappears from the new 398 Elements of Marine Ecology underside and the upper parts develop the characteristic pigmentation and spots of the adult. During this period of metamorphosis, the fish becomes demersal and grows to about 14 mm in length. Once they have settled on the seabed, they feed at first on a variety of small benthic organisms including annelids, harpacticoid copepods, amphi- pods and small decapods. While very small, a significant food source is the siphons of bivalve molluscs which are bitten off but then regenerate. As the young fish grow larger, molluscs form an increasingly important part of the diet, although this varies from place to place according to availability. Newly metamorphosed fish remain in shallow nursery areas along coastlines. In the North Sea extensive sandy expanses along the shores of west coast Denmark and the Wadden Sea form a major nursery area for young plaice as do estuaries such as the Wash. Considerable fluctuations occur from year to year in the numbers of young plaice which successfully complete their metamorphosis. Several factors may influence survival including the strength and direction of the wind, upon which depend the speed and direction of drift of the surface water. Poor survival may occur in years when a large proportion of the larvae fail to reach, or are carried beyond, the sandy regions required by the young fish. Tagging experiments have shown that during their first year of life, the majority of plaice live close inshore, mainly in water of less than 5 m depth. With increasing size they migrate into deeper water but remain in depths of less than 20 m until they are about 20 cm long (around 3 years old). Thereafter, they move increasingly further off- shore. The size reached by a particular age depends partly on the food supply. North Sea plaice reach a size of between 35 and 45 cm by about their sixth year of life. In most areas the females are slightly larger than males of the same age. Plaice occasion- ally hybridize with both flounder and dab. Atlantic herring (Clupea harengus) Whilst herring provide substantial fisheries on both sides of the North Atlantic, in the British Isles and particularly in the North Sea, the herring fishery has a long and important social and economic history. Its ups and downs have influenced the way of life of whole communities and the development of towns and ports along the east coast. In spite of its once huge abundance, it is an example of a species that has under- gone dramatic stock collapses from over-fishing (see Section 8.2.1). Its place in the his- tory and culture of NE Atlantic fishing nations is documented in an amusing and informative recent online encyclopaedia (Rigby, 2020). Distribution: Herring (Fig. 8.3) are widely distributed in shelf areas on both sides of the North Atlantic. In the NE Atlantic they extend from the Arctic, south to the area of Gibraltar on European coasts and in the NW Atlantic from Labrador to Cape Hatteras on the North American coast. However, it is only in northern areas that they are sufficiently abundant to be commercially valuable. They have now become scarce  Human impacts 1: sea fisheries and aquaculture 399 Figure 8.3 Atlantic Herring (C. harengus). Identification features: Single dorsal fin in middle of back; rounded belly with no keel; smooth, unridged gill covers. From Shutterstock, Ilya Akinshin. in some areas, such as the North Sea where they were once immensely abundant and they now no longer support the great fisheries they once did (see Section 8.3.2). With such a wide distribution, it is not surprising that the herring exhibits a number of local races, distinguished by features such as the number of vertebrae and by differences in spawning times and growth rates. A very similar form, Clupea pallasii, extends in the North Pacific from Japan to the coast of British Columbia and also occurs in the North Atlantic. Herring-like fish (Clupeidae) are found in many other parts of the ocean and also in freshwater. Herring are pelagic fish of mainly offshore areas, making inshore migrations in great shoals for spawning in certain coastal localities, after which the shoals disperse and the spent fish move out to deeper water, reassembling offshore as feeding shoals. They are therefore caught mainly using ring nets and purse seine nets. Life history (British Isles): When herring are preparing to spawn, they congregate in huge shoals as they approach their spawning grounds. They are the only commercially important ray-finned fish in British and Irish waters to lay demersal eggs. A spawning female deposits 10,000
60,000 eggs on inshore banks of small stones and gravel. The eggs are heavier than water and form sticky masses which readily adhere to seabed stones and seaweeds and to each other. Most races spawn in depths from 15 to 40 m, but some oceanic races spawn on offshore banks as deep as 200 m (Hodgson, 1967; Parrish and Saville, 1965). Because they lay demersal eggs, locating herring spawning grounds is particularly important in order to protect them from trawling. Apart from dredging which is not very effective, spawning grounds have been found in the past from observing the posi- tion of ripe and newly spent herring, the occurrence of herring larvae in tow net hauls and by noting the places from which trawlers bring up ‘spawny’ haddock, that is had- dock engorged with herring eggs, on which they prey. The distribution of spawning grounds in the North Sea can also be very roughly equated with the distribution of predominantly gravelly areas, which in turn can be identified using sonar techniques (see Ellis et al., 2012 for map of areas around the British Isles). 400 Elements of Marine Ecology The major spawning shoals are of two principal races, oceanic and shelf herring. Oceanic herring have an extensive range in deepwater in the NE Atlantic, Arctic and Norwegian seas. In the latter, spring-spawning shoals move towards the Norwegian coast, whilst those which approach the British coasts form winter-spring spawning shoals between February and April around the Hebrides, Orkney and Shetland Isles and along the Irish and Scottish coasts. These shoals occur mainly in water of oceanic- neritic type (see Section 2.7.7) where the temperature is between about 5 C and 8 C. In contrast, the shelf herring of the North Sea, English Channel, Minch and Irish Sea form mainly summer-autumn spawning shoals. The North Sea shoals spawn progres- sively later from north to south. In the Shetlands to the east coast of Scotland spawn- ing is from July to September, off the East Anglian coast between October and December and in the eastern English Channel near the north French coast from December to January. These shoals are found in neritic water at 8 C
12 C. Slight morphological differences have been noted between the two groups. Both show variation in the number of vertebrae between 54 and 59, with a mean vertebral count slightly above 57 for the oceanic herring and below 57 for the shelf herring. There are also slight differences in the number of gill rakers, number of keeled scales on the ventral surface and in the structure of the otolith. The oceanic herring are slower growing, later maturing, longer lived and reach a larger size than the shelf herring. It is now generally considered that the two groups, both of which can be further subdivided into several more or less distinct stocks, are biologically separate units with no appreciable interbreeding. In water of 5 C
6 C, herring hatch in about 22 days, at 11 C
12 C in 8
10 days. Newly hatched larvae swim to the surface and are about 6
