Lean Lake Trout Research Articles

Sea lamprey (Petromyzon marinus) size trends in a salmonid stocking context in Lake Superior

Fish community objectives for Lake Superior call for restoration such that it resembles its historical species composition, to the extent possible, yet allow for supplementation of naturalized Pacific salmonids (Oncorhynchus spp.). To achieve these goals, managers strive to control the sea lamprey (Petromyzon marinus) to levels that cause insignificant (<5%) mortality to host species. While control efforts have been successful, sea lamprey size has increased during the control period. We analyzed long-term sea lamprey size trends and found a significant increase from 1961 to 2003 (F = 36.76, p < 0.001, R2 = 0.473). A local regression revealed two significant size increase periods. We used Bayesian model averaging to find the relationship between sea lamprey size and the stocking of salmonids (lean lake trout (Salvelinus namaycush) and Pacific salmon). Bayesian model averaging identified 91 models, and several regressors were common features in many of the models. Sea lamprey weight was related to stocked lake trout lagged 3, 9, 11, and 13 years, and stocked Pacific salmon lagged 4 years. If sea lampreys can achieve larger sizes attached to Pacific salmonid hosts, and thus inflict more damage, there may be a trade-off for managers in achieving the fish community objectives for Lake Superior.

Growth potential and host mortality of the parasitic phase of the sea lamprey (Petromyzon marinus) in Lake Superior

Landlocked Lake Superior sea lampreys (Petromyzon marinus) cause a significant but uncertain amount of mortality on host species. We used a sea lamprey bioenergetics model to examine the scope of host sizes vulnerable to death as a consequence of sea lamprey feeding and incorporated the bimodal lake-ward migration of parasitic sea lampreys. At their peak feeding rate and maximum size (P = 1.0, proportion of maximum consumption), spring migrants were capable of killing lean lake trout (Salvelinus namaycush) hosts ≤ 2.0 kg, which was larger than fall migrants (1.8 kg). Spring migrants feeding on Pacific salmon (coho (Oncorhynchus kisutch), Chinook (Oncorhynchus tshawytscha), and steelhead (Oncorhynchus mykiss)) killed hosts ≤ 2.0 kg, but fall migrants killed hosts as large as 2.8 kg. Although there is no direct empirical evidence, bioenergetics modeling suggests that it is plausible that some of the largest sea lampreys in Lake Superior spent more than one summer as parasites. Two-summer parasites readily attained sizes of sea-run adult anadromous sea lampreys and killed hosts from 3 to >5.5 kg in size. The maximum upper limit number of 2-kg hosts killed by two-summer parasites was nearly twice that of one-summer parasites.

Trophic Relationships among Lean and Siscowet Lake Trout in Lake Superior

Lake trout Salvelinus namaycush occur in several different morphotypes in Lake Superior, including nearshore “leans” and deepwater “siscowets.” Siscowets, which are highly abundant and commercially undesirable, have been hypothesized to constrain populations of the more desirable lean lake trout through competition for prey. The stable isotope ratios (δ13C and δ15N) of leans and siscowets differed in several regions of U.S. waters, implying differences in both production base and trophic level. Spatial differences were partly a result of large-scale variation in food web structure, as stable isotope analysis of fishes in the genus Coregonus revealed different site-to-site patterns. In western Lake Superior, δ13C was similar among small leans and siscowets (implying a common production base) but diverged as the fish grew larger. The value of δ15N was greater for siscowets than for leans at all sizes, further implying that siscowets feed on a different prey assemblage than leans and are preying on another predatory fish, burbot Lota lota. A bioenergetics-based stable isotope model showed that siscowets in western Lake Superior may rely on nearshore prey for up to 25% of their production until they reach relatively large sizes. The large biomass of siscowets thus may exert strong predation pressure on nearshore communities in some regions. Evidence from central U.S. waters of Lake Superior, however, implies that siscowet predation on nearshore prey has not had a direct negative effect on lean lake trout stocks.

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...

