Lake Trout














June 2005

Bob Snyder
Montana Fish, Wildlife and Parks
Helena, Montana
and
Dick Oswald
Montana Fish, Wildlife and Parks
Dillon, Montana



Taxonomy

Lake trout (Salvelinus namaycush) were first described from a specimen from Hudson Bay (Walbaum 1792 cited by Scott and Crossman 1973). Salvelinus is the generic name for arctic chars; namaycush is a North American Indian name for �dwellers of the deep� (NSDAF 2004). Lake trout are typically trout-like in shape. Like other chars, lake trout have light spotting on a dark background that can range from light green to almost black. A narrow, sometimes indistinct, white anterior border is present on the pectoral, pelvic and anal fins (NSDAF 2004). Lake trout can be distinguished from other chars by their deeply forked tail (Holton and Johnson 1996). Lake trout display considerable genotypic and phenotypic variation throughout their range, with many local color and body morphological differences (Page et al. 2003; NatureServe 2004). Lake trout and bull trout (S. confluentis) are the only members of the genus Salvelinus native to Montana (Holton and Johnson 1996).

Historic Range

Lake trout are native to northern North America, with their historic range determined primarily by the extent and dynamics of the Pleistocene glaciations (Lindsey 1964; Wilson and Hebert 1998). The lake trout�s native range includes portions of all of the Canadian provinces except Newfoundland and Prince Edward Island, and the states of Alaska, Maine, Illinois, Indiana, Massachusetts, Michigan, Minnesota, Montana, New Hampshire, New York, Ohio, Pennsylvania, Vermont and Wisconsin (NatureServe 2004).

During the Pleistocene glaciations, a lake trout refugium was formed in the lakes occurring along the ice sheet in what is now Montana. As the Wisconsin ice sheet withdrew, fish from the Alberta/Montana refugium recolonized the newly exposed deep lakes (Wilson and Mandrak 2004). Montana�s native lake trout populations are remnant examples of this important refugium (Wilson and Hebert 1998).

In northwestern Montana, native lake trout populations occur in Waterton Lake, Glenns Lake, Cosley Lake, and St. Mary Lake in Glacier National Park and Lower St. Mary Lake on the Blackfeet Indian Reservation (Figure 1). All of these waters are in drainages that eventually reach Hudson Bay. Although there are records of some stocking of lake trout into Cosley, Glenns and Lower St. Mary lakes, mitochondrial DNA (mtDNA) analysis by Wilson and Hebert (1998) gives evidence of the native status of these populations. In southwestern Montana, native lake trout populations are found in Twin Lake in the Big Hole River drainage and Elk Lake in the Red Rock River drainage, which are both tributaries to the upper Missouri River drainage (Figure 1). There are no records documenting the stocking of lake trout into Twin and Elk Lakes. Other lake trout populations in Montana are the result of legal and illegal introductions and are not remnant native populations.

The lake trout of Elk and Twin Lakes in southwestern Montana have long been accepted as representing native populations (Brown 1971). Recent examination of the literature and genetic analysis lend credence to this assumption. Vincent (1963) cited numerous authors from the late 1800's that described collections of native lake trout from Elk Lake prior to 1890, when the species was first brought to the intermountain west for introduction. He also referenced a type collection of three lake trout sent to the U.S. National Museum listing Elk Lake as the specimen locality. He concluded that the lake trout of Elk Lake were a glacial relict population and native in origin. Vincent (1963) also cited early naturalists� reports from the late 1800's which described visits to Twin Lakes and references to a native lake trout population prior to 1890. Khan and Quadri (1971), in a meristic and morphological examination of lake trout from across its range of distribution, determined that the lake trout of Elk and Twin Lakes represented a glacial refuge population in the upper Missouri River drainage. Genetic examination of a small collection of lake trout from Twin Lakes in 1994 led Wilson and Hebert (1998) to conclude that the fish were of a distinct haplotype and represent a glacial relict native population. This C3 haplotype originated in northernmost Alaska and Canada and is currently distributed in western Montana and southern Alberta. More recent genetic examination of 14 fish from Elk Lake and 14 fish from Twin Lakes concurred with Wilson and Hebert�s findings and the conclusion that the lake trout populations of both lakes represent native populations of the same glacial origin (M.B. Curtis, US Geological Survey, personal communication 2000). This expanded examination also eliminated the possibility that the native Elk Lake stock had been contaminated by introduced lake trout of Great Lakes origin that were stocked in Yellowstone Park in 1890. Examinations of mtDNA by Curtis also suggest that low variation is associated with a genetic �bottleneck� caused and maintained by low populations of breeding individuals in both lakes. This is substantiated by lake trout collection densities in Elk and Twin Lakes and also by the age and size structure of the sample populations.





























