Many Vulnerable or a Few Resilient Specimens? Finding the Optimal for Reintroduction/Restocking Programs.

Area of Science:

• Fisheries science
• Population dynamics
• Conservation biology

Background:

• Reintroduction and restocking programs often release captive-raised juveniles, but their effectiveness is debated.
• Evaluating restocking success relies on accurate mortality estimation, which is challenging.
• Current statistical models for tag recovery data assume constant mortality and non-reporting rates.

Purpose of the Study:

• To develop a novel statistical method for estimating mortality rates in released fish, accounting for age/size-dependent variability.
• To differentiate between natural and fishing mortality using temporal capture data from multiple release events.
• To assess the effectiveness of restocking programs, using the meagre (Argyrosomus regius) as a case study.

Main Methods:

• Proposed a statistical model that incorporates age/size-dependent mortality rates.
• Analyzed temporal capture data from fishermen to disentangle natural and fishing mortality.
• Applied the method to a meagre restocking program in the Balearic Islands.

Main Results:

• Estimated high natural mortality (m0 = 0.126 day-1) for young meagre juveniles (180 days old), decreasing significantly with age/size.
• Identified a trade-off: older/larger fish exhibit lower natural mortality but higher vulnerability to fishing.
• Determined an optimal release age of 1,173 days to maximize one-year survival, but noted high production costs.

Conclusions:

• Restocking success for meagre may not improve by simply increasing the number of released fish due to high production costs of older juveniles.
• Reducing natural mortality in young juveniles and/or decreasing the time to improve fish resilience are recommended strategies.
• The developed method provides a more realistic assessment of restocking program effectiveness by accounting for variable mortality.