Optimizing marine stock enhancement through modeling: A sex-specific application with California halibut Paralichthys californicus

Abstract

There is interest in stock enhancement to support fisheries for California halibut Paralichthys californicus, a regionally important yet depleted species in the Southern California Bight (SCB), U.S. A quantitative model was developed to assess the cost of increasing harvestable California halibut in the SCB via releases of cultured juveniles. Reduced post-release survival of cultured juveniles was incorporated into the model, as was sex-specific lifetime survival, growth, and maturity. Simulations examined how adult California halibut population metrics were influenced by key release-strategy variables (i.e., size, number, survival, and sex ratio of released juveniles). A target increase in abundance of harvestable (≥559 mm total length; TL) California halibut by 5% in the entire SCB could likely be achieved by releasing large numbers (0.1–1.6 million per year) of larger-sized juveniles (40–200 mm TL). However, this was strongly mediated by their survival and sex ratio; fewer and smaller juveniles were required when releases comprised better surviving and / or more female individuals. Depending on the survival and sex ratio of released juveniles, increasing harvestable abundance in the SCB to achieve this 5% target was estimated to cost US$ 0.2–2.9 million annually, or US$ 14–219 per additional harvestable California halibut in the wild. This exploratory modeling study demonstrates that releasing juvenile California halibut could be a feasible management tool in the SCB, and provides a theoretical basis for field experiments. More broadly, it suggests that the allocation of resources to the production of larger and / or better surviving juveniles, in addition to optimizing release sex ratios, should be explored in stock enhancement programs.