Fish Early Life Stages and Recruitment

L3: EARLY LIFE HISTORY STAGES AND RECRUITMENT

Early Life History Stages

• Developmental stages include egg to larva to juvenile.
• Basic life history phases include egg, larva, and then juvenile.
  • Some newly hatched fish are tiny and undeveloped, resembling miniature adults while others don't.
  • Freshwater teleosts transition gradually from the juvenile stage, normally as a smaller version of the adult.
  • Marine species may undergo pronounced metamorphosis before becoming juveniles.
• The first larval stage is the yolk sac larva.
  • This stage is dependent on yolk for nutrition.
  • Fry is a general term for larvae.
  • Alevins are yolk sac larvae in salmon and trout and only if the species has a yolk sac.

Early Feeding

• Early feeding is a critical transition period, moving from yolk dependence to active feeding
  • This is the most vulnerable stage in a fish's life.
  • High mortality often occurs due to starvation.
• Sensory system and feeding apparatus development (mouth size and mobility) limits feeding ability
  • Many larvae are planktonic, drifting with currents rather than actively seeking food.
• Prey density relates to survival.
  • High prey density means higher survival chances.
  • Low prey density results in likely mortality.
• Initial food sources include early food like phytoplankton, copepods, ciliates, and mollusk larvae.
  • Zooplankton quickly becomes the primary food source for most species.

Predation on Fish Larvae

• Key threats to fish larvae include larger fish as primary predators and invertebrate predators which also pose a threat.
• A survival strategy is using rapid growth and development to reduce vulnerability to predators.

Abiotic Factors Affecting Early Life Stages in Fish

• Environmental sensitivities affect early life stages
  • These include temperature fluctuations, acid runoff, and wave action.

Recruitment: Conversion of Eggs into Fish

• Recruitment refers to the conversion of eggs into fish that reproduce in the next generation.
• It involves the relative survival of fish during their early life stages.
• Recruitment importance is determined by
  • The estimation of how many fish are added to the overall population in a given year.
  • Fish population assessment and fisheries management.
• There are different definitions for recruitment
  • Marine fisheries biologists consider it the age where fish are first harvested (fishing occurs).
  • Salmon biologists define it as the return of spawning adults (adults returning to their natal waters to spawn).
• Year class strength indicates the relative success of recruitment in any given year
  • It is strong when certain age groups (high proportion of 4-year-olds) are abundant, leading to high recruitment.
• Factors influencing recruitment include both density-dependent and density-independent elements.
  • Density-dependent factors include competition and predation.
  • Density-independent factors include weather and environmental conditions.
• Fisheries biologists study correlations between strong year classes and environmental factors, such as summer temperatures and prey abundance.
• Recruitment models relate the number of spawning adults to the number of recruits (offspring surviving to adulthood).
  • Different models account for various issues.
  • These models reflect different factors and conditions that each model aims to address.
  • Model choice depends on specific issues that affect recruitment in a given fishery.
  • Abiotic factors include temperature and water quality.
  • Biotic factors include prey availability, predation, and competition.
• Understanding the relationship between spawning adults and the number of recruits is important to
  • Manage fisheries and predict how many fish can be harvested each year without depleting the population.
  • Evaluate recruitment success and assess the health and sustainability of fish populations.
• Recruitment can be improved by
  • Recreational fisheries applying pressure to maximize the number of adults.
  • Spawning areas can be enhanced.
  • Reducing pressures that interfere with spawning success.
• Example of bass larvae survival.
  • High survival rates are seen with stable and consistent warming in early summer (May/June).
  • Low survival rates occur with fluctuating temperatures (warming followed by sudden cold snaps).
  • Currents matter.

Exponential Growth Model

• This model has a low population size.
• It predicts exponential growth where each pair of spawners produces many recruits to the population. This can lead to a large number of recruits and a steep growth curve.

Beaverton-Holt Model

• In this model, growth starts exponentially when the population is small (few spawners).
• The slope near the origin represents the maximum reproductive rate.
• The maximum carrying capacity is where no more recruits can be added regardless of the increase in spawners.
• Some type of juvenile density dependence is presumed to be a limiting factor (finite resources).
  • For example, in a salmon stream, once all nursery areas are occupied, additional recruits will not contribute to the population because the stream has reached its capacity to produce young fish.
• The maximum reproductive rate is the slope, and that's how rapidly recruits are produced as spawners increase.

Ricker Model

• Recruitment initially increases with an increasing number of spawners but begins to decline after reaching a maximum number of recruits.
• As the number of recruits peaks, further increases in spawners lead to a decline in recruits.
  • This is caused by limited food supply, meaning fewer, large individuals survive, or a higher number of smaller individuals may not.
  • Cannibalism is a more significant factor in dense populations.