Lake Trout Ecotypes

Evolutionary & ecological genetics, Paul Bentzen

Lake Trout (Salvelinus namaycush), also known as Lake Char, is an iconic species of pristine northern lakes. In these systems, trout typically occupy the top level in the food web and eat fishes. In most systems, trout exhibit slow growth and a late age at maturation (7-20 years of age). Lake Trout spawn in the fall (mid August-late October, depending on latitude). Unlike most salmonids, they do not construct redds (nests) and instead spawn over cobble and small boulders in lakes. This unique breeding system has been implicated in their ability to colonize and be successful in hostile post-glacial lakes. It is known from previous research that Lake Trout colonized their present distribution from four glacial refugia and used enormous pro-glacial lakes as conduits of dispersal.

As is the case with some other northern freshwater fishes, several Lake Trout populations occur as sympatric morphs that utilize different ecological niches. The best-known example is Lake Superior, where three forms are currently recognized: ‘siscowet’, ‘humpers’ and ‘leans’. These morphs differ with respect to the depth of water they occupy as well as other phenotypic and genotypic attributes.

Siscowet live in deepwater zones of the lake, humpers occur in mid depth habitats associated with offshore ‘humps’ and leans occupy the shallower areas. These morphs differ with respect to the dietary niche they fill but it is the physiological adaptations for living at different depths that most clearly distinguish these morphs. Previous research has shown that high fat content and gas bladder retention (two traits believed to be adaptations to living at great depth with regular ascending and descending in the water column for feeding) are heritable traits.

Other lakes, including Great Bear Lake (Northwest Territories), Great Slave Lake (Northwest Territories), and Lake Mistassini (Quebec) also harbour sympatric morphs of Lake Trout. Great Slave Lake has leans and siscowet while Lake Mistassini contains leans and humpers.

Great Bear Lake is an ecological exception where most of the phenotypic variation is split between two shallow water morphs: a large fish-eating morph and a smaller invertebrate-eating morph.
There is evidence for further variation within these morphs but the major split among life history types relates to diet and maximum size attained.
The two morphs differ greatly in body morphology with fish-eaters having a more streamlined body shape, larger mouths, smaller fins and fewer gill rakers (used for filtering prey out of the water) than the smaller invertebrate eater. Although the most striking phenotypic variation is found in the shallow water habitats of the lake, recent observations suggest that there is a ‘humper’ form in Great Bear as well. Why similar Lake Trout morphs occur in a number of geographically widespread lakes, and how these morphs arose, is currently unknown.

Current research

Morphs of lake trout arose by one or a combination of two processes: historical divergence in isolated glacial refugia followed by post-glacial dispersal, or by contemporary in situ ecological divergence . Our research utilizes both phenotypic and genotypic approaches to disentangle the evolutionary processes responsible for Lake Trout morphs. Our molecular tools include both mitochondrial (mtDNA) and nuclear loci, and these are being used to distinguish historical versus contemporary divergence. We are further investigating the hypothesis of contemporary evolution by contrasting patterns of neutral genetic variation with those potentially of adaptive significance and hence under natural selection. This project is a collaboration with researchers at the Great Lakes Fisheries Commission as well as Plummer’s Lodges, a tourism operator on Great Bear Lake.

On Great Bear Lake, we collected lake trout at spawning time to investigate whether morphs segregate at spawning time and we investigated this question on different spatial scales within the lake (i.e. within and among geographically distinct ‘arms’ of the lake.
Preliminary results indicate that morphs segregate at spawning time. Morphological analyses show that fish of similar phenotype aggregate in different areas during their breeding period. Microsatellite DNA estimates of population structure show weak levels of differentiation although there is evidence for greater gene flow within a particular morph rather than between morphs. We are currently examining mtDNA to determine whether morphs are associated with different mitochondrial lineages, a result that would suggest that the morphs are at least partially the result of allopatric divergence.

Conservation issues

Due to the extremely nutrient poor environments in which they live, their slow growth and late age at maturity, Lake Trout populations tend to have low productivity. Many populations have experienced sharp declines, stemming from over-fishing, pollution and ecological effects of introduced species. The most notable example of the latter has occurred in the Laurentian Great Lakes where the accidental introduction of sea lamprey (Petromyzon marinus) decimated Lake Trout stocks in the mid 1900s. There is considerable interest in restoring Lake Trout morphs to the Laurentian Great Lakes, a prospect rendered all the more challenging by the fact that much of the phenotypic variation was lost before active research on these morphs began. This is a major impetus for the study of Lake Trout morphs in lakes outside the Great Lakes, as these systems are comparatively pristine, and may offer indications of the ecological conditions in the Laurentian Great Lakes prior to sea lamprey invasion, pollution and over-fishing. Knowledge of lake trout diversity in pristine lakes can contribute to informed decisions for Lake Trout restoration. Furthermore, the pristine systems such as Great Bear Lake in our study are not without their own conservation issues. There is growing concern over how expanded uranium and diamond mining may affect populations in Great Bear Lake.