Evaluating a Protected Slot Limit On Smallmouth Bass Size Structure and Angler Harvest in a Large Missouri River Reservoir

In an effort to increase size structure of smallmouth bass Micropterus dolomieu in Lake Sharpe, the South Dakota Game, Fish and Parks instituted two protected slot limits: restricted (305-457 mm) beginning 2002 and relaxed (355-457 mm) beginning 2007. Estimated harvest during the regulation periods was higher for smallmouth bass <300 mm (P<0.001) and 250-350 mm (P<0.001) during the restricted and relaxed slot limits, respectively, compared with pre-regulation estimates. Additionally, an increase in PSD-P, number >355 mm, and angler catch of trophy smallmouth bass was observed suggesting an effective regulation. Our goal was to determine if the smallmouth bass size structure changes could be attributed to the regulation, so we compared population trends of smallmouth bass from Lake Sharpe with two unregulated populations in adjacent Missouri River reservoirs. A before-after-control-impact (BACI) study design and analysis indicated the slot limit regulation was not contributing to the observed increases in smallmouth bass size structure. Indeed, similar changes in size structure were observed in abutting Lakes Oahe and Francis Case suggesting a Missouri River system wide affect was responsible for observed population changes. Subsequently, the protective slot limit regulation was removed from Lake Sharpe in 2012.