Predicted Effects of Angler Harvest on Largemouth Bass Populations in Northern Wisconsin Lakes

Largemouth bass Micropterus salmoides abundance has increased in many northern lakes over the last decade and this trend may continue based on projected changes in climate. 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 14-in total length (TL) limit for bass has been removed from some northern Wisconsin lakes. However, low rates of exploitation may prevent significant reductions in largemouth bass abundance. My 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 under different harvest regulations. During 2012 and 2013, I described population demographics and dynamics of largemouth bass populations in Big Arbor Vitae, Big Sissabagama, Little John, and Teal Lakes in northern Wisconsin and used this information to formulate population models for each lake. Models were used to simulate effects of F between 0 and 0.9 on predicted abundance of largemouth bass ≥ 8 in TL, relative stock density of largemouth bass ≥ 15 in TL (RSD-15”), and spawning potential ratio (SPR) under the following harvest regulations: 1) current statewide minimum length limit of 14-inches; 2) 14-in maximum length limit; 3) no minimum length limit; 4) 12- to 15-in harvest slot length limit (i.e., fish between 12- and 15-in can be harvested); 5) catch-and-release and 6) 18-in minimum length limit. No minimum length limit had the greatest potential for reducing largemouth bass abundance by ≥ 25%, but relatively high levels of fishing mortality for Wisconsin bass fisheries (F ≥ 0.2) were necessary to achieve this reduction. Abundance was reduced ≥ 25% under other harvest regulations, but only at rates of F ≥ 0.3. Similarly, reducing SPR to ≤ 30% was more likely to occur under a 14-in maximum length limit or no minimum length limit, but only if F ≥ 0.15. Catch-and-release and an 18-in minimum length limit maximized RSD-15”. However, RSD-15” differed among harvest regulations by < 10% when F was ≤ 0.10, which suggests that changing harvest regulations may have little effect on size structure in most Wisconsin largemouth bass fisheries because available data suggests exploitation rates are typically ≤ 10%. A 14-in maximum length limit and a 12- to 15-in harvest slot limit provided the most equitable trade-offs between reductions in abundance and maintaining size structure, which is of great interest to fishery managers. My results suggest that altering harvest regulations for largemouth bass in these 4 Wisconsin lakes would not likely change largemouth abundance and size structure if rates of F are ≤ 0.10. Consequently, if reducing largemouth bass abundance is a primary management objective, targeted removal of bass or angler education or incentive programs may be necessary to achieve levels of F predicted to achieve this objective.Wisconsin Department of Natural Resources (WDNR) through Sportfish Restoration Fund