Michael J. Hansen

Lake Trout (Salvelinus namaycush) Populations in Lake Superior and Their Restoration in 1959–1993

Naturally-reproducing populations of lake trout (Salvelinus namaycush) have been reestablished in most of Lake Superior, but have not been restored to 1929-1943 average abundance. Progress toward lake trout restoration in Lake Superior is described, management actions are reviewed, and the effectiveness of those actions is evaluated; especially stocking lake trout as a tool for building spawning stocks, and subsequently, populations of wild recruits. Widespread destruction of lake trout stocks in the 1950s due to an intense fishery and sea lamprey (Petromyzon marinus) predation resulted in lower overall phenotypic diversity than was previously present. Stocking of yearling lake trout, begun in the 1950s, produced high densities of spawners that reproduced wherever inshore spawning habitat was widespread. Sea lampreys were greatly reduced, beginning in 1961, using selective chemical toxicants and barrier dams, but continue to exert substantial mortality. Fishery regulation was least effective in Wisconsin, where excessive gillnet effort caused high by-catch of lake trout until 1991, and in eastern Michigan, where lake trout restoration was deferred in favor of a tribal fishery for lake whitefish (Coregonus clupeaformis) in 1985. Restoration of stocks was quicker in offshore areas where remnant wild lake trout survived and fishing intensity was low, and was slower in inshore areas where stocked lake trout reproduced successfully and fishing intensity was high. Inshore stocks of wild lake trout are currently about 61 % of historic abundance in Michigan and 53% in Wisconsin. Direct comparison of modern and historic abundances of inshore lake trout stocks in Minnesota and Ontario is impossible due to lack of historic stock assessment data. Stocks in Minnesota are less abundant at present than in Michigan or Wisconsin, and stocks in Ontario are similar to those in Michigan. Further progress in stock recovery can only be achieved if sea lampreys are depressed and if fisheries are constrained further than at present.

First-Year Growth, Recruitment, and Maturity of Walleyes in Western Lake Erie

Abstract In some lakes, first-year growth of walleyes Stizostedion vitreum has been identified as an important factor governing recruitment of juveniles to the adult population. We developed a regression model for walleye recruitment in western Lake Erie by considering factors such as first-year growth, size of the spawning stock, the rate at which the lake warmed during the spring, and abundance of gizzard shad Dorosoma cepedianum. Gizzard shad abundance during the fall prior to spring walleye spawning explained over 40% of the variation in walleye recruitment. Gizzard shad are relatively high in lipids and are preferred prey for walleyes in Lake Erie. Therefore, the high degree of correlation between shad abundance and subsequent walleye recruitment supported the contention that mature females needed adequate lipid reserves during the winter to spawn the following spring. According to the regression analysis, spring warming rate and size of the parental stock also influenced walleye recruitment. Our reg...

Factors Affecting Recruitment of Walleyes in Escanaba Lake, Wisconsin, 1958–1996

Abstract We modeled recruitment variation of age-0 walleyes Stizostedion vitreum in Escanaba Lake, Vilas County, Wisconsin, to determine factors regulating their abundance. Abundance of age-5 and older walleyes (spawning population), variation in May water temperatures, and abundance of 152.4-mm total length and longer yellow perch Perca flavescens explained 89% of annual variation of age-0 walleye abundance from 1958 through 1995. Abundance of age-5 and older spawners accounted for 32% of the recruitment variation and indicated that maximum numbers of age-0 recruits were produced at low numbers of spawners (fewer than 1,000 individuals). The coefficient of variation of May water temperatures improved the model fit to 56% of recruitment variation and indicated that increased variation of May water temperatures greatly reduced recruitment. The abundance of yellow perch 152.4 mm and longer further improved the model fit to 78% of recruitment variation and also reduced recruitment, though not as much as vari...

Catch Rates and Catchability of Walleyes in Angling and Spearing Fisheries in Northern Wisconsin Lakes

Abstract We examined relationships between angling and spearing catch rates (catch/h) and walleye population density (number/acre) in 118 northern Wisconsin lakes to determine if walleye catchability in these fisheries was density dependent. The densities of both adult and total walleye populations were unrelated to lake surface area. Similarly, the catchability of walleyes in angling and spearing fisheries was unrelated to lake surface area. Angling catch rates of walleyes were linearly related to total walleye population density, whereas spearing catch rates of walleyes were exponentially related to adult walleye population density. Walleye catchability in the angling fishery was not significantly related to population density, whereas walleye catchability in the spearing fishery was inversely related to population density. We conclude that walleye angling is density independent and is therefore self-regulating, whereas walleye spearing is density dependent and is therefore not self-regulating.

