Detecting Largemouth Bass Population Responses to a Minimum Length Limit: Effects of Variable Recruitment and Duration of Evaluation

Mike S. Allen and William E. Pine III., 
Department of Fisheries and Aquatic Sciences, The University of Florida, 7922 NW 71st Street, Gainesville, FL 32653. msal@gnv.ifas.ufl.edu, 352-392-9617 ext. 252.

We used a simulation model to evaluate how population density, population biomass, total catch (fish harvested and released), yield, and proportional stock density (PSD) of largemouth bass Micropterus salmoides would differ in response to a single three-year or five-year length limit evaluation at various recruitment variabilities. Recruitment variation modeled (coefficient of variation, CV= SD/mean * 100% in recruits to age-1) ranged from 20-100%. Minimum length limits modeled were 305, 356, and 457-mm. Simulations revealed that largemouth bass populations would not exhibit detectable differences in any population parameter (all P > 0.1) unless recruitment variability was <=40% for a 305-mm and <=65% for a 356-mm length limit, respectively. Values of CV in recruits to age-1 for largemouth bass populations averaged 66% (range 11-189, N=13). A 457-mm length limit provided detectable differences in total biomass and PSD up to CV in recruits of 100%. Five-year evaluation periods yielded more detectable differences than three-year evaluations. Proportional Stock Density (PSD) was the variable most likely to change in response to the size limit. However, at recruitment variabilities > 90%, detectable differences did not occur unless the size limit was 457-mm. Fishery managers should consider effects of variable recruitment and duration of evaluation period when evaluating the success of a length limit.