North American Journal of Fisheries Management 20:672–682, 2000 q Copyright by the American Fisheries Society 2000 Detecting Fish Population Responses to a Minimum Length Limit: Effects of Variable Recruitment and Duration of Evaluation

Abstract.—We used a simulation model to evaluate how recruitment variability and evaluation duration would affect fisheries managers ’ ability to detect fish population responses to a minimum length limit. Length limits modeled were 254 mm for white crappie Pomoxis annularis and 305, 356, and 457 mm for largemouth bass Micropterus salmoides. Simulations were conducted at recruitment variation (coefficient of variation, CV 5 100 3 SD/mean) of 20–100 % for age-1 recruits. We evaluated how population density, population biomass, total catch (fish harvested and released), yield, and proportional stock density (PSD) would differ in response to a single 3-year or 5-year length limit evaluation. For white crappies, simulations suggested that a 254-mm length limit would not provide detectable differences (P. 0.10) in any population parameter if recruitment variability exceeded 90 % for either evaluation period. Mean CV in recruits to age 0 or age 1 for empirical white crappie populations was 82 % (range 5 55–124, N 5 14). Simulations revealed that largemouth bass populations would not exhibit detectable differences unless recruitment var-iability was 40 % or less for a 305-mm length limit and 65 % or less for a 356-mm length limit. Values of CV in recruits to age 0 or age 1 for largemouth bass populations averaged 66 % (range 5 11–189, N 5 13). A 457-mm length limit for largemouth bass provided detectable difference