Optimal fishery policy with artificial enhancement through stocking: California's white sturgeon as an example

Abstract Although optimal fishery policy has been derived from different kinds of economic and biological models, the interaction of fishing policy with artificial stocking policy has not been explicitly considered. We here determine optimal size limits, fishing effort, and stocking rate for three cases of interest: (1) recruitment-limited population, pre-recruitment stocking; (2) adult biomass-limited population, post-recruitment stocking; and (3) adult biomass-limited and recruitment-limited population, post-recruitment stocking. Results show that lower size limits should be set at the size at which the current market value exceeds the total future value of an individual, both to the fishery and to reproduction. Imposition of upper size limits is rarely optimal. Stocking is advisable when the hatchery cost times the relative contribution of stocking to recruitment is less than the contribution to the value of the catch. Optimal policy ranges from infinite effort at a specific size limit with maximum stocking when the cost of stocking is zero, to lower values of size limit and effort as stocking costs increase, the amount of stocking decreases, and more natural reproduction is optimal. Thus, as hatchery costs decline (or value of captured fish increases), optimal stocking/fishery policy varies from an unstocked fishery to a “put and take” fishery. The results are applied to the sturgeon fishery in the San Francisco Bay Estuary as an example. They imply that a reduced lower size limit and greater fishing mortality together with stocking would be optimal, but that current levels are conservative. The stocking decision depends critically on the values of parameters that are currently poorly known, such as: hatchery costs, survival to the fishery and the mechanisms controlling the sturgeon population.