Assessment of detecting minimum length limit changes for crappie in two Alabama reservoirs

Abstract

Abstract The objectives of this study were to compare population metrics prior to and after the initiation of a minimum length limit for black crappie Pomoxis nigromaculatus and white crappie Pomoxis annularis in two reservoirs in Alabama, USA. In addition, we estimated recruitment variability and incorporated this stochastic component into a simulation model to assess the effects of three different minimum length limits on population and fishery metrics. Fish were sampled from October 1990–1991 to 2000 with trap nets, aged with otoliths, and population metrics including recruitment (age-0 catch), catch rates of age-2.5 and older fish, and length-based stock density indices were determined. We found no differences in population metrics between pre- and post-length limit time periods. We compared the effects of 203, 229, and 254 mm minimum length limits and predicted the long-term response of abundance of age-3 and older fish, angler catch, and stock density indices. Low, medium, and high levels of exploitation were explored and recruitment was varied at rates similar to observed data. Power analysis was performed using modeling results to estimate the number of years of sampling required to detect differences in population and fishery metrics in relation to changes in minimum length limits. Differences in population parameters resulting from a change in a minimum length limit were not likely to be detected with a reasonable amount of sampling effort in our populations where the recruitment CV was 70% or greater. The ability to statistically detect population and fishery changes in response to minimum length limits was dependent upon recruitment variation, exploitation rate, the magnitude of the change in a minimum length limit, and sampling duration.