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 = 100 × 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 = 55–124, N = 14). Simulations revealed that largemouth bass populations would not exhibit detectable differences unless recruitment variability 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 = 11–189, N = 13). A 457-mm length limit for largemouth bass provided detectable differences in total biomass and PSD up to recruitment variabilities of 100%. Detectable differences were more likely under 5-year evaluations than 3-year evaluations. Proportional stock density was the variable most likely to change in response to the size limit for both white crappies and largemouth bass. However, at recruitment variabilities greater than 90%, detectable differences did not occur in 3-year or 5-year evaluations, unless the size limit was 457 mm for largemouth bass. Fishery managers should consider effects of variable recruitment and duration of evaluation period when evaluating the success of a minimum length limit.