8 mm long. At first they depend on the food reserves of the yolk sac whilst they develop a mouth and can begin to feed on very small plankton such as diatoms, and early copepod larvae. As the herring larvae grow, they take larger plankton including adult copepods. For their first 3 months or so and until they reach about 45 mm long, the larvae resemble tiny, almost transparent eels. The larval fish are pelagic and drift with the currents. There is some mixing from different areas and a high proportion of larvae from the North Sea drift considerable distances into nursery grounds situated in the shallow central and southern North Sea (Heath and Richardson, 1989). Extensive nursery areas exist along the shallow coast- line of the Netherlands, Germany and Denmark and in large estuaries such as the Thames Estuary, the Wash, the Moray Firth and the Firth of Forth. When the young fish reach about 5 cm long, they are able to swim more strongly and form large shoals known as ‘whitebait’, which often contain a mixture of young herring and young Atlantic Sprat (Sprattus sprattus). Areas such as the Dogger Bank support large numbers of young fish. The young herring remain inshore until near the end of their first year of life and then start to move offshore. By now the fish are mostly some 4
8 cm in length and during their second year these fish grow to some 13
18 cm in length but are still extremely thin.  Human impacts 1: sea fisheries and aquaculture 401 In their third year the herring start to fatten and feeding shoals of the immature fish often approach the English coast during mid-summer. Throughout life the food is predominantly planktonic, mainly copepods, but also chaetognaths, pteropods, appen- dicularians, decapod larvae and fish eggs. The type of food varies seasonally and geo- graphically depending on availability. By the end of their third year they have reached the ‘fat herring’ stage and the flesh has become rich in oily and nutritious fatty acids. The quality of the fish depends upon the abundance and nature of their food. The valuable oiliness of the herring reflects the fat content of the plankton, which in turn depends to some extent upon climatic conditions. Generally, a warm, sunny summer produces a rich plankton and provides fish in excellent condition for the autumn fish- ery. A copious diet of copepods produces a specially oily flesh. If the food consists mainly of pteropods, the fat content of the flesh is poorer. During their fourth year, that is as Group III fish, the majority of southern North Sea herring become sexually mature. The oceanic fish of the northern part of the North Sea and the Norwegian coast mainly reach maturity later, between their 5th and 8th years. The maturing fish leave the young fish shoals and join the adult spawning shoals. 8.1.3 Anadromous and catadromous species Some species of fish spend part of their life cycle in freshwater and part in the ocean. This way of life involves migration over considerable distances and physiological adapta- tion when moving from one medium to another. Species that spend their adult life in the ocean and move up rivers to spawn are termed anadromous and include all species of salmon (Salmonidae) as well as shads (Alosa spp.), which belong to the herring family (Clupeidae) and some lampreys (Petromyzontidae). In contrast, species in which the adults live in freshwater and migrate out into the ocean to spawn are termed catadro- mous. A prime example of catadromous species is freshwater eels (Anguilla spp.). Wild salmon from several different genera are important commercial species in the North Atlantic and North Pacific Oceans. Atlantic Salmon (Salmo salar) spend about 3 years in the ocean before they are ready to spawn. In autumn and early winter they return to the river where they were born and travel far upstream, negotiating water- falls and rapids until they reach headwaters. Here they spawn in pairs, the female lay- ing her eggs in fast-flowing gravelly areas. At this stage many die from exhaustion, but some are strong enough to return to the sea. Young Atlantic Salmon spend up to 3 years in freshwater before migrating downriver to the ocean. In other species such as Pacific Sockeye Salmon (Oncorhynchus nerka) virtually all the adults die after spawning. However, their bodies provide a vital food resource for predators such as bears and their remains, dragged into nearby forest, providing soil nutrients. Out at sea on their feeding grounds, salmon are caught using purse seine nets and gill nets (see Section 8.3.3). There is an obvious vulnerability in catching salmon as 402 Elements of Marine Ecology they congregate in estuaries and the lower reaches of rivers to acclimatise to their new salinity regime, before running upstream to spawn. Many wild salmon stocks have declined dramatically and international, national and local management of stocks are all necessary for sustainable fisheries. European and American Eel (Anguilla anguilla and A. rostrata) fisheries have also declined significantly. These two species have an extremely complex life cycle involv- ing several different adult, juvenile and larval stages. Adults live in freshwater for any- thing up to 30 years, the time depending on many environmental and geographical factors, but once they approach maturity they move downriver and migrate thousands of miles to the Sargasso Sea in the deep waters of the North Atlantic to spawn. In good conditions, females may be ready to make the journey by about 8 years old. The adults never return, but their leaf-shaped leptocephalus larvae drift back over months and years to the coasts of Europe and North America, respectively. The larvae develop into small transparent glass eels with the shape and ability to wriggle their way up riv- ers and streams, often moving together in their thousands. This is when they are most vulnerable and also most valuable and in spite of strict controls in most European countries, millions are caught illegally. Eels are an example of just how important it is to know the full life history details (though some aspects of their migration and spawning still remain a mystery). Management of fisheries for species with such com- plex lives must involve not only how, when and where they are caught, but also con- siderations of access past constructions such as dams and weirs, availability and condition of freshwater habitats and international controls on trade. 8.1.4 Stocks Whilst many commercial species of fish have a wide geographical distribution, this does not mean that the whole population can intermingle. There is a semiseparation into a number of stocks and separated stocks of the same species evolve their own var- iations in terms of life history, spawning migrations, and so on. The fish in a biological stock live in a geographical area within which they are able to mix and breed. In terms of implementing fisheries management it is therefore important to know about these variations and to treat each separate stock unit accordingly. In the UK there is an obvious geographical separation of the North Sea from the eastern Atlantic. This results in a separate North Sea stock of many important commercial species. For example two stocks of Atlantic Mackerel (Scomber scombrus) are found in northwest European waters: a western stock which spawns mainly along the shelf edge west of Great Britain in the Celtic Sea and Bay of Biscay and a North Sea stock which spawns in the central North Sea and off southern Norway. During their spawning period, mackerel congregate on the spawning grounds in huge shoals. The western stock spawns between February and July, the most intensive  Human impacts 1: sea fisheries and aquaculture 403 spawning occurring during April in a spawning area some 50
