Allee effects and the Allee-effect zone in northwest Atlantic cod

Area of Science:

• Marine Biology
• Fisheries Science
• Ecology

Background:

• The theory of compensatory dynamics posits that depleted populations recover once threats are removed, assuming highest per capita growth at smallest viable sizes.
• However, many depleted fish stocks, including Atlantic cod (Gadus morhua) off Newfoundland, have not recovered despite reduced fishing mortality and closures.
• This suggests other factors beyond fishing regulate growth in collapsed populations, hindering recovery.

Purpose of the Study:

• To investigate the presence and impact of Allee effects in a south Newfoundland Atlantic cod population.
• To infer the Allee-effect threshold below which population recovery is impaired.
• To highlight the importance of low-population size data for understanding population dynamics and recovery potential.

Main Methods:

• Utilized Bayesian inference to analyze data on recruitment and spawning stock biomass.
• Focused on data from low population sizes to accurately model Allee effects.
• Compared findings with prevailing fisheries management models assuming compensatory dynamics.

Main Results:

• Presented strong evidence for Allee effects in the studied south Newfoundland cod population.
• Inferred a specific Allee-effect threshold, indicating a critical low abundance level for recovery.
• Demonstrated that Allee effects are significant in commercially exploited fish populations and can prevent recovery from overfishing.

Conclusions:

• Allee effects are a critical factor inhibiting the recovery of collapsed fish populations, contrary to common assumptions in fisheries management.
• Prevailing management practices may overestimate recovery potential by underestimating the impact of Allee effects at low population densities.
• Accurate assessment of Allee effects requires data specifically from low population abundance scenarios.