Figure 1. Current distribution of native lake trout populations in Montana.

Life History and Ecology

Whereas lake trout can be found in cold rivers and shallow lakes in the northern portion of its range (Scott and Crossman 1973), in Montana native lake trout inhabit a few relatively deep, cold lakes remaining from the Pleistocene glaciations. Lake trout typically use water temperatures in the 10-12 �C range (Magnuson et al. 1990) and, therefore, spend most of their lives in the deeper habitats associated with these water temperatures. Lake trout can occasionally be found in shallow water habitats during seasons when surface waters are within their preferred temperature range.

Lake trout are long-lived and reach sexual maturity relatively late, as early as age 4 and as late as age 17 (NatureServe 2004). The oldest estimated age for lake trout is 65 years (NatureServe 2004). Lake trout are the largest freshwater salmonid native to North America, with specimens over 45.4 kg caught by commercial fishermen (NSDAF 2004). The current Montana state record for lake trout is a 19.4 kg fish from Flathead Lake that measured 108 cm (Easterling 2004).

Lake trout spawn during the fall when water temperatures decline to 8-14 � C, which typically coincides with the breakdown of thermal stratification (Gunn 1995). Lake trout do not demonstrate distinct sexual dimorphism (Martin and Olver 1980) and are the only salmonid that exhibits exclusively nocturnal spawning behavior (Gunn 1995). Lake trout are broadcast spawners. Eggs are deposited on boulder or cobble habitats in depths usually less than 15 m, although spawning has been documented to occur at depths as shallow as 0.3 m and greater than 36.6 m (Scott and Crossman 1973). Eggs and fry develop in the interstitial spaces of the substrate and emergence typically occurs 4 to 6 months after spawning (Scott and Crossman 1973).

Juvenile lake trout feed on a variety of zooplankton and aquatic invertebrates. Adult lake trout are predacious, feeding upon a wide range of organisms including crustaceans, aquatic and terrestrial insects and fish (Scott and Crossman 1973). Faster growth rates and larger sizes occur when abundant fish forage is available. The importance of benthic cottids as a forage to some lake trout populations has been documented (Scott and Crossman 1973). As a top trophic level predator with a mostly piscivorous diet, non-native lake trout have caused catastrophic declines in native fish assemblages where they have been introduced (Cordone and Frantz 1966; Donald and Alger 1993; Stapp and Hayward 2002).

Substantial genotypic and phenotypic variability can be found in lake trout populations throughout their range, with many unique local adaptations including various life histories, differing ability to avoid lamprey predation, and varieties of body form, coloration, and spotting patterns (Page et al. 2003).

Status

The Natural Heritage Program classifies native lake trout in Canada and the United States as an N5 (Secure-common, widespread, and abundant in the nation; NatureServe 2004). The NHP conservation status rank for Montana native lake trout populations is currently SU (status under review; NatureServe 2004).

Within the United States, introduced lake trout populations are widespread and, in some cases, expanding. Native lake trout populations, however, are in a more tenuous status. The lake trout�s long generation time, low fecundity, specific adult and spawning habitat requirements and status as a top-end predator make the species vulnerable to anthropogenic disturbances (Wilson and Mandrak 2004). Lake trout populations have been decimated by over-harvest, habitat loss, pollution, and introductions of non-native fish species, especially sea lampreys (Petromyzon marinus) into the Great Lakes. Efforts to re-establish native lake trout populations have generally not met management goals (Wilson and Mandrak 2004).