80 miles west of the Isles of Scilly, mainly along the line of the continental edge extending southwards into the Bay of Biscay. As the season proceeds, spawning fish are found in progressively shallower water further to the east, until by July they are spawning in St. George’s Channel, the Bristol Channel and the western part of the English Channel. The North Sea stock spawns from May to July, peaking in June. The annual movements of the fish within these stocks, from overwintering grounds to and from spawning and feeding grounds, are complex and lead to some intermingling of the two stocks (Box 8.3). This species has a wide range extending BOX 8.3 Atlantic Mackerel migrations around Great Britain. At the end of the spawning period and towards the end of the autumn mackerel disappear from coastal regions and return to their overwintering grounds. It is thought that the fish tend to return to the same overwintering sites from which they left the previous spring. Part of the western stock migrates up the west coast of Britain to the northern North Sea and Norwegian Sea to overwintering grounds there. There is also some migration through the English Channel from the west into the southern North Sea. Other major overwintering shoals are thought to extend well down the continental slope to the west of the Isles of Scilly and there are also other concentrations along the south coast of Cornwall. The timing of these migrations has changed over the decades, probably as a result of shifts in major currents and may continue to change in the light of global warming. Overwintering fish are demersal, congregating near the seabed and feeding on any avail- able food such as small crustaceans, polychaetes and small fish. Cascade currents (see Section 3.5.3) may carry food down from the surface in some areas. Towards the end of winter, fish begin to leave their overwintering concentrations. At first they still keep to the bottom but soon begin to perform diurnal vertical movements, ascending during darkness and eventually forming surface shoals. During the early months of the year the surface plankton is sparse and the majority of the fish are still without food, but they readily feed when suitable food is found, for example on shoals of small fish such as pearlsides (Maurolicus muelleri). During the late winter and spring the surface shoals begin to move towards the spawn- ing areas. Those which overwintered in the English Channel and southern North Sea move westwards, those from the Irish Sea move south-westwards, while those which wintered on the continental slope move towards the east, all tending to converge towards the west of the Isles of Scilly. From the northern North Sea fish move towards the Norwegian coast, or southwards into the central North Sea. After spawning the shoals move away from the spawning grounds, some over consider- able distances. Many that spawn in the Celtic Sea move eastwards towards the English and French coasts. Some pass up the English Channel into the North Sea and others go north- wards west of Ireland or through the Irish Sea to the Hebrides, or even beyond to the Shetlands, where there is some mingling of the western and Norwegian stocks. Fish that spawned in the central North Sea return mainly northwards towards the Norwegian coast. 404 Elements of Marine Ecology from northern Norway as far south as the Canaries. Separate stocks also occur in the Mediterranean and on the western side of the Atlantic from south Labrador to North Carolina. 8.2 Overfishing Overfishing is a complex subject but in simple terms, occurs when too many of a par- ticular species of fish are caught, without allowing an opportunity for the fish to natu- rally replenish themselves. Too many fish can be taken, leaving insufficient numbers to build up the stock again in a sensible time scale. In addition too many immature fish can be caught before they have had a chance to reproduce. The latter is the thinking behind imposing lower limits on the size of an individual that can be caught. This is possible for example, in fisheries such as potting for crabs and lobsters see also Box 8.4. BOX 8.4 Early evidence of overfishing in the North Atlantic. Addressing the International Fishery Exhibition in London in 1883, T. H. Huxley said: I believe that it may be affirmed with confidence that, in relation to our present modes of fishing, a number of the most important fisheries, such as the cod fishery, the herring fishery and the mackerel fishery, are inexhaustible. And I base this conviction on two grounds, first, that the multitude of these fishes is so inconceivably great that the number we catch is rela- tively insignificant; and, second, that the magnitude of the destructive agencies at work upon them is so prodigious, that the destruction effected by the fisherman cannot sensibly increase the death rate. Shortly afterwards the landings of fish from the seas around northwest Europe increased to an extent that Huxley could not have envisaged. Sailing vessels were superseded by ships with powerful steam engines, enabling the use of much larger nets and the replacement of the old beam trawl by the far more effective otter trawl and giving fishermen relative inde- pendence from wind and tide so that they could fish for longer and more often. Within a few decades of Huxley’s pronouncement there was already evidence of reduction of stocks of certain favoured demersal species such as cod, haddock and plaice, in the more inten- sively fished areas of the northeast Atlantic. In 1942 E. S. Russell (Russell, 1942) wrote: A state of overfishing exists in many of the trawl fisheries in northwest European waters. Two things are wrong. First, there is too much fishing, resulting in catches below the possible steady maximum, and second, the incidence of fishing falls too early in the fishes’ life result- ing in a great destruction of undersized fish which ought to be left in the sea to grow. From the late 1940s the catching power of fishing vessels was further augmented by several innovations. The change from steam to diesel power raised the power and speed of ships and by the 1960s, steam trawlers were fast disappearing. The development of nets (Continued)  Human impacts 1: sea fisheries and aquaculture 405 BOX 8.4 (Continued) made of stronger, lighter and longer-lasting synthetic materials allowed the use of larger and more sophisticated designs of nets. Another step forward was the further development of sonic techniques for the detection of fish shoals and for net handling. Modern navigation systems such as GPS (see Section 7.5.5) allow accurate return to good fishing areas and accu- rate deployment of nets. Refrigeration equipment installed on fishing boats now allows them to range far afield and continue fishing until full without deterioration of the catch. The rise in world landings of fish since the end of World War II has been remarkable (Fig. 8.4). Over the period 1948
68 the increase was around 7% per year, bringing the world total annual catch of marine fish from under 20 million tonnes in 1948 to over 60 million tonnes in 1970. Around 1970, the annual catch stopped rising and between 1980 and 1989 it rose very slowly. It continues to rise but the flattening of the curve, even with greater fish- ing efforts, is an indication of current overfishing problems. In order to understand what is involved in ‘overfishing’, it is useful to consider the effects that fishing is likely to have on the size and composition of fish populations. In the case of a stock of fish subjected only to very light fishing, the population can be regarded as having grown to the limits imposed by the available food supply, which restricts both the number of fish surviving and the size to which the individual fish grow. Scarcity of food prevents all the fish from making as much growth as they could if they were better fed and the slow-growing, older fish compete for food with the rapidly growing younger specimens. A stock of fish in this condition could be described as ‘underfished.’ The population is overcrowded and an undue proportion of the food is consumed by old fish of poor market quality at the expense of young fish and none can realize its full growth potential. This stock could support a larger, more profitable fishery of better-quality fish if more fish were caught. A reduction in the size of the population, particularly by the removal of older fish, could promote a better growth rate throughout the remaining stock and improve the condition of the fish, the process being analogous to a gardener thinning out his plants to encourage the best growth and quality of his specimens. Considering now the opposite case, a stock of fish is subject to extremely heavy fishing. This population is likely to consist mainly of young, small specimens because the fish are caught as soon as they reach catchable size. This stock is obviously ‘overf- ished.’ Too many fish are caught too early in life. Young fish make rapid growth and if left longer in the sea, would soon reach a more valuable size, providing heavier landings and better prices. Productivity and profits would eventually improve if the amount of fishing was reduced. However, in these conditions the fisherman is tempted to try to increase his profits by catching even more fish. This is a vicious spiral which can only lead to a further reduction in the size of the stock, and a further dwindling of the fisherman’s income. 