Cooperative Fish and Wildlife Research Units Program: Wisconsin Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Largemouth bass Micropterus salmoides abundance has increased in many northern Wisconsin lakes over the last decade. Density-dependent effects on largemouth bass growth and size structure and the potential for bass interactions with other popular sport fish such as walleyes Sander vitreus are concerns among anglers and biologists. To reduce largemouth bass abundance, the statewide minimum 15-in total length (TL) limit for bass has been removed from some northern Wisconsin lakes. However, available information suggests that exploitation of largemouth bass is generally low (i.e., < 10%) and it is unclear whether angler harvest will be sufficient to actually affect bass abundance in these lakes. Furthermore, several recent studies suggest that anglers voluntarily release most of the black bass they catch, so liberalization of harvest regulations may not result in substantial increases in bass exploitation. Our objective was to use predictive modeling to determine if largemouth bass abundance, recruitment potential, and size structure in four northern Wisconsin lakes would change in relation to instantaneous fishing mortality rates (F) and changes to harvest regulations. During 2012 and 2013, we collected information on population demographics and dynamics of largemouth bass populations in Big Arbor Vitae, Big Sissabagama, Little John, and Teal lakes in northern Wisconsin. We used this information to formulate population-specific models that were used to simulate the effects of: 1) the current 14-in minimum length limit; 2) a 14-in maximum length limit; 3) no minimum length limit; 4) a 12- to 15-in harvest slot length limit (i.e., fish between 12- and 15-in can be harvested); 5) catch-and-release; 6) an 18-in minimum length limit. No minimum length limit was most effective at reducing abundance by ≥ 25%, although high levels of fishing mortality for Wisconsin anglers (F ≥ 0.2) were necessary to achieve this reduction. A 14-inch maximum length limit or 12-15 inch harvest slot length limit were predicted to provide the most equitable tradeoff between reductions in abundance and size structure, but improvements in RSD-15” ≥ 10% were only observed if F was ≥ 0.10.