Stock Characteristics of Lake Whitefish in Lake Michigan
Belnap, Matthew J.
University of Wisconsin-Stevens Point, College of Natural Resources
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Lake whitefish Coregonus clupeaformis support important recreational, commercial, and tribal fisheries in the Great Lakes, including Lake Michigan. Genetic analyses indicate at least six distinct lake whitefish stocks exist in Lake Michigan, and subsequent analyses show a mixed-stock fishery exists where multiple stocks are often harvested within a single management zone. Biological characteristics could vary among genetic stocks, but these differences may not be accounted for in current monitoring efforts that rely on capture location to assign fish to individual stocks. This could ultimately affect the utility of statistical catch at age (SCAA) models used to manage the lake whitefish fishery. The objective of my research was to determine if weight-length relationships, growth, age structure, and measures of reproductive investment (i.e., gonad weight-fish weight relationships, fecundity, and egg diameters) differed among genetic stocks and capture locations for lake whitefish in Lake Michigan collected during October, the month when the majority of commercial harvest occurs. A total of 1,528 lake whitefish were collected from whitefish management zones (WFM) 08, 07, 05, 03, 01, and WI-2 in Lake Michigan in 2012 (N = 700) and 2013 (N = 828) for genetic and demographic analyses. Sex, total length, weight, otoliths, gonad weight, ovaries (females only), and maturity were collected from lake whitefish. Only one genetic stock was represented in samples obtained from capture locations in four management zones (WFM-08, WFM-07, WFM-05, and WFM-01), and only two stocks were represented at capture locations in management zones WFM-03 and WI-2. At the two locations where more than one genetic stock was represented, stocks were geographically proximal and biologically similar. Consequently, using October capture location to assign lake whitefish collected to genetic stock would likely provide fishery managers with realistic estimates of biological metrics for each genetic stock. Based on similarities in the biological characteristics I measured, lake whitefish in Lake Michigan could be divided into four groups. Specifically, the Elk Rapids (ER) and Southern stocks (SOU) appear to be biologically distinct from all other stocks, the Northeast (NOE) and Northern (NOR) stocks were biologically similar to one another, but distinct from all other stocks and the Big Bay de Noc (BBN) and Green Bay (GB) stocks were biologically similar to one another, but distinct from all other stocks. While stock assignments based on October capture location appears useful for describing the biological characteristics of specific genetic stocks, using capture location alone is not sufficient to determine stock-specific contributions to harvest within each zone throughout the entire year.