406 Elements of Marine Ecology Indeed a point may be reached at which so many fish are caught before they have lived long enough to spawn that the reproductive capacity of the stock becomes severely impaired, leading to a catastrophic decline in numbers through failure to pro- duce enough young and even a danger of extinction. We can therefore distinguish two aspects of overfishing. There is what may be termed growth overfishing, where catches are poor because too many fish are caught before making optimum growth, but recruitment of young fish is not seriously affected. There may also occur recruitment overfishing when the stock fails through depressed intake of recruits resulting from a reduction in the numbers of spawners. The above description is of course a great over-simplification. However, it is increasingly being realised that when stocks do recover with efficient management schemes in place, then much more account may need to be taken of species and eco- system interactions when formulating and imposing management. A simple example might be of a rebuilt cod stock, as a result of restrictions on fishing. Cod are predators of other high-value fishery species such as Nephrops (see Section 8.3.5) and conceivably Nephrops stocks in a particular area could be negatively impacted as cod stocks recover. However, currently the problem of overfishing throughout the ocean far outweighs the ‘problem’ of underfishing. 8.2.1 Why and how The total global capture fisheries production in 2018 was the highest ever recorded at 96.4 million tonnes (FAO, 2020b). Of this, 84.4 million tonnes were from marine sources and 12 million tonnes from freshwater (Fig. 8.4). Fish in this context includes fish, crustaceans, molluscs and other aquatic invertebrates (but not seaweeds, seagrasses, reptiles and mammals). When overfishing is allowed to continue in the face of declin- ing stocks, dramatic stock collapses have and can occur of which three examples are North Atlantic Herring (C. harengus), Atlantic Mackerel (Scomber scombrus) and Canadian Grand Banks Atlantic Cod (G. morhua). Herring collapse For many years it was thought that the stocks of North Atlantic herring were so large as to be virtually inexhaustible no matter how intensively fished (Box 8.4). As long as fisheries were conducted mainly by drift nets exploiting only local shoals, this may well have been true. Although the size and movement of herring shoals were subject to many fluctuations, appearing and disappearing in unpredictable ways, with the con- sequent rise and decline of particular local fisheries, there was seldom any overall shortage of herring. However, during the years following 1948 an intensive trawl fish- ery for immature herring developed on the North Sea nursery grounds, capturing huge numbers of herring in age groups I, II and III (see Section 8.4.4) for conversion  Human impacts 1: sea fisheries and aquaculture 407 Chart Title 90 Millions of metric tonnes 80 70 60 50 40 30 20 10 0 Years FAO stascs Marine capture FAO stascs Marine aquaculture Figure 8.4 Global landings of marine capture fisheries (blue) and marine aquaculture production (red) 1950
2018, both excluding marine mammals, seaweeds and plants. Between about 1950 and 1970 world landings of marine capture rose steeply, but between 1970 and 1980 landings hardly rose at all, then rose slowly again and since the 1990s landings have remained relatively constant despite increased fishing efforts. Freshwater capture reached 12 million tonnes in 2018 and fresh- water aquaculture reached 51 million tonnes (neither shown). Data from FAO yearbooks (FAO, 2020b. FAO Yearbook. Fishery and Aquaculture Statistics 2018. Rome. https://doi.org/10.4060/cb1213t). to fishmeal for animal feed. Far more fish were destroyed in this way prior to spawn- ing than had ever previously been taken from the adult shoals by drift nets. In the following decades drift nets were largely replaced by ring nets, purse seines and pelagic trawls, which enclose shoals that have been located by accurate sonic methods. In the 1930s although herring were heavily fished according to the methods of those times, the total annual catch of herring from the northeast Atlantic then aver- aged under 1.5 million tonnes. By 1965 and 1966 the landings obtained by newer methods were over 3.5 million tonnes per year. These huge catches were followed by a dramatic decline of landings. In 1969 the catch had fallen to 1.4 million tonnes and by 1976 to less than 800,000 tonnes. The devastating effects of a combination of the industrial fishery for immature her- ring and the more intensive fishing of adult shoals caused the North Atlantic herring stocks to collapse and in 1977 a total ban was imposed on the fishing of herring over virtually the whole area in the hope of some recovery of stocks. By 1983 the popula- tion had partially recovered and fishing began again with limits on size and numbers taken. The world catch of Atlantic Herring in 2018 was 1.8 million tonnes. The sustainability of herring stocks today (2020) varies widely and there is still cause for concern. Even in those stocks that are fished within sustainable limits, there have been some worrying declines in spawning stock biomass within the past few years. In spite 408 Elements of Marine Ecology of adequate management, other factors such as seabed disturbance from aggregate dredging can impact this bottom-spawning species. Stocks such as those in the Baltic remain in an over-fished state. The history of northeast Atlantic herring fishing is a sorry example of failure to uti- lize rationally a major natural resource which, if properly managed, could provide large quantities of highly nutritious food for direct human consumption. A major part of the excessive landings of the mid-1960s was processed as fishmeal for animal rearing. Atlantic mackerel stock collapse Mackerel are caught by trawling in the winter, when they are demersal and through most of the year by various pelagic fishing methods, mainly purse seines and pelagic trawls. Some small boats still use handlines in winter in areas such as along the Cornish coast. During the mid-1960s the North Sea fishery underwent enormous expansion from normal levels of 100,000 tonnes or less to nearly a million tonnes in 1967. This was partly as a result of the purse seine fleet transferring most of its attention to mackerel after the herring fishery collapsed. The catch was used mainly for fishmeal and oil. During the early 1970s the landings declined rapidly as the stock diminished and the Norwegian government instituted controls to limit mackerel fishing. However, the size of the spawning stock in the North Sea is presently less than 50,000 tonnes and biologists believe the North Sea mackerel will probably become effectively extinct, apart from influxes from Atlantic waters in the north. The south-western fishery has traditionally been of interest mainly to British, French and Spanish ships, and landings were always less than from the North Sea and remained fairly stable at around 30,000 tonnes per year. The importance of this fishery increased with the decline in herring landings, and in 1968 Soviet ships began to fish the south-western stock. By 1975 annual landings from this fishery had increased to about 500,000 tonnes, with ships from the USSR taking over half this total. In 1977 Soviet ships were excluded from EEC waters. The western spawning stock was still relatively large in 1989, approximately 2 million tonnes and it is mainly this stock that the international fishery now exploits. Canadian Grand Banks cod fishery collapse One of the best-known collapses of a fish stock, principally as a result of overfishing and lack of scientific-based management, is the Canadian Grand Banks cod fishery. Environmental conditions in this area made it a rich feeding ground for many species of fish including Atlantic Cod. The area is relatively shallow (,100 m) and the con- junction of the warm Gulf Stream and the cool Labrador current ensure a good supply of nutrients and plankton, ultimately resulting in plenty of food for an omnivore like  Human impacts 1: sea fisheries and aquaculture 409 