Predicted Effects of Angler Harvest on Largemouth Bass Populations in Northern Wisconsin Lakes
File(s)
Date
2014-12Author
Schnell, Kaitlin E.
Publisher
University of Wisconsin-Stevens Point, College of Natural Resources
Metadata
Show full item recordAbstract
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.