Delineation of Sympatric Morphotypes of Lake Trout in Lake Superior

Abstract Three morphotypes of lake trout Salvelinus namaycush are recognized in Lake Superior: lean, siscowet, and humper. Absolute morphotype assignment can be difficult. We used a size-free, whole-body morphometric analysis (truss protocol) to determine whether differences in body shape existed among lake trout morphotypes. Our results showed discrimination where traditional morphometric characters and meristic measurements failed to detect differences. Principal components analysis revealed some separation of all three morphotypes based on head and caudal peduncle shape, but it also indicated considerable overlap in score values. Humper lake trout have smaller caudal peduncle widths to head length and depth characters than do lean or siscowet lake trout. Lean lake trout had larger head measures to caudal widths, whereas siscowet had higher caudal peduncle to head measures. Backward stepwise discriminant function analysis retained two head measures, three midbody measures, and four caudal peduncle measu...

Sustainability of the Lake Superior Fish Community: Interactions in a Food Web Context

The restoration and rehabilitation of the native fish communities is a long-term goal for the Laurentian Great Lakes. In Lake Superior, the ongoing restoration of the native lake trout populations is now regarded as one of the major success stories in fisheries management. However, populations of the deepwater morphotype (siscowet lake trout) have increased much more substantially than those of the nearshore morphotype (lean lake trout), and the ecosystem now contains an assemblage of exotic species such as sea lamprey, rainbow smelt, and Pacific salmon (chinook, coho, and steelhead). Those species play an important role in defining the constraints and opportunities for ecosystem management. We combined an equilibrium mass balance model (Ecopath) with a dynamic food web model (Ecosim) to evaluate the ecological consequences of future alternative management strategies and the interaction of two different sets of life history characteristics for fishes at the top of the food web. Relatively rapid turnover rates occur among the exotic forage fish, rainbow smelt, and its primary predators, exotic Pacific salmonids. Slower turnover rates occur among the native lake trout and burbot and their primary prey—lake herring, smelt, deepwater cisco, and sculpins. The abundance of forage fish is a key constraint for all salmonids in Lake Superior. Smelt and Mysis play a prominent role in sustaining the current trophic structure. Competition between the native lake trout and the exotic salmonids is asymmetric. Reductions in the salmon population yield only a modest benefit for the stocks of lake trout, whereas increased fishing of lake trout produces substantial potential increases in the yields of Pacific salmon to recreational fisheries. The deepwater or siscowet morphotype of lake trout has become very abundant. Although it plays a major role in the structure of the food web it offers little potential for the restoration of a valuable commercial or recreational fishery. Even if a combination of strong management actions is implemented, the populations of lean (nearshore) lake trout cannot be restored to pre-fishery and pre-lamprey levels. Thus, management strategy must accept the ecological constraints due in part to the presence of exotics and choose alternatives that sustain public interest in the resources while continuing the gradual progress toward restoration.

A stable isotope evaluation of the structure and spatial heterogeneity of a Lake Superior food web

We used stable isotope analysis to derive trophic relationships and movement patterns for components of the western Lake Superior food web. Trophic linkages implied by previous gut content studies were only marginally supported by stable isotope data. Siscowet lake trout (Salvelinus namaycush siscowet) were the top predators, and trophic overlap between siscowet and lean lake trout (Salvelinus namaycush) was low. Exotic Pacific salmon (Oncorhynchus spp.) occupied a lower trophic position than native piscivores because the latter relied more on coregonids. To evaluate spatial heterogeneity of the food web, we assumed that the adjacent cities of Duluth and Superior (DS) were a point source of 15N, and we measured isotopes of organisms close to and far from DS. Slimy sculpin (Cottus cognatus) were enriched in the DS area relative to other sites, implying that they are relatively sedentary. Rainbow smelt (Osmerus mordax) showed no differences at any sites, implying high vagility. Other organisms showed differences that could not be attributed to DS, implying that other mechanisms, such as trophic ontogeny, were influencing their isotopic signatures.

