Changes in walleye (Stizostedion vitreum vitreum) stocks in the Great Lakes from 1800 to 1975 were linked to proliferation of foreign species of fish and culturally induced sources of stress — exploitation, nutrient loading, alteration of spawning habitat, and toxic materials. During the 1800s, three small spawning stocks (and probably many others) were damaged or destroyed because of either overfishing or elimination of spawning habitat through logging, pollution, or damming.During 1900–40, stocks in the Michigan waters of Lake Superior, southern Green Bay, the Thunder Bay River of Lake Huron, the North Channel of Lake Huron, and the New York waters of Lake Ontario declined gradually. Pollution, in general, and degradation of spawning habitat, in particular, probably caused three of the declines and overexploitation was suspected in two instances. In addition, the decline of three of these stocks occurred when rainbow smelt (Osmerus mordax) were increasing.During 1940–75, stocks in seven areas declined abruptly: Saginaw Bay (1944), northern Green Bay (1953), Muskegon River (mid-1950s), western Lake Erie (1955), Nipigon Bay (late 1950s), Bay of Quinte (1960), and Black Bay (mid-1960s). The decline of each stock was associated with a series of weak year-classes. The stocks were exposed to various sources of stress, including overexploitation, pollution, and interaction with foreign species, which, if not important in the decline, may be suppressing recovery. Only the western Lake Erie stock recovered, in part due to a reduction in exploitation and, possibly, because of the relatively low density of smelt and alewives (Alosa pseudoharengus) in the nursery areas.Relatively stable stocks persisted in five areas: Wisconsin waters of Lake Superior, Lake St. Clair — southern Lake Huron, eastern Lake Erie, northern Lake Huron, and parts of Georgian Bay. Pollution problems were relatively minor in these areas and exploitation was light during recent decades. Apparently these stocks were more capable of withstanding the additional stresses exerted by alien species. Key words: population fluctuations, Percidae, Stizostedion, Great Lakes walleye, history of fisheries, summary of stresses, harvests, management implications
Late-summer phytoplankton in western Lake Erie (Laurentian Great Lakes): bloom distributions, toxicity, and environmental influences