cod. The history and ultimate demise of the fishery is a long and complex one involv- ing politics, international agreements and disagreements, advances in fishing technol- ogy and a lack of adequate management, at least partly due to an unwillingness to accept and act on evidence of declining stocks. The final collapse of the stock and a moratorium on fishing came in 1992 and was a social and economic disaster for local communities in Newfoundland and Labrador. The story is well-told in a popular book by Kurlansky (1997). In an ecological context it is the lack of recovery of the fishery that is of interest. Atlantic Cod are extremely fecund (see Section 8.1.1) but the stocks have not yet (2020) recovered to the stage where a fishery could be reopened to any great extent. The most likely reason behind the lack of recovery is that intense and prolonged sea- bed trawling has changed the ecosystem to the extent that it now cannot recover to anything like its previous state. Cod depend to a large degree on food sourced from the seabed and trawling can be very destructive to bottom habitats. Recovery of sea- bed habitats and communities and restoration of links with the pelagic systems above may be a very slow process. Seabed trawling and impacts are discussed in Section 8.3.4 and it is now increasingly being understood that fisheries management needs to con- sider whole ecosystems in its approach. Another factor in the slow recovery of stocks may be that Atlantic Cod are a long-lived species and can potentially grow to a large size (see Atlantic Cod in Section 8.1.2). Large females produce proportionately greater numbers of eggs and a lack of older (10
15 year old) ‘motherfish’ means many fewer eggs and larvae available to build up the stocks. Constant heavy fishing pressure means that few fish survive to reach this size and age. It is also possible that such heavy fishing may exert evolutionary pressure for the fish to mature earlier at a smaller size. Since the mid-2000s, there have been small signs that stocks may at last be recover- ing, but stocks are still calculated to be only around 10% of what they were in the 1960s. Wars and fish stocks During the first half of the 20th century the fisheries of the northeast Atlantic were twice interrupted by war. Analyses of various fishery statistics reveal the dramatic impact the cessation of fishing had on heavily pressured stocks, which showed indica- tions of underfishing during war years and severe overfishing in peacetime. For exam- ple, the North Sea provides an important fishery for Haddock (Melanogrammus aeglefinus). During the years prior to 1914, the total landings of Haddock from this area showed a fairly steady decline. During the period of the 1914
18 war, fishing in the North Sea was greatly reduced. When normal fishing was resumed after the war, greatly increased yields were at first obtained, amounting to approximately double the pre-war landings, but during subsequent years the catches gradually diminished. By the later 1930s the annual landings of haddock had fallen below their pre-1914 level 410 Elements of Marine Ecology to about a quarter of their 1920 weight. During the war of 1939
45, fishing in the North Sea virtually ceased. When normal fishing was re-established, there was at first again a marked increase in haddock landings, but subsequently the yield of the fishery again fell (Fig. 8.5). These diminishing peacetime yields have not been the result of any reduction of fishing effort, rather the reverse. Furthermore, as the yields decline the percentage of the catch consisting of small fish shows a great increase (Fig. 8.6). In the European Plaice (P. platessa) fishery, comparable changes occurred (Fig. 8.7). The larger size categories of plaice in the North Sea were abundant only in the imme- diate post-war years (Fig. 8.8) whilst before and after that, the catch consisted mainly of smaller fish. Although the total weight of plaice landed from the North Sea remained fairly constant over the first half of the 20th century, the effort and expense of catching them greatly increased, that is the yield per unit of fishing effort has gone down. During the 1960s there was some reduction of fishing for plaice in the North Sea because of poor returns. Attention turned to other species and some old vessels were laid up and not replaced. Consequently, there was some recovery of plaice stocks in the late 1960s and landings considerably improved. In normal peacetime operation, plaice and haddock fisheries of the North Sea have therefore shown major features of overfishing, namely, declining yields and a prepon- derance of small fish. Similar trends have been observed in many areas for such impor- tant species as cod, plaice, haddock, halibut and hake. The war years, on the other hand, produced some symptoms of underfishing. The stock of fish increased greatly Wt. landed in cwt/days absence from port Wt. landed in cwt/100 h fishing 20 200 15 150 War years War years 10 100 5 50 1905 10 15 1920 25 30 35 40 1945 1950 Years Figure 8.5 North Sea Haddock catch per unit of fishing effort by Scottish trawlers, before, between and after the two world wars. Statistics for the years 1905
15 show landings per days absence from port (red) and for years 1920
50 landings per 100 h (blue). Modified from Graham, M. (Ed.), 1956. Sea Fisheries. Their Investigation in the United Kingdom. London, Arnold (Graham, 1956). Courtesy Edward Arnold.  Human impacts 1: sea fisheries and aquaculture 411 Figure 8.6 North Sea Haddock catch per unit of fishing effort in size categories and percentage of the total in the ‘small’ category, for the years 1923
38. Modified from Graham, M. (Ed.), 1956. Sea Fisheries. Their Investigation in the United Kingdom. London, Arnold. Courtesy Edward Arnold. War years War years 5 Yield in cwt x 105 4 3 2 1906 8 10 12 14 1920 22 24 26 28 30 32 34 36 38 1946 48 Years Figure 8.7 Catches of North Sea plaice by first-class English steam trawlers, before, between and after the two world wars. Modified from Wimpenny, R.S., 1953. The Plaice. Buckland Lecture. London, Arnold. Courtesy The Buckland Foundation. 412 Elements of Marine Ecology 60 50 Percentage of landing 40 30 20 10 20/24 25/29 30/34 35/39 40/44 45/49 Length groups (cm) Figure 8.8 Size distribution of plaice landings at Lowestoft, UK before and after World War II. Red is catch in 1938. Blue is catch in 1946. Modified from Wimpenny, R.S., 1953. The Plaice. Buckland Lecture. London, Arnold. Courtesy The Buckland Foundation. and this was reflected in the heavier catches obtained when fishing was resumed. These early post-war catches contained a high proportion of large fish of the older age groups, but some were diseased and of poor quality. In the case of plaice, there is some evidence that the stock increased to such an extent that the average growth rate of the fish was reduced, presumably by food shortage. Stock collapses from other causes The classic history of the Peruvian fishery for anchoveta (E. ringens) has been a remarkable example of the rapid development of a primitive fishery to become one of the world’s major sources of fish, unfortunately followed by a dramatic decline. During the period 1948
