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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. • Stock enhancement of California halibut was simulated by quantitative modeling. • Pre-defined target was achieved by releasing 0.1–1.6 million larger-sized (40–200 mm) juveniles. • Outcomes were strongly mediated by survival and sex ratio of released fish. • Annual costs to achieve the target were US$ 0.2–2.9 million. • Provides guidance for the allocation of resources in stock enhancement programs. [ABSTRACT FROM AUTHOR]

Abstract: We evaluate post-release dispersal, growth and survival of hatchery-reared white seabass (Atractoscion nobilis) released as part of an experimental stock enhancement program in California. Releases of hatchery white seabass have been carried out since 1986, year round either directly from the hatchery or after acclimation in net pens. All released fish are implanted with coded wire tags. Post-release monitoring is carried out through a research gillnet fishery for juveniles, and tag returns from the commercial and recreational fisheries. Fish dispersed from the release sites at a diffusion rate D 795km2 year−1, such that 50% of fish remained within 47km and 95% within 135km of the release site at the end their third year at large. The von Bertalanffy growth parameters for released hatchery fish were estimated at L ∞ =975mm SL and K =0.21year−1. A set of alternative survival models accounting for dispersal and size- and time-dependent natural and fishing mortality patterns was confronted with the mark-recapture data using model selection based on the Akaike Information Criterion (AIC). The model that fit the data best accounted for short-term post-release mortality dependent on season and release method in addition to a long-term size-dependent mortality. Survival of released hatchery fish was highest in Spring, moderately lower in Summer and Autumn, but much lower in Winter releases. Acclimatisation in net pens had a substantial, positive effect on survival relative to direct releases. Survival of hatchery fish to legal minimum length (600mm SL) in the fishery was estimated at 1.5% for a release size of 200mm, rising to 13.8% for a release size of 400mm, under optimal conditions (Spring releases with net pen acclimatisation). Mortality rates of hatchery white seabass under optimal release conditions were substantially below average for other hatchery fish released into the wild, but remained above those expected for wild fish in both, the short-term and long-term components. [Copyright &y& Elsevier]