Lake Sturgeon Age Validation using Bomb Radiocarbon and Known-Age Fish

Abstract Pectoral fin spines have been the accepted structure for estimating the age of various sturgeon species for nearly 100 years, though other structures have also been used (otoliths, pectoral girdle, scutes, and caudal fulcra). Accuracy of age estimates using any of these structures has not been validated, so we report the first use of bomb radiocarbon (14C) assays to assess the validity of ages estimated using growth increments on pectoral fin spine and otolith frontal cross sections from lake sturgeon Acipenser fulvescens; we also assessed age estimates from pectoral fin spines of known-age lake sturgeon. Growth increments on pectoral fin spine cross sections underestimated true age of fish older than 14 years and error increased with age, whereas otoliths accurately estimated true age up to at least 52 years. Increment formation on pectoral fin spine and otolith cross sections from juvenile lake sturgeon (ages 2-11) was similar, although pectoral spines were clearer and easier to interpret. A po...

Sea Lamprey Abundance and Management in Lake Superior, 1957 to 1999

The international sea lamprey (Petromyzon marinus) control program successfully laid the foundation for rehabilitation of lake trout (Salvelinus namaycush) in Lake Superior and was well coordinated among management agencies during 1957–1999. The lampricide TFM was the primary control tool, with recurring treatments in 52 larval-producing streams. Barriers and sterile-male-release, as alternative control technologies, were significant elements of the program. Barriers blocked spawning sea lampreys from substantial areas of habitat for sea lamprey larvae during 1966–1999, and the sterile-male-release technique was used to reduce larval production during 1991–1996. Sea lamprey control resulted in the suppression of sea lamprey populations in Lake Superior, as evidenced by the linear decline in spawner abundance during 1962–1999. However, sea lamprey abundance was not as low as the targets specified in the fish community objectives. Most of the parasitic sea lampreys in Lake Superior probably originated from survivors of lampricide treatments. Self-sustaining populations of lake trout were restored in most of the lake by 1996, although many were killed annually by sea lampreys. Economic injury levels for damage to fish populations by sea lampreys are being developed and will be used to distribute sea lamprey control resources among the Great Lakes.

Development of a Regional Stock–Recruitment Model for Understanding Factors Affecting Walleye Recruitment in Northern Wisconsin Lakes

Abstract We used data from 162 lakes in northern Wisconsin during 1990–1999 to develop a stock–recruitment model for walleye Stizostedion vitreum and to identify important factors affecting recruitment. We used the linear version of the Ricker stock–recruit model and regression tree analysis to model age-0 walleye density, which ranged from 0.24 to 505 per km of shoreline (mean = 51.2). Adult walleye density ranged from .040 to 5.9 per surface hectare (mean = 1.5). The density of adult (sexually mature) walleyes, calendar year, and angling harvest rate of yellow perch Perca flavescens were significant descriptors of age-0 walleye density. Adult walleye density explained 10% of the variation in age-0 walleye abundance, which exhibited significant compensatory density dependence. Calendar year and yellow perch density explained nearly 32% of the residual variation in age-0 walleye density. The average age-0 walleye density in high-recruitment years (1991, 1994, and 1995) was 74.2/km, whereas the average age...

Variation in Walleye Abundance with Lake Size and Recruitment Source

Abstract We quantified the relationship between lake size and abundance of walleyes Stizostedion vitreum at two life stages, age 0 and adult, in 172 northern Wisconsin lakes. We also determined if the relationship varied with recruitment source (stocked or natural) in order to evaluate the current system of management. For adult walleyes, as estimated by mark–recapture in spring, abundance was linearly related to lake surface area. Age-0 walleye abundance estimated by fall electrofishing catch was also linearly related to the miles of shoreline surveyed. Lake surface area explained 59% of the variability in adult walleye abundance across lakes ranging in size from 93 to 15,300 acres. Lake surface area explained 61% of the variation in abundance of self-sustaining walleye populations and 65% of stocked populations. Walleye abundance was higher in lakes sustained by natural reproduction than in lakes sustained by stocking. These patterns in abundance based on recruitment source were also apparent at age 0. ...

Catchability of Walleyes to Angling in Escanaba Lake, Wisconsin, 1980–1995

Abstract Angler catch rates were modeled as a nonlinear function of population density of adult walleyes Stizostedion vitreum to determine if walleye angling catchability was density dependent in Escanaba Lake, Wisconsin, during 1980–1995. Relations between angler catch rates (number fish/angler-hours) and catchability (q) of walleyes were modeled from data derived from compulsory creel censuses and mark–recapture estimates of walleye abundance (number/acre). For age-3 and older walleyes combined and for individual ages 3–8, angler catch rate was linearly related to population density, whereas catchability was not significantly related to population density. We conclude that angling catchability of walleyes in Escanaba Lake is not density dependent and therefore that angling can safely be regulated using indirect management strategies such as creel limits, size limits, and closed seasons.

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

Abstract 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 t...