Growth and Contaminant Dynamics of Lake Superior Lake Trout

Lean lake trout ( Salvelinus namaycush) and the endemic siscowet are ecologically distinct phenotypes of char inhabiting Lake Superior. To examine growth and contaminant concentration dynamics of these fishes, and to assess their utility as indicators of environmental contamination, length, weight, estimated age, and muscle tissue lipid and organochlorine (OC) chemical concentration data were analyzed. Lean lake trout grew faster in length and weight than siscowet. Siscowet muscle tissue lipid concentrations were on average greater than in lean lake trout and increased with length and age, while lean lake trout muscle tissue lipid concentrations decreased in larger, older fish. Polychlorinated biphenyls (PCBs) were the most often detected contaminant, found in 95% of the lean lake trout analyzed and 100% of the siscowets. PCBs attained the highest concentration of the OC compounds quantified, followed by p,p’-DDE, dieldrin, and trans-nonachlor, respectively. Siscowet had greater total concentrations of PCBs at length than lean lake trout, but siscowet and lean lake trout had similar concentrations of PCBs at age. Based on 1990s growth and contaminant data, it is estimated that an 88 cm total length (16-year-old) lean lake trout and a 62 cm total length (15-year-old) siscowet from Lake Superior will exceed the 2.0 mg/kg “Action Level” for total concentrations of PCBs set by the United States Food and Drug Administration. Study results show phenotypic differences in growth and contaminant dynamics between lean lake trout and siscowet, findings that may improve our understanding and management of Lake Superior's aquatic resources.

A stable isotope evaluation of the structure and spatial heterogeneity of a Lake Superior food web

We used stable isotope analysis to derive trophic relationships and movement patterns for components of the western Lake Superior food web. Trophic linkages implied by previous gut content studies were only marginally supported by stable isotope data. Siscowet lake trout (Salvelinus namaycush siscowet) were the top predators, and trophic overlap between siscowet and lean lake trout (Salvelinus namaycush) was low. Exotic Pacific salmon (Oncorhynchus spp.) occupied a lower trophic position than native piscivores because the latter relied more on coregonids. To evaluate spatial heterogeneity of the food web, we assumed that the adjacent cities of Duluth and Superior (DS) were a point source of 15N, and we measured isotopes of organisms close to and far from DS. Slimy sculpin (Cottus cognatus) were enriched in the DS area relative to other sites, implying that they are relatively sedentary. Rainbow smelt (Osmerus mordax) showed no differences at any sites, implying high vagility. Other organisms showed differences that could not be attributed to DS, implying that other mechanisms, such as trophic ontogeny, were influencing their isotopic signatures.

Ontogenetic and Seasonal Patterns in the Energy Content of Piscivorous Fishes in Lake Superior

Ontogenetic, seasonal, and sex-related patterns in energy density (J/g wet mass) was studied in burbot ( Lota lota), lean lake trout ( Salvelinus namaycush namaycush) and siscowet lake trout ( Salvelinus namaycush siscowet) collected from Lake Superior in 1996 and 1997. Energy density was strongly negatively correlated with water content for all fishes (r 2 = 0.86 to 0.99). For fishes of comparable size, energy density of siscowet (10.8 kJ/g) was significantly higher than lean lake trout (7.8 kJ/g) and both were significantly higher than burbot (5.1 kJ/g). Energy density in spring was higher in lean and siscowet lake trout, and lower in burbot than in other seasons. No significant differences were apparent between sexes within a species. Energy density increased in a predictable fashion with body mass for lean and siscowet lake trout, but varied without trend for burbot. The regression common to the three species (kJ/g wet mass = 36.78 – 0.41(% water), p < 0.001) provides a straightforward method for estimating ontogenetic and seasonal energy density in these fishes.

Otoliths Reveal a Diverse Age Structure for Humper Lake Trout in Lake Superior

Humpers are one of at least three morphological variants of wild lake trout Salvelinus namaycush that maintain self-sustaining populations in Lake Superior. In an early study, bumpers from Isle Royale were shown to have a sharply truncated age distribution that was attributed to high mortality after age 11, but we suspected that these fish were underaged. In August of 1989 and 1992 we collected spawning humper lake trout from the same area and estimated their ages using both scales and sagittal otoliths. Humpers in our sample ranged from 5 to 13 years, based on scale annuli, but counts of sagitta annuli revealed ages of 8 to 28 years. Individual discrepancies between ages from scales and sagittae varied from –2 to 20 years, but differences between scale and otolith ages did not increase with individual age. We applied the von Bertalanffy growth model to the humper length-at-age data to indirectly assess the accuracy of aging estimates. The model significantly overestimated mean asymptotic length when scale ages were used, but the mean asymptotic length estimate was more similar to observed lengths when sagitta ages were used. Our results corroborate evidence that bumpers in Lake Superior grow more slowly and mature at a smaller size than lean lake trout; however, the age composition of bumpers is more diverse than previously thought. This particular population experiences little or no exploitation; the presence of older fish provides one standard by which the success of lake trout rehabilitation programs can be evaluated and emphasizes the need for accurate aging techniques.

