Do Higher Size and Reduced Bag Limits Improve Northern Pike Size Structure in Wisconsin Lakes?

Abstract

Minimum length limits (MLLs) combined with reduced bag limits are frequently used by fisheries managers to improve size-structure in Northern Pike Esox lucius populations. We evaluated the effectiveness of a relatively moderate 660-mm MLL and a two-fish-daily bag limit (hereafter, “moderate limit”) and a relatively conservative 813 mm MLL and one-fish-daily bag limit (hereafter, “conservative limit”) on size structure of Northern Pike in Wisconsin lakes. We used a before–after-control–impact design on a series of treatment and reference lakes to test the efficacy of each regulation. Across seven moderate-limit lake pairs, mean and mean maximum total length (i.e., mean of the 10 longest fish) in treatment lakes did not differ relative to reference lakes, where there was no MLL and a five-fish-daily bag limit (relatively liberal; hereafter, “liberal limit”). Across five conservative treatment lakes, there was a significant reduction in mean size relative to references, but mean maximum size increased significantly by an average of 104 mm. A linear mixed-effect regression of 990 Northern Pike populations similarly suggested that higher size and reduced bag limits were related to improved size structure. Northern Pike populations in the moderate-limit lakes and conservative-limit lakes increased by an average of 35 mm and 45 mm, respectively, compared with the liberal-limits lakes. Size structure was significantly and positively related to increased angler effort and reduced Northern Pike densities and was negatively related to longitude and mean Secchi depth, revealing the importance of lake characteristics in mediating regulation efficacy. Proportional size structure differed significantly between the southern and northern management zones of Wisconsin for the 20 years before and after implementation of regional regulations (moderate limit in south versus conservative limit in north). Analysis of long-term creel data revealed that catch rates of Northern Pike have been increasing over time, and anglers have focused on fewer, larger fish. We conclude that higher MLLs and reduced bag limits are effective at improving availability of large Northern Pike in Wisconsin lakes but are best utilized in certain lake and population types.

Received September 4, 2015; accepted April 11, 2016 Published online August 23, 2016

Acknowledgments

We thank a large number of WDNR biologists and technicians that through the years collected and contributed to the data used in this study. We also thank members of the Northern Pike team and other biologists at WDNR that provided useful ideas and feedback throughout the development of this analysis, including Jennifer Bergman, Brian Brecka, Benjamin Heussner, Robert Hujik, John Kubisiak, Randal Larson, Terry Margenau, Thomas Meronek, Tammie Paoli, Laura Stremick-Thomspon, Max Wolter, Steve Gilbert, Lawrence Eslinger, and Zachary Lawson. We thank Greg Sass for his review of this manuscript. We also thank Lori Tate and Jon Hansen for data management and Paul Rasmussen for statistical support. Financial support for this study was provided by the Wisconsin Department of Natural Resources, Bureau of Science Services, Federal Aid in Sport Fish Restoration (Project F-95-P).