Recruitment in Fish Populations (Lecture 11)

ICHPTHYOLOGY

What is recruitment in fish biology?

The number of fish that survive to enter a fishery or reach a defined life stage (e.g., maturity, metamorphosis, or catchable size).

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are The number of fish that survive to enter a fishery or reach a defined life stage (e.g., maturity, metamorphosis, or catchable size).

No. Survival during planktonic and juvenile stages heavily influences recruitment

What are the three critical stages influencing recruitment?

Egg production, survival in the plankton (larvae), and survival of juvenile fish.

Why is spawning stock biomass (SSB) a poor predictor of recruitment?

Real-world data often show weak or no correlation between SSB and actual recruitment.

What causes recruitment variability?

Annual fluctuations in environmental conditions and survival rates during early life stages.

What does the Ricker–Foerster Thesis state?

Small changes in larval mortality rates can lead to huge differences in recruitment.

What are the main factors affecting larval fish growth?

Temperature and food availability.

What did Brett et al. (1969) discover about sockeye salmon

Growth rates increase with both higher temperature and food (ration) levels.

What is otolith microstructure analysis used for?

Determining larval age and estimating growth rate using daily growth rings.

What did Pannella (1971) confirm?

Otoliths contain daily growth rings.

What did Mehot & Kramer (1979) conclude about anchovies?

Larval growth is more influenced by food than temperature.

How is larval mortality estimated

By analyzing size-frequency histograms and correcting for net avoidance using day/night comparisons.

What are common prey for larval fish?

Dinoflagellates, nauplii (copepod larvae), and copepodites.

What is gape limitation in larval fish?

Small larvae can only eat prey small enough to fit in their mouths.

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What does Lasker’s Stability Hypothesis propose?

Calm water supports larval survival by concentrating dinoflagellate prey; turbulence disperses prey and increases mortality.

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What is the Match–Mismatch Hypothesis

Larvae must hatch during peak prey abundance for survival; mismatches lead to high mortality.

What did Beaugrand et al. (2003) find in their study of North Sea cod?

Recruitment declined due to changes in plankton size, timing, and abundance linked to climate change—not just overfishing.

What is density-dependent mortality

High larval densities increase competition for limited prey, raising mortality rates.

What did Cushing & Horwood (1994) show about larval feeding?

Larvae consume ~3% of encountered prey per day, leading to intense competition under food scarcity.

Why can a large spawning event be self-defeating

Too many larvae deplete prey quickly, leading to increased mortality.

Why is juvenile survival important for recruitment

Juvenile mortality is more predictable and can better forecast future population strength.

What species shows a strong link between post-larval survival and recruitment?

: Arcto-Norwegian cod—post-larval numbers predict 0-group abundance.

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How do oceanographic conditions influence larval fish

ability promotes prey aggregation; turbulence disperses food and increases mortality.

What tool was essential in Beaugrand's study?

Continuous Plankton Recorder (CPR)—provided long-term plankton data (>245,000 samples).

What is a strong year-class?

A cohort of fish that survives at much higher rates and can sustain fisheries for several years.