68 this fishery increased over a 100-fold to 10 million tonnes per year (more than one-sixth the entire world catch), providing Peru with a larger catch than any other nation. In 1970 the fishery took 12.5 million tonnes, but by 1973 landings had fallen to little more than 2 million tonnes. This failure exemplifies some of the problems of distinguishing the effects of overfishing from those of concur- rent environmental changes. The collapse of the fishery occurred at a time when changes to the circulatory pattern of the area reduced the extent of upwelling (see Section 2.7.4) of deepwater along the continental slope, upon which the area’s prolific production of marine life depends. There had previously been signs of impending threat to adult stocks because fewer young fish were entering the shoals, probably the result of over-exploitation. Although there was then some recovery of the fishery, the 1987 catch over a decade later was only 2.1 million tonnes. Landings in 2018 amounted to 7 m million tonnes, up from 3 million tonnes in 2014. The fishery con- tinues to be affected by changes in upwelling and current patterns, resulting from  Human impacts 1: sea fisheries and aquaculture 413 incidences of ‘El Nino’ (see Section 2.7.5) the number and strength of which seem to be increasing as a result of climate change. 8.2.2 Optimum fishing rates and maximum sustainable yield The management of any wild capture fishery relies on statistics, both landing/catch statistics and scientific survey data such as size and ages of the fish making up a stock, spawning stock biomass and recruitment, in other words, stock analysis (Section 8.4.4). Provided there is market demand for the product, then a well- managed, wild capture fishery might aim to take as many fish as possible without affecting the ability of the stock to replace those taken. This is essentially the concept of maximum sustainable yield (MSY), the maximum catch of fish by weight or num- bers that can be taken from a stock over an indefinite period of time. In European waters, most commercial fish stocks are thought to be outside safe biological limits. This is another way of saying that a stock is overfished and occurs when mortality through fishing exceeds recruitment and growth of the stock (see Box 8.5). Given good environmental conditions, there will naturally be a balance between predators and prey and predation is an essential part of maintaining healthy populations of prey fish. Fishing is essentially a form of predation and at an optimal level could theoretically help maintain the maximum possible sustained natural yield, which is an BOX 8.5 Russell’s equation. The Scottish biologist and philosopher E. S. Russell was considering the problem of overfish- ing as long ago as the late 1930s. Given an optimal environment the weight of a stock of fish tends to increase as the fish grow and young fish join the stock. The effects of fishing and natural mortality operate against this tendency to natural increase. Russell (1942) com- bined these factors into a simple equation: S2 ¼ S1 þ ðA þ GÞ
ðC þ MÞ where S1 is the weight of stock at the beginning of a year, S2 is the weight of stock at the end of that year, A is the annual increment by recruitment of young fish to the stock, G is the annual increment due to growth of all fish in the stock, C is the total weight of fish removed during the year by fishing, and M is the weight of fish lost during the year by death from all the other causes, that is the natural mortality. The amount by which the stock weight would increase if no fishing took place, A+G
M, can be termed the natural yield. In the special case where the total stock remains unchanged, C+M must equal A+G. In these conditions the fishery is said to be ‘stabilized’, and fishing removes an ‘equilibrium catch’, that is a weight of fish that exactly corresponds with the natural yield of the stock. In theory, equilibrium conditions might be established for any level of stock weight, but the natural yield will vary for different weights and composi- tions of stock. 414 Elements of Marine Ecology optimum yield. Such a fishing intensity could be termed the biological optimum fish- ing rate. Underfishing and overfishing are extremes both of which result in low natural yields. However, there are also economic factors to be considered. Highest profits do not necessarily result from the heaviest landings. If a market is glutted, prices collapse. A biological optimum fishing rate, giving the heaviest possible sustained landings, may not be the same as an economic optimum fishing rate, that is one giving the greatest financial returns. The best utilization of a fish stock must be one that is acceptable from the viewpoint of both the fishing industry and society in general. It is only through the regulation of fishing activity (Section 8.5) that there is a practical possibility of achieving optimum yields. The relationships between fishing, stock, yields and profitability are by no means simple. The same weight of fish can be taken in innumerable ways: as a small number of large fish, a large number of small ones, or any combination of different sizes. Every variation in the composition of the catch will have a different effect upon the composition of the stock and its natural yield. To achieve anything approaching optimum yields it would therefore be neces- sary to control not simply the gross weight of fish landed but also the numbers of each size of fish. There are broadly two ways of studying the dynamics of fish stocks relative to fish- ing activities, both of which are used as bases for the formulation of fishery regulations. They are generally termed respectively the surplus production approach and the ana- lytical or yield-per-recruit approach. Recruits are young fish joining the adult fish stock. The surplus production approach visualizes the stock as a single unit, adopting the simple concept of fisheries already outlined. The effect of fishing is regarded as cropping the natural increase of the stock, thereby reducing the stock to a level below the limit set by the environment and promoting the production on which the fishery depends. Equilibrium yields are considered to be determined mainly by the size of the stock which can be controlled by varying the fishing intensity. The data required are fairly simple, mainly the statistics of fish catches and fishing effort. This information indicates general trends of the fishery from year to year with different intensities of fishing and enables predictions to be made of the effects of changing the fishing effort. The alternative analytical, yield-per-recruit method attempts a more fundamental elucidation of all factors producing changes in the size and yield of fish stocks. Instead of regarding the population as a single unit it is analysed in terms of all the individual fish constituting a number of separately recruited units, the annual year-classes of the stock. The objective is to estimate the contribution to the yield of the fishery at vari- ous fishing intensities from each year-class, or from one year-class throughout its life- span in the stock. This requires much more detailed data over a wider range of fish biology than for surplus production studies, especially with respect to the relationships of stock size and composition to growth, mortality and recruitment. The analytical approach offers possibilities of greater precision of prediction. It separates the effects of  Human impacts 1: sea fisheries and aquaculture 415 changes in the total amount of fishing from those of changes in the selectivity of fish- ing gear, such as are obtained from alterations in mesh size of nets or hook size on lines. The size or age at which fish first become liable to capture must profoundly influence the stock size, recruitment rate and potential yield of a fish population. With sufficient information it is theoretically possible to compute MSY for every combination of fishing effort (boat hours) and gear restrictions, particularly in net mesh size. This leads to the concept of eumetric fishing, which in very simple terms may be defined as the optimum combination of effort and gear giving the MSY. 8.2.3 Bycatch A major problem faced by many fisheries is that of incidental catch or bycatch. Most fisheries target one species or group of species, but the nature of fishing gear means that other unwanted species are also unintentionally caught. Bycatch also includes juveniles and undersized specimens of target species. When bottom gear is used, bycatch can include sessile species ripped off the seabed. Whilst reliable statistics are difficult to obtain, some estimates suggest as much as 10% of the total global catch may be bycatch. Bycatch is usually discarded at sea, but it is difficult to estimate per- centage survival rates. Bycatch species may die not only due to damage suffered but also to additional predation after return to the sea. Both these factors will vary hugely depending on the species involved. Various studies have been done to estimate sur- vival rates, through assessing the levels of potentially fatal damage to bycatch species and through retaining bycatch and measuring its survival in laboratory tanks. Mitigation measures aim to minimise bycatch and increase the chances of its sur- vival through improvements and modifications to fishing gear and fishing methods. Regulatory interventions in terms of banning or regulating discards (of e.g., undersized fish) are imposed on some fisheries in some countries which aim to minimise waste of resources. The reform of the European Union Common Fisheries Policy (see Section 8.5.3) in 2013 included a gradual phasing out of discarding and an introduc- tion of a ‘landing obligation’, whereby all fish of any regulated commercial species that are caught, must be kept and landed by the fishing boat and counted against quotas. Problems associated with specific gears, fishery methods, target species and bycatch species and some potential solutions are described in Section 8.3. 