Pesticide residues, PCBs and PAHs in baked, charbroiled, salt boiled and smoked Great Lakes lake trout

Skin-off lean lake trout ( Salvelinus namaycush namaycush) from Lakes Huron, Michigan and Ontario as well as siscowets (fat lake trout) ( Salvelinus namaycush siscowet) from Lake Superior were cooked by baking and charbroiling to determine the potential of processing/cooking on reducing the levels of pesticides and total PCBs in fish at the dinner table. Lake trout from Lakes Michigan and Superior also were salt boiled and skin-on fillets smoked. All fish analyzed were below the action level except skin-on siscowets with > 0.3 ppm action level for chlordane. Cooked lake trout had significantly less residue than raw. Smoking resulted in significantly greater losses of pesticides and total PCBs than other cooking methods but analyses of polynuclear aromatic hydrocarbons showed significant compound formation during smoking with higher levels occurring in high fat siscowets. Overall losses of pesticides and total PCBs ranged from a low of 21% for dieldrin to a high of 39% for chlordane complex. Most of the total losses were about 30% establishing that cooking is effective in reducing residues in these Great Lakes fish.

Diets of Siscowet Lake Trout from the Apostle Islands Region of Lake Superior, 1993

Stomach contents of 384 siscowet lake trout ( Salvelinus namaycush siscowet) from the Apostle Islands region of Lake Superior were examined to determine if diets exhibited seasonal trends or shifted with predator size. The information was also used to qualitatively compare siscowet and lean lake trout ( S. namaycush) diets. Stomachs from siscowet less than 500-mm total length (TL) contained primarily macroinvertebrates (clams, Mysis relicta, and insects), sculpins (Cottidae; mean TL = 45 mm), and coregonines ( Coregonus spp.; mean TL = 120 mm). Stomachs from siscowet between 500- and 699-mm TL contained items similar to smaller siscowet, but burbot ( Lota lota; mean TL = 162 mm and a 254-mm siscowet were also present. Stomachs from siscowet longer than 699-mm TL contained fishes such as burbot (mean TL = 246 mm) and coregonines (mean TL = 240 mm). Dietary proportions of coregonines increased significantly with increasing siscowet length groups. The proportion of the total number of identified taxa and frequency of occurrence of terrestrial insects and sculpins declined significantly across all length groups between June and December. Siscowet and lean lake trout diets were not found to be substantially different and a potential for competition exists.

Density-Independent Survival of Wild Lake Trout in the Apostle Islands Area of Lake Superior

The lake trout (Salvelinus namaycush) stock at Gull Island Shoal in western Lake Superior was one of only a few stocks of lean lake trout in the Great Lakes that survived overfishing and predation by the sea lamprey (Petromyzon marinus). Since the mid 1960s, the abundance of wild recruits measured at age 0 and the number of age-7 to -11 wild fish recruited to the fishable stock have increased. We used the Varley-Gradwell method to test for density-dependent survival between these life stages. Survival from age-0 to ages 7–11 was not affected by increasing density, which suggests that further increases in recruitment and stock size are still possible. We suggest that testing for the existence of density-dependent survival can be used to indicate when lake trout populations are rehabilitated.

Evaluation of Age-0 Survival and Its Effect on Lake Trout Rehabilitation in the Michigan Waters of Lake Superior

Efforts to rehabilitate lake trout in the upper Great Lakes have met with limited success. One key factor hypothesized to impede progress towards rehabilitation is inadequate age-0 survival (from egg deposition to age-1). We evaluated age-0 survival rates for lean lake trout populations in the Michigan waters of Lake Superior during three periods of significantly different abundance levels and mortality regimes: pre-sea lamprey (1929–1950), sea lamprey-dominant (1951–1962), and current (1988–1993). Current age-0 survival rate was calculated to be 0.00118 which is lower than age-0 survival rates corresponding to the pre-sea lamprey (0.00372) and sea lamprey-dominant (0.00416) periods. Population growth rates of lake trout were found to be extremely sensitive to changes in age-0 survival rates. We found that if the current lake trout population in the Michigan waters of Lake Superior exhibited an age-0 survival rate similar to that of the pre-sea lamprey population, the current population would be increasing in numbers rather than decreasing. As increasing age-0 survival will greatly increase the population growth rate and facilitate the rehabilitation process, we recommend that future research and management efforts place a high priority on evaluating the factors that are currently depressing survival rates of age-0 lake trout in Lake Superior in order to develop strategies to overcome these impediments.

