Harvest and Relative Abundance of Siscowet Lake Trout in Michigan Waters of Lake Superior, 1929–1961

—Siscowet, a deepwater morphotype of lake trout Salvelinus namaycush and the top predator in Lake Superior, currently makes up most of the lake trout biomass in this lake. Anecdotal accounts indicate that siscowets made up some portion of the historical lake trout commercial fishery, but estimates of harvest and relative abundance are lacking. By using the location information provided by historical fishers on monthly catch reports and past and contemporary knowledge of the depth distribution of siscowets, we provide the first estimates of historical siscowet commercial harvest, fishing effort, and changes in relative abundance for Michigan waters of Lake Superior from 1929 to 1961. Siscowets made up about 27% of the historical yield of lake trout in Michigan waters during this period, but the composition varied greatly among management units. The relative abundance of siscowet in its principal habitat (waters deeper than 80 m) generally declined in most management units before the increase in fishing effort in the mid to late 1940s and the invasion of sea lamprey Petromyzon marinus during the 1950s. These factors led to the collapse of nearshore lean lake trout populations by the late 1950s. Modest levels of fishing effort (around 2,000 km annually) before sea lamprey invasion were sufficient to cause declines in siscowet and were probably related to the low production rates associated with the k-selected life history attributes of this deepwater morphotype. Lake Superior had a substantial commercial fishery for lake trout Salvelinus namaycush until the 1950s. The fishery concentrated principally on lean lake trout—one of three principal morphotypes found in Lake Superior—occupying the nearshore area. The other types were the siscowet and the humper lake trout. These three morphotypes are similar in gross appearance (see Figure 1 in Moore and Bronte 2001) but differ in head and caudal peduncle shape, growth, fat content, water depth preference, age at maturity, mortality, diet, and spawning time and location (Eschmeyer and Phillips 1965; Khan and Qadri 1970; Lawrie and Rahrer 1973; Bronte 1993; Ray et al. 2007). Lawrie and Rahrer (1973), Burnham-Curtis (1993) and Hansen et al. (1995) provide concise descriptions of this diversity. By 1960 the lake trout fishery collapsed from a combination of overfishing and predation by sea lamprey Petromyzon marinus (Hile et al. 1951; Lawrie and Rahrer 1973; Pycha and King 1975; Baldwin et al. 1979; Figure 1). Before 1950, when sea lampreys were not abundant enough to cause significant mortality, increasing commercial fishing intensity reduced lean lake trout populations in Michigan waters of Lake Superior (Hile et al. 1951; Pycha and King 1975; Wilberg et al. 2003) and probably elsewhere. Controls on sea lampreys (Smith and Tibbles 1980) and exploitation in the early 1960s and intensive stocking of hatchery-reared lake trout thereafter restored lean lake trout throughout most of Lake Superior (Hansen et al. 1995; Bronte et al. 2003; Wilberg et al. 2003; Richards et al. 2004; Sitar et al. 2007). Siscowet, a deepwater morphotype (Sweeny 1890; Khan and Qadri 1970; Lawrie and Rahrer 1973; Moore and Bronte 2001), has also recovered under these conditions of lower mortality (Bronte et al. 2003; Sitar et al. 2007). Siscowets are found principally in deep, offshore waters greater than 80 m, as consistently observed for over 100 years (Sweeny 1890; Montpetit 1897; Jordan and Evermann 1911; Eschmeyer 1955; Lawrie and Rahrer 1973; Bronte et al. 2003). Many siscowet stocks persisted after overfishing and sea lamprey predation decimated most lean lake trout populations. Siscowet lake trout make up most of the lake trout biomass in Lake Superior at present, as was probably true historically (Ebener 1995; Bronte et al. 2003). The siscowet, with its high fat content (30–90% by weight; Eschmeyer and Phillips 1965) that makes them neutrally buoyant, is adapted for vertical migration (Crawford 1966; Eshenroder and Burnham-Curtis * Corresponding author: [email protected] Received April 25, 2007; accepted November 20, 2007 Published online May 1, 2008 916 Transactions of the American Fisheries Society 137:916–926, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/T07-096.1 [Article]