8.2.4 Trophic cascades and fisheries Fisheries that target top predators such as sharks may trigger an ecological phenome- non termed a trophic cascade. The removal of a top predator or predators within a system (e.g., a kelp forest) can initiate changes in the relative abundance of predator and prey that can ultimately lead to irreversible (or nearly so) changes in whole 416 Elements of Marine Ecology ecosystems and potentially in a whole ocean context. It is termed a cascade because the effects can cascade down through multiple trophic levels. A well-studied example in a relatively simple ecosystem is the past over-exploitation of Sea Otters (Enhydra lutris), which ultimately led to the destruction of entire kelp forests. The destruction was caused mainly by large rises in numbers of sea urchins, normally kept in control by various predators, particularly sea otters (see Box 7.7 Keystone species). However, compared to the terrestrial environment, it is more difficult to collect long-term marine population data on predator and prey populations and without such data, wrong assumptions can easily be made. This can lead to poor fishery manage- ment decisions. Grubbs et al. (2016) discuss this, particularly in relation to a predator- control fishery for cownose rays (Rhinoptera bonasus). Intense fisheries for large, coastal sharks in the northwest Atlantic had been associated with an apparent increase in the rays, which in turn were blamed for the collapse of some commercial bivalve stocks. This was used as justification for encouraging a fishery for the rays. However, in their study Grubbs et al. (2016) found that the data used to support the hypothesis for a tro- phic casquade initiated by over-fishing of sharks were not sufficient to prove the case. They define the following five diagnostic criteria necessary to support the occurrence of a trophic cascade, following predator depletion: temporally correlated inverse abundance trends between predators and prey, spatiotemporal overlap between predators and prey, prey populations that grow rapidly compared to their predators, specified prey must be a significant component of the predator’s diet, and specific predators included are the primary source of predation mortality on the prey. Since the early 2000s, there has been a noticeable increase in the abundance of jel- lyfish throughout the world’s oceans and some huge blooms have been reported. Although over-fishing and decline of jellyfish predators such as tuna and Leatherback Turtles (Dermocheles coriacea), respectively, may be implicated to varying degrees with the increase in some particular jellyfish species, predator removal (over-fishing) is unlikely to be the whole story when discussing worldwide jellyfish increases in gen- eral. Pitt et al. (2018) discuss the available published evidence for whether anthropo- genic stressors cause jellyfish blooms. The public perception lumps all jellyfish together, whereas in reality only a few species are capable of forming massive blooms. 8.3 Some fisheries and fishing methods
impacts and solutions The examples of life histories given in Section 8.1 indicate the huge variation and complexities of fishery targeted finfish. Understanding of life histories is essential for effective management of stocks and the following examples were chosen as examples of fisheries for species groups which require very different approaches to the way they  Human impacts 1: sea fisheries and aquaculture 417 are caught and to conservation of stocks. Readers should also refer to Section 8.1.3 for information on fisheries problems connected with anadromous and catadromous salmon and eels. 8.3.1 Shark and ray fisheries Sharks and rays are particularly vulnerable to fishery exploitation mainly due to their biology, especially with respect to their reproductive strategies. In general, cartilagi- nous fishes (sharks, rays and chimaeras) grow slowly, have late sexual maturity and produce relatively few young (see Section 8.1.1). This means that populations are eas- ily over-fished and if this happens then recovery is slow, even when fishing pressure is greatly reduced. The global (reported) catch of elasmobranchs (sharks, rays and chi- maeras) in 2018 was nearly 700,000 tonnes (FAO, 2020b) with the real figure likely to be very much higher, as there is a lack of global data reporting and extensive illegal fishing. Possibly a quarter of all cartilaginous fishes are endangered at some level, and 105 are listed in the IUCN Red List of threatened species in the top two categories (Critically Endangered and Endangered). This is almost 10% of known species. However, 472 of the 1038 species documented within this list have not yet been assessed. Sharks are top predators and play a vital role in balancing the ocean food web (see Section 8.2.4) and there is increasing concern over unregulated fisheries. A further problem is impact on, and loss of, nursery grounds such as mangrove areas and coastal bays. As small juveniles, sharks are as vulnerable to predators as any other fish. Many large oceanic sharks spend their juvenile years in nursery grounds, such as mangrove swamps and inshore bays where they are safe from large predators. For example Scalloped Hammerhead (Sphyrna lewini) populations around seamounts off the west coast of Costa Rica use sheltered inshore bays such as Golfo Dulce as nursery areas. Nursery areas for Blue Shark (Prionace glauca) are known from sheltered coastal areas of Spain, Portugal and the eastern Mediterranean and Lemon Sharks (Negaprion brevirostris) give birth to their live young in the sheltered waters of Bimini Lagoon, Bahamas. Identification of such areas through satellite tagging schemes and subsequent protection, even at a very local scale, can be a very effective management tool. Small benthic and demersal species such as Lesser Spotted Catshark (Scyliorhinus canicula) and Piked Dogfish (Squalus acanthias) are caught using bottom trawls, whilst large pelagic species are taken commercially on longlines. There are also recreational and competitive fisheries for large pelagic species. Many recreational shark fishers do so on a catch and release basis and take part in tagging projects (Section 8.4.2). Competitive fishing, where the catch is killed, can have significant effects on local shark populations because the aim is to catch the largest individuals. Illegal fishing for large oceanic sharks is a significant problem due to the high value of the catch and even protected areas such as the Galapagos islands are targeted. 