From top to bottom: Do Lake Trout diversify along a depth gradient in Great Bear Lake, NT, Canada?

Abstract

Depth is usually considered the main driver of Lake Trout intraspecific diversity across lakes in North America. Given that Great Bear Lake is one of the largest and deepest freshwater systems in North America, we predicted that Lake Trout intraspecific diversity to be organized along a depth axis within this system. Thus, we investigated whether a deep-water morph of Lake Trout co-existed with four shallow-water morphs previously described in Great Bear Lake. Morphology, neutral genetic variation, isotopic niches, and life-history traits of Lake Trout across depths (0–150 m) were compared among morphs. Due to the propensity of Lake Trout with high levels of morphological diversity to occupy multiple habitat niches, a novel multivariate grouping method using a suite of composite variables was applied in addition to two other commonly used grouping methods to classify individuals. Depth alone did not explain Lake Trout diversity in Great Bear Lake; a distinct fifth deep-water morph was not found. Rather, Lake Trout diversity followed an ecological continuum, with some evidence for adaptation to local conditions in deep-water habitat. Overall, trout caught from deep-water showed low levels of genetic and phenotypic differentiation from shallow-water trout, and displayed higher lipid content (C:N ratio) and occupied a higher trophic level that suggested an potential increase of piscivory (including cannibalism) than the previously described four morphs. Why phenotypic divergence between shallow- and deep-water Lake Trout was low is unknown, especially when the potential for phenotypic variation should be high in deep and large Great Bear Lake. Given that variation in complexity of freshwater environments has dramatic consequences for divergence, variation in the complexity in Great Bear Lake (i.e., shallow being more complex than deep), may explain the observed dichotomy in the expression of intraspecific phenotypic diversity between shallow- vs. deep-water habitats. The ambiguity surrounding mechanisms driving divergence of Lake Trout in Great Bear Lake should be seen as reflective of the highly variable nature of ecological opportunity and divergent natural selection itself.

Habitat overlap of juvenile and adult lake trout of Great Bear Lake: Evidence for lack of a predation gradient?

“…Bear Lake followed the same trend previously reported for adults, with a higher δ 15 N (trophic level) compared to their shallow-water counterparts (Chavarie et al, 2018). Differences in asymptotic length in relation to depth could support the assumption of different foraging opportunities encountered by individuals Hansen et al, 2012;.…”

“…Lake (Chavarie et al, 2018;Zimmerman et al, 2009). The source of the difference in adult distribution may help explain differences in juvenile distribution between the lakes.…”

“…Adult lake trout distributions in Great Bear Lake have been reported previously (Chavarie et al, 2018) and were used for this study to compare against juvenile distributions. G-tests (Zar, 1999) were used to determine whether the proportions of juveniles and adults, based on our net catches, differed among the three depth zones (0-20 m, 21-50 m and 51-150 m).…”

“…Lake, based on the detection of a small genetic cline in profundal deep-water adults in comparison with their shallower counterparts (Chavarie et al, 2018), although this pattern was weaker for juveniles than for the adults. Another ecological depth-based trend, detected both in adults and juvenile, was related to lipid accumulation.…”

“…However, an isotopic enrichment signal linked with depth could also lead to the observed isotopic difference in juvenile individuals (Post, 2002;Sierszen et al, 2014;Stockwell, Yule, Hrabik, Sierszen, & Isaac, 2014). One way or another, limited movement among depths, would enforce an ecological cline, where tensions between homogenising and divergent evolutionary forces arise (Chavarie et al, 2018). One way or another, limited movement among depths, would enforce an ecological cline, where tensions between homogenising and divergent evolutionary forces arise (Chavarie et al, 2018).…”