Historic and Modern Lake Trout Abundance, Effects of Fishing on Lake Trout, and Dynamics of the Commercial Lake Trout Fishery in Michigan Waters of Lake Superior

Abstract

For more than one hundred years Lake Superior supported one of the largest lake trout (Salvelinus namaycush) fisheries in the world. Lake trout populations in Lake Superior collapsed in the late 1950s due to the combined effects of overfishing and sea lamprey (Petromyzon marinus) predation. A binational effort to restore lake trout began with chemical control of sea lamprey, stocking of hatchery-reared lake trout, and closure of the commercial fishery. The objectives of this thesis were to compare historic and modem lake trout abundance, to quantify the effect of large mesh (>/= 114-mm stretch-measure) gill net fishing on lake trout recruitment, and to determine how the behavior of commercial lake trout fishermen changed over time. I compared historic and modern lake trout density with t-tests and Welch's approximate t-tests, and determined whether yield was an accurate index of lake trout density with linear and nonlinear regression. Modern (1984-1998) lake trout CPE was equal to or significantly higher than historic (1929-1943) CPE off Ontonagen (MI-2), in waters west of the Keweenaw Peninsula (MI-3), in Keweenaw Bay (Ml-4), off Marquette (Ml-5), and off Munising (MI-6). Relative variation in CPE was similar between historic and modern periods. Modern CPE was higher than historic CPE, because lake trout density is now higher in most areas than during 1929-194 3. Gill net yield was maintained across a wide range of lake trout densities and was best described by a saturation curve. Historic lake trout yield should not be used as the lake trout restoration goal for Lake Superior, because it is not a good index of lake trout abundance. I fit Ricker stock-recruit models with terms for parental stock size and large mesh (>/= 114-mm stretch-measure) gill net effort to lake trout CPE data for Michigan waters of Lake Superior to determine if and when recruitment overfishing occurred. Large mesh gill net effort likely suppressed lake trout recruitment in five Michigan management areas of Lake Superior. Lake trout were overexploited before 1929 near Isle Royale (MI-1), in MI-3, and in Whitefish Bay (MI-8), during 1929-1959 in MI-4, MI-5, and Mi-6, and not overexploited in MI-2 and near Grand Marais (MI-7). Historic abundance indices should not be used as lake trout restoration goals in the areas that were overfished before 1943. I examined dynamics of the commercial lake trout fishery that occurred in Michigan waters of Lake Superior during 1929-1961. The number of full-time commercial fishermen increased during 1933-1943 and decreased during 1945-1957. Increases in the number of fishermen were related to past yield, market prices, and fishing opportunities on Lake Huron. Decreases in the number of fishermen corresponded with decreases in lake trout abundance. The amount of large mesh gill net fished increased during 1929-1951. Fishermen tended to fish more net near shore as lake trout density declined even though lake trout CPE was often higher in deeper waters. The most common gill net mesh size increased over time from 114-mm to 120-mm. Mean fishing location changed in MI-1, Ml-2, MI-4, MI-5, MI-6, and MI-8. More effort was fished near shore than offshore and near shore effort was more stable over time than offshore effort. Relatively stable yield was maintained by increasing gill net effort and by moving some effort to better grounds. Abundance estimates derived from CPE may be biased high, because of changes in fishing locations within management areas and increases in mesh size over time. The yield during 1929-1943 should not be used as the lake trout restoration goal for Lake Superior, because it is probably an overestimate of maximum sustainable yield.