418 Elements of Marine Ecology Mitigation: Where regulation of shark and ray fisheries is in place, it is generally implemented through quotas and limits (see Section 8.5.1). However, within Europe and some other countries a number of other measures specific to sharks or rays are used. Shark finning is widely seen as the most damaging and wasteful of all shark fish- eries and takes place worldwide, including Europe. The use of regulations that make the landing of unattached shark fins illegal, or impose limits on the ratio of fins to car- cass landed, can be effective both in limiting catches and reducing waste. Fins-attached policies also allow species-specific statistics to be collected which in turn can lead to better management and enforcement. A requirement to provide landings data by spe- cies, for skates and rays, can provide far more management information than just lumping all species together. The development of simple guides to identification for use on fishing vessels can greatly assist compliance. However, many shark fisheries throughout the world remain unregulated. In areas where diving tourism is important, the realisation that a live shark can be more valuable than a dead one has led to a shift from shark fishing to shark tourism. Currently (2020), 32 species of sharks and rays (Elasmobranchii) are protected through trade restrictions by listing on CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora). Illegal trading of listed species remains a problem. This includes two species of Manta, 11 Mobula, three Alopias and seven Pristidae that are not listed individually. Migratory species of sharks face particular pro- blems because their protection must rely on international cooperation. Forty shark and ray species are listed on CMS (Convention on the Conservation of Migratory Species of Wild Animals). Lists of protected species and details of restrictions and protection levels can be found on the respective websites (https://cites.org and http://www.cms.int). 8.3.2 Herring and cephalopod fisheries Clupeid fishes (Clupeidae), commonly known as herrings and sardines, sprats and pil- chards, together with anchovies (Engraulidae), make up some of the largest and most important commercial fisheries throughout the oceans. In 2018 world landings of ‘her- rings, sardines and anchovies’ was 19.8 million tonnes (FAO, 2020b). These relatively small fish form large shoals that feed on plankton in the epipelagic zone near the sur- face. Nearly 70% of all anchovies caught commercially are the Peruvian Anchoveta (E. ringens) which has the dubious honour of being possibly the world’s most exploited fish species. The major problem with fisheries for clupeids is simply one of over- fishing. In the 19th and early 20th centuries, fishing vessels were much smaller and slower and onboard technology was nonexistent or minimal. Catches whilst large were mostly sustainable. However, today there are large fleets of vessels capable of staying at sea for extended periods, able to handle large nets and with computer tech- nology to navigate and pinpoint shoals. So although the fish occur in immense  Human impacts 1: sea fisheries and aquaculture 419 numbers, many stocks are fished at unsustainable levels. Plankton-feeding fish such as herring cannot be caught using baited hooks and these stocks are mostly caught by surrounding shoals with nets, which are then drawn in and hauled up. Purse seine nets are commonly used and these nets are also used for smaller tuna species. Bycatch of turtles and dolphins is a particular problem using these nets, but there are ways to mit- igate this as discussed below. Stocks of plankton-feeding fish are also affected by changes in the distribution of their target plankton, resulting from climate change. Peruvian anchoveta and other populations dependent on upwelling (see Section 3.3.3 Upwelling and turbulence) to provide nutrients for their plankton food are also subject to periodic population crashes as a result of severe El Niño weather events (Section 2.7.5). Purse seine netting A purse seine net is effectively a curtain of fine-mesh net hung vertically in the water to encircle near-surface shoals of fish such as herring and mackerel in the UK and anchovies in warmer waters. Detecting a suitable shoal of the target species is an important part of the process, nowadays usually done using sonar. The net is deployed in a circle that encompasses a shoal and the bottom of the net is then drawn closed by drawing in a strong cable (purse line) that runs through rings on the bottom lead line. This action is similar to closing a draw-string purse, hence the name (Fig. 8.9). Purse seines can be deployed in various sizes and mesh sizes to suit the particular target spe- cies and fishery, whether small-scale local or huge commercial operations. Large com- mercial purse seiners can have nets as much as 700 m in length and 200 m in depth. The advantage of purse seines and other types of ‘ring nets’ is that they are not towed through the water and do not touch the seabed. The operator can be selective in Figure 8.9 Diagrammatic depiction of a typical purse seine net. 420 Elements of Marine Ecology choice of shoal to target, thereby reducing bycatch. Bycatch does remain a problem, especially with the larger nets where the enclosed circle is so large that animals such as cetaceans and turtles do not notice they are being surrounded. In warm seas such as the eastern tropical Pacific, purse seines are extensively used to encircle shoals of tuna. The problem is that dolphins and porpoises follow the tuna shoals and are often caught incidentally and eventually drown in the nets. One solution to this problem is the incorporation of a ‘Medina panel’ into the net. This is an area of fine mesh, sewn into the net around what forms the back of the net as it is drawn in. This is where dolphins are most likely to end up and the small mesh prevents them from becoming entangled. It is also much more visible to them, so as the net tightens, the dolphins can jump over the net rim. The operating vessel may also back down or pause to slacken the net and allow escape. The disadvantage of really large purse seine nets in terms of conservation of fish stocks is the obvious one that they are capable of taking huge quantities of fish from one area in one go. This is where fishery management and regulation come in (see Section 8.5). The size of mesh used is also important as very small mesh nets in the wrong place or at the wrong time are likely to catch an undue number of juvenile fish. Purse seine and other ring nets can be laid by a pair of vessels operating together, which may be more efficient with large nets or when operating in certain areas. When a shoal of fish has been detected, one end of the net is secured to one vessel while the other steers a semicircular course paying out the net. The two ves- sels then steam on parallel courses, towing the net and finally turn to meet and enclose the shoal. In some operations the nets are not hauled on board but the concentrated catch is scooped out of the net when it is alongside the boat using a dip-net known as a brailer. Pelagic mid-water trawls Herring and other pelagic, shoaling fish can also be caught using mid-water trawls and these can also be used for demersal species during periods when they leave the seabed. A pelagic trawl is usually a conical net kept open by floats on the headrope, a weighted footrope and by various otterboards, elevators and depressors attached to the mouth (Fig. 8.10). The depth of trawl can be regulated by the length of the warps and the speed of the vessel, but its control requires considerable skill. Modern nets may be under computer control which may greatly increase their efficiency. Sensors can show height above the bottom, depth below the surface, vertical mouth opening and data on fish entering the net. Accurate location of the fish shoals is obviously essential. Sonic techniques are used to enable the position of the net and its relation to the fish shoals to be accurately known and this too can be fed into the computer sys- tem. Hybrid semipelagic nets which can be used either on the bottom or in mid- water have also been developed.  Human impacts 1: sea fisheries and aquaculture 421 Figure 8.10 Diagrammatic depiction of a pelagic trawl net. Modern systems have various sensors on the net that indicate the position of the net and the size of the catch. Although shoals are targeted, there is no control over what is caught during the trawl operation, although this can be partly overcome with mesh size adjustments. In the UK, pair trawling for European Seabass (Dicentrarchus labrax) has led to problems with dolphins becoming trapped and pair trawling is now not permitted in inshore waters. Acoustic pingers can be attached to pelagic trawls to deter cetaceans from approaching the area where the boats are operating. Large mesh at the mouth of the net helps to herd shoals in but allows smaller species to escape. 8.3.3 Tuna: pelagic long lines and drift nets Pelagic long lines Pelagic long lines are used for capturing large predatory species that do not form dense surface shoals, particularly tuna and bil