Historic and Modern Abundance of Wild Lean Lake Trout in Michigan Waters of Lake Superior: Implications for Restoration Goals

—Populations of lake trout Salvelinus namaycush in Lake Superior collapsed in the late 1950s due to overfishing and predation by sea lampreys Petromyzon marinus. A binational effort to restore the lean morphotype of lake trout began with the stocking of hatchery-reared fish followed by the chemical control of sea lampreys and closure of the commercial fishery. Previous comparisons of the contemporary abundance of wild lean lake trout with that from historic commercial fishery statistics indicate that abundance was higher historically. However, this conclusion may be biased because several factors—the inclusion of siscowet (the ‘‘fat’’ morphotype of lake trout) in the catch statistics, the soak time of nets, seasonal effects on catch per effort, and the confounding effects of effort targeted at lake whitefish Coregonus clupeaformis—were not accounted for. We developed new indices of historic lean lake trout abundance that correct for these biases and compared them with the assessment data from 1984 to 1998 in Michigan waters of Lake Superior. The modern (1984–1998) abundance of wild lean lake trout is at least as high as that during 1929– 1943 in six of eight management areas but lower in one area. Measures to promote and protect naturally reproducing populations have been more successful than previously realized. Historically, lake trout Salvelinus namaycush were the predominant piscivore in Lake Superior (Lawrie and Rahrer 1972). These lake trout have differentiated into many discrete and semidiscrete stocks that use different habitats, both temporally and spatially (Goodier 1981). Three morphotypes are recognized in Lake Superior: the lean morphotype, the siscowet or ‘‘fat’’ morphotype, and the humper or banker morphotype (Khan and Qadri 1970; Lawrie and Rahrer 1973; Pycha and King 1975; Moore and Bronte 2001). In spring, lean lake trout usually inhabit waters shallower than 80 m (Dryer 1966; Selgeby and Hoff 1996), siscowet lake trout usually inhabit waters deeper than 60 m (U.S. Geological Survey [USGS], Great Lakes Sci* Corresponding author: [email protected] 1 Present address: Department of Fisheries and Wildlife, Michigan State University, 13 Natural Resources Building, East Lansing, Michigan, 48824, USA. 2 Present address: U.S. Fish and Wildlife Service, Green Bay Fishery Resources Office, 2661 Scott Tower Drive, New Franken, Wisconsin, 54229, USA. Received May 21, 2001; accepted April 25, 2002 ence Center, unpublished data), and humper lake trout inhabit isolated offshore reefs (Eschmeyer and Phillips 1965; Rahrer 1965). During 1913–1950, the annual lake trout yield of Lake Superior was relatively stable and averaged 2.0 million kg (Hile et al. 1951; Baldwin et al. 1979; Hansen et al. 1995b). In the late 1950s, lake trout populations collapsed to near extinction because of overfishing and predation by sea lampreys Petromyzon marinus (Lawrie and Rahrer 1972; Pycha and King 1975; Hansen et al. 1995a). Stocking of hatchery-reared lean lake trout began in 1952 (Lawrie and Rahrer 1972), and chemical control of sea lampreys began in 1958 with treatments in larval nursery areas in streams (Smith and Tibbles 1980). Commercial lake trout fisheries were closed in the summer of 1962, when the sea lamprey abundance had been reduced 87% in Lake Superior and stock rebuilding seemed possible (Pycha and King 1975). Lake trout restoration has focused mainly on the lean morphotype, but the siscowet and humper morphotypes have also benefited from reduced fishing and sea lamprey mor101 ABUNDANCE OF WILD LEAN LAKE TROUT tality. During the 1960s, the abundance of hatchery-reared lean lake trout increased greatly; 95% of the lake trout caught inshore were of hatchery origin (Lawrie and Rahrer 1972). The abundance of wild lean lake trout increased in the 1970s and early 1980s but declined somewhat during the late 1980s and 1990s (Hansen et al. 1995b). Millions of dollars have been spent on lake trout restoration and progress has been promising, with natural reproduction occurring throughout most of Lake Superior (Hansen et al. 1995a). The current objective for lake trout management on Lake Su-