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Morphological, Physiological and Genetic Differentiation of Lake Trout Morphotypes from Lake Superior
1NOAA Northwest Fisheries Science Center
Manchester Research Station
7305 Beach Drive East
Port Orchard, WA 98366
2Marquette Fisheries Research Station
Michigan Department of Natural Resources
484 Cherry Creek Rd.
Marquette, MI, 49855
Results of a past study (Goetz et al. 2010: Molecular Ecology 19 Suppl. 1: 176-196) on lean and siscowet lake trout reared under identical conditions from conception, indicated that differences in growth and morphometry between lean and siscowet lake trout have a genetic basis. Using these cultured lake trout, in the present study we found that siscowet lake trout had higher lipid levels and lower glycogen levels as compared to lean lake trout in skeletal muscle and liver. Lean lake trout also had higher circulating levels of lipids (total lipid, low density, and high density lipoproteins) and glucose compared to siscowet lake trout. Analysis of the progeny from crosses of the cultured morphotypes showed that progeny of crosses between siscowet females and siscowet males had significantly higher lipid levels than all other crosses. Further, results of the reciprocal cross between female siscowets and lean males indicated that the lipid effect could be associated with the female siscowet. Siscowets had higher levels of the polyunsaturated fatty acids, eicosapentaenoic and docosahexaenoic acids, and of myostatin protein and myostatin 1b mRNA. The combined results indicate that the lake trout morphotypes differ substantially in the storage of energy that may be related to their specific life histories. Siscowets store energy preferentially as lipid and appear to be more efficient in moving lipid from the blood into the muscle and liver. The lipid in siscowets may be adaptive for regulating buoyancy as well as an essential energy reserve for reproduction.
The current study shows that a percentage of lean (12.2 %) and siscowet (58.0 %) lake trout (Salvelinus namaycush) in populations in southern Lake Superior do not reproduce each year (“skipped spawning”) even though they have reached puberty. We measured skipped spawning in lake trout in southern Lake Superior and assessed its impact on reproduction by developing length- and age-based ogives, and evaluated the influence of skipped spawning on a key stock assessment quantity, spawning stock biomass (SSB), by applying maturity and reproduction ogives to simulated populations. Furthermore, we evaluated whether skip spawning was associated with lower energy reserves by testing for differences in hepatosomatic index (HSI) according to maturity status. In populations where skipped spawning is negligible, a maturity ogive (probability of being mature) can be considered equivalent to a reproduction ogive (probability of being reproductive). However, in populations where skipped spawning is present, such as lean and siscowet lake trout in southern Lake Superior, maturity and reproduction ogives are very different. Our population simulations indicate that SSB can be overestimated (8.3 % for leans and 475 % for siscowets) when skipped spawners were assumed to be a part of the spawning stock. An inflated reproductive potential of the population could allow for excessive fishing if quotas are developed based on these biased SSB estimates. Siscowet lake trout that were skipping spawning had significantly lower energy reserves (assessed by HSI) than mature and immature fish. The high incidence of skip spawning in siscowets may be related to reported slowed somatic growth, high population abundance, and low prey resources in southern Lake Superior, which supports the hypothesis that skip spawning is a density-dependent response to limited energy resources.