Elk Lake, in the Red Rock River drainage, is a mesotrophic 114.5 ha lake at 2,034 m elevation with a maximum depth of 21.3 m (USFS 2004). The lake trout population in Elk Lake has been most recently described by Oswald (2000; 2002; 2004). Lake trout numbers appear relatively stable at low density; varying between about 0.8 and 1.6 fish per net since 1991. Analyses of mean total length, length range, and composite length frequency over the 1991-2001 period demonstrate stability within the sample population. Mean length varied only slightly, with length-frequency analysis demonstrating that the majority of lake trout sampled range between 41.9 and 49.5 cm in length. Juvenile fish only appeared in the 1994, 1998, and 2001 samples, suggesting limited recruitment into the population. It is estimated that the Elk Lake population is small (250-1000 fish) with an irregular age structure. Lake trout inhabit Elk Lake along with native burbot (Lota lota), white sucker (Catostomus commersonii), and mottled sculpin (Cottus bairdii). A lacustrine population of Arctic grayling (Thymallus arcticus) also occupied the lake until the mid 1990�s when persistent drought dewatered spawning streams, resulting in a lack of recruitment and ultimate extirpation. Currently, the most abundant salmonid inhabiting Elk Lake is a stocked population of westslope cutthroat trout (Oncorhynchus clarkii lewisi) that provides a popular sport fishery. The lake trout population of Elk Lake has been managed under catch and release regulations since 2000.

Twin Lakes, in the Big Hole River drainage, is an oligotrophic 30.4 ha lake at 2,205 m elevation with a maximum depth of 22 m (USFS 2004). The fish populations of Twin Lakes have been sampled sporadically since 1964 and have been reported in detail by Oswald (2000; 2002; 2004). To better determine the genetic status of the lake trout of Twin Lakes, an intensive sampling effort was conducted in 1998 (Oswald and Roberts 1998). This effort consisted 20 overnight experimental gill net sets between July 17 and October 8, 1998. Lake trout CPUEs varied between 1.0 and 5.0 per net during the pre-1998 period, with the highest CPUEs in the 1964, 1986 and 1990 samples. In contrast, the more intensive sampling in 1998 produced a CPUE of only 0.6 fish per net. The length-frequency distribution of fish collected in the 1998 sampling program revealed the presence of only two size classes of fish. The smaller size class was composed of fish in the 30.5-35.3 cm length range, while the larger size class consisted entirely of fish in excess of 76.2 cm in length. The disparate size classes indicative of sporadic success in lake trout recruitment was also observed in the 1964 and 1986 samples. In contrast with Elk Lake, the length range and mean length of the lake trout from Twin Lakes samples has varied widely over the sample period. We estimate that the Twin Lakes population is perhaps 50 � 250 fish and is declining due to extremely sporadic and limited recruitment.

Little is known about the status of Montana�s northern native lake trout populations. The populations in Waterton, Cosley, Glenns and St. Mary lakes are afforded the protection of their location within Glacier National Park. The Waterton Lake population, in the Waterton River drainage, is believed to be abundant and stable (L. Marnell, National Park Service, personal communication).

There is some uncertainty about whether the Glenns and Cosley lakes populations, in the Belly river drainage, are native due to the location of a downstream barrier falls (L. Marnell, National Park Service, personal communication). Holton and Johnson (1996) did not list these as native populations; however, Wilson and Hebert (1998) found that there is genetic evidence that the Cosley Lake haplotype is consistent with the other populations that formed the Alberta-Montana refugium. The Cosley and Glenns lakes populations are believed to be stable, but there is no data to support this presumption. There are records of stocking lake trout of unknown origin into Cosley and Glenns lakes, so the genome of these populations may contain introduced alleles. Genetic analysis has been performedto identify the source stock for these introductions, but results are not yet available (L. Marnell, NPS, personal communication; R. Wagner, US Fish and Wildlife Service, personal communication).

The St. Mary Lake population is also believed to be abundant and stable. Lake trout are the second most abundant fish species after lake whitefish in St. Mary Lake. There are records of stocking lake trout into St. Mary Lake, so the genome of this population may contain exotic alleles. Genetic analysis has been performed to identify the source stock for these introductions, but results are not yet available (L. Marnell, NPS, personal communication; R. Wagner, USFWS, personal communication).

Lower St. Mary Lake is located within the Blackfeet Indian Reservation. This population is stable and abundant. Lake trout are the most abundant fish species after lake whitefish and comprise 10-30% of the commercial lake whitefish catch. Again, there are records of stocking lake trout of unknown origin into Lower St. Mary Lake. Water level fluctuations and dewatering due to lake management for irrigation impacts this population (R. Wagner, USFWS, personal communication).

The genetic uniqueness and significance of Montana�s native lake trout populations to the post-glacial distribution of the species mandate that these remnant native populations be conserved.

Acknowledgements

This paper was reviewed by C. Barfoot, A. Dux, and T. McMahon. Comments were provided by R. Bramblett, W. Fredenberg, G. Hoffman, K. Knudsen, R. Leary, J. Lovell, B. Maxell, and B. Shepard.


References

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UPDATED September 30, 2010

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