The Biology and Ecology of Polymorphic Lake Trout, Salvelinus namaycush, in Great Bear Lake, Northwest Territories

• Author / Creator

  Chavarie, Louise

• Post-glacial lakes, a common feature in northern landscapes, provide favorable ecosystems for studying intra-specific diversity in fishes. Great Bear Lake, with its large size and virtually pristine, recently colonized cold water habitats, provides unique opportunities for Lake Trout (Salvelinus namaycush) diversification. This thesis presents a new case of exceptional intraspecific diversity of Lake Trout in Great Bear Lake, featuring four co-existing shallow-water morphotypes. In chapter 2, I combined classical morphometric/meristic measures with shape analysis (geometric morphometrics) to quantify morphological differences among adult and juvenile shallow-water Lake Trout. Head and fin measurements best discriminated the adult morphotypes whereas little differentiation was found in body shape. No consistent patterns of variation were found among juveniles, suggesting that divergence develops at later stages. The lack of body shape variation among morphs combined with the size the lake, led me to investigate geographic-based morphological patterns within the five arms of Great Bear Lake in Chapter 3. Within each of the three more common morphotypes, morphological measures, particurlaly body shape differences, were found to vary among lake arms. Genetic and morphological distance matrices were also compared to investigate potential parallel patterns, and suggested observed morphological variation is a phenotypically plastic response to distinct environments. In Chapter 4, I analyzed stomach contents and fatty acids to investigate diet partitioning among the four sympatric shallow-water morphs of Lake Trout as a potential explanatory mechanism for diversification since trophic polymorphism is common among post-glacial fishes. Results suggested that polymorphism in the Lake Trout of Great Bear Lake is partially maintained by diet differences and by some habitat partitioning, but some overlap and seasonality in resource use were also found among morphs in this northern lake. Finally, in Chapter 5, I investigated potential variation in life history traits among the four morphotypes, a commonly observed response to inhabiting different freshwater habitats and exploiting different resources. Growth rate, age-at-maturity, size-at-maturity, and survival differed among morphs, representing trade-offs between reproduction and somatic growth. However, unexpected results, such as high proportions of resting individuals, early maturation of Morph 2 (piscivorous form), and a lack of variation in fecundity, were also found, some of which reflect the complexity of making predictions in any specific case. Great Bear Lake is a distinctive case that will be used as a benchmark for intraspecific diversity in Lake Trout, and for sympatric polymorphism and parallel adaptive radiation more generally. With its exceptional diversity and pristine environment exempt from problems that typically confound investigations (e.g., declines in fish diversity and anthropogenic impacts), Great Bear Lake provides a reference against which to compare other lakes, such as the Laurentian Great Lakes. This thesis addressed some key concepts (e.g., mode of diversification, ecological relationships) related to phenotypic diversity in northern Lake Trout and the extent of their polymorphism that can occur in large northern Canadian lakes. However, several gaps in our knowledge, e.g. the mechanism(s) of intraspecific diversity and its geographic context, remain in understanding polymorphism.

• Subjects / Keywords

  • Great Bear Lake
  • Lake Trout
  • Aquatic ecology

• Graduation date

  Fall 2014

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R38G8FQ3F

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.