Patterns of Organochlorine Contamination in Lake Trout from Wisconsin Waters of the Great Lakes

To investigate spatial and temporal patterns of organochlorine contamination in lake trout from Wisconsin waters of the Great Lakes, we examined laboratory contaminant analysis data of muscle tissue samples from Lake Michigan (n = 317) and Lake Superior (n = 53) fish. Concentrations of polychlorinated biphenyls (PCBs), chlordane, and dieldrin, reported as mg/kg wet weight in 620 mm to 640 mm mean length Lake Michigan lake trout, decreased over time. Mean total PCB concentration declined exponentially from 9.7 in 1975 to 1.9 in 1990. Total chlordane concentration declined 63 percent from 0.48 in 1983 to 0.18 in 1990, and dieldrin declined 52 percent during this same period, from 0.21 to 0.10. The bioaccumulation rate of PCBs is significantly lower for lake trout inhabiting Lake Michigan's midlake reef complex, compared to lake trout from the nearshore waters of western Lake Michigan. Organochlorine compound concentrations were greater in Lake Michigan lake trout than Lake Superior fish. Lake Superior lean lake trout and siscowet exhibited similar rates of PCB bioaccumulation despite major differences in muscle tissue lipid content between the two subspecies. The lack of a significant difference in the PCB bioaccumulation rates of lean trout and siscowet suggests that lipid content may not be an important factor influencing PCB bioaccumulation in lake trout, within the range of lipid concentrations observed. Relative concentrations of the various organochlorine contaminants found in lake trout were highly correlated, suggesting similar mass balance processes for these compounds. Evidence presented revealing spatial and temporal patterns of organochlorine contamination may be of value in reestablishing self-sustaining populations of lake trout in Lake Michigan.

Chlorinated Organic Residues in Fish, Water, and Precipitation from the Vicinity of Isle Royale, Lake Superior

Studies of persistent organic contaminants for the Upper Lakes Reference Group of the International Joint Commission indicated increased levels of organic contamination in the flesh of fish taken from the vicinity of Isle Royale on Lake Superior. Significant increases in levels of organic residues in Lake Superior fish near Isle Royale were observed in lean lake trout, Salvelinus namaycush, and in fat lake trout, Salvelinus namaycush siscowet.Siskiwit Lake, a deep cold lake on Isle Royale, well removed from the direct influences of man, was selected as a control site for studies on Lake Superior. Values for several organic residues in the flesh of fish from Siskiwit Lake were significantly higher than corresponding fish from Lake Superior. Polychlorinated biphenyls were nearly double the Lake Superior mean value, and p,p-DDE showed a more than 10 fold increase in Siskiwit Lake.These findings lead to a preliminary study of the transport of polychlorinated biphenyl compounds in atmospheric precipitation. Data from the Isle Royale area are compared with metropolitan sites and with those of other investigators.

Age, Growth, Maturity, and Fecundity of Humper Lake Trout, Isle Royale, Lake Superior

Abstract Humper lake trout are one of the several races or subpopulations of lake trout in Lake Superior. This study is based on 3,705 fish collected on a reef south of Isle Royale near the eastern end. The mean lengths of humper trout from commercial gill nets were smaller than those of lean lake trout. Members of age-groups VII, VIII, and IX represented 81.5 per cent of the commercial humper catch. The body-scale relation was described by two intersecting straight lines. The weight of humper trout increased as the 3.282 power of the length. Growth in length was slow and ranged from 1.6 to 3.5 inches per year. Annual increments were greatest in the first, sixth, and seventh years. Growth in weight was also slow but increased each year. Humper trout became legal (1 1/2 pounds) in the eighth year of life and reached 5 pounds in 11 years. All fish longer than 19.1 inches and older than age-group VIII were mature; the shortest mature fish were: males, 12.7 inches; females, 14.7 inches. At minimum legal size,...