AUTHORS: Thomas S. Jones, Minnesota Department of Natural Resources Mark Luehring, Great Lakes Indian Fish and Wildlife Council Heidi Rantala, Minnesota Department of Natural Resources John M. Hoenig, Virginia Institute of Marine Sciences Patrick J. Schmalz, Minnesota Department of Natural Resources
ABSTRACT: Abundance of Walleye in Mille Lacs Lake, Minnesota, has decreased since the 1990s. Two important environmental changes likely contributed to these declines. First, water clarity increased abruptly in the mid-1990s and increased clarity continues through the present day. Second, zebra mussels and spiny waterflea invaded in the late 2000’s, leading to a 90% reduction in zooplankton. Efforts to rebuild the stock through low harvest since 2013 have met with limited success. This study used multiple approaches to compare Walleye productivity changes associated with three time periods (pre-water clarity, post-water clarity, and post-invasive species). First, we estimated annual harvestable surpluses by summing Walleye fishing mortality and the resultant change in population size estimated from statistical catch-at-age models. These data were fit to surplus production models making various assumptions about the shape of the sustainable yield curve. Second, we estimated total annual Walleye production using an instantaneous growth model. All models showed reduced production after each ecological event. Recognition of reduced productivity may have ramifications for future Walleye management in Mille Lacs Lake.
