Negative Implications of Large Minimum Size Regulations on Future Mean Size at Age: An Evaluation Using Simulated Striped Bass Data
Abstract.—One component of the management regimen employed to rebuild the Atlantic coastal migratory stock of striped bass Morone saxatilis was the imposition of large size limits to reduce fishing mortality to zero, on the 1982 and subsequent year-classes, until 95% of the females had an opportunity to reproduce at least once. Such size limits introduce size-selective mortality that favors the survival of slower-growing members of the population and may select for slower growth in succeeding generations. Size-at-age data from the Hudson River population were used to examine the effect of alternative size limits on the length distributions of survivors by age using simulation. The results indicate that minimum sizes currently in use can cause profound changes in the size composition of the spawning stock. The mean asymptotic length of unfertilized eggs under equilibrium conditions was estimated as an index of the extent to which these changes may influence future growth. With the current best estimates of growth parameters and fecundity at size, this index was reduced by only about 3%. However, because of the size-selective mortality in the existing fishery and uncertainty arising from sampling difficulties, growth is probably poorly characterized by the existing data. Sensitivity analyses revealed that some reasonable combinations of growth and minimum sizes can induce changes in the mean asymptotic length of unfertilized eggs by more than 10%, which would be about 30% in terms of asymptotic mean weight. The extent to which this selective force might be expressed in succeeding generations is uncertain. However, reliance on minimum sizes to constrain catch has the potential to cause profound changes in growth and probably should not be adopted for long-term management of striped bass until this problem is better understood.