Effects of a protected slot limit on smallmouth bass size structure and angler harvest

Abstract

In an effort to improve size structure of smallmouth bass (Micropterus dolomieu) in Lake Sharpe, a large Missouri River impoundment, the South Dakota Department of Game, Fish and Parks instituted 2 protected slot limits: restricted (305–457 mm) beginning in 2003 and relaxed (355–457 mm) beginning in 2008. We examined the effects of these regulations on smallmouth bass harvest and population characteristics and compared creel and population trends of Lake Sharpe smallmouth bass to adjacent reservoirs where smallmouth bass harvest was not regulated by length limits. Prior to the slot limit, the majority of the smallmouth bass harvest from Lake Sharpe was for individuals 250–400 mm in length (P < 0.001). Estimated harvest after regulation implementation was higher for smallmouth bass 250–350 mm (P < 0.001). Additionally, an increase in proportional stock density–preferred (PSD-P), number >355 mm, and angler catch of trophy smallmouth bass was observed, suggesting an effective regulation. However, a before-after-control-impact (BACI) study design and analysis indicated changes in size structure were observed in abutting Lakes Oahe and Francis Case, suggesting a Missouri River system-wide affect was likely responsible for observed population changes. Because of the lack of biological support and the negative feedback by anglers, the protective slot limit regulation was removed from Lake Sharpe in 2012. Further, when considering length limit regulations, we recommend gathering data on similar “control” systems in an effort to document changes resulting solely from the impacts of the regulation.