Introduction:
Phenotypic diversity within fish species that have colonized post-glacial lakes often represent early stages of species diversification (Snorrason et al., 2004). Many fishes that have colonized post-glacial freshwater systems are assumed to have been plastic generalists (i.e., flexible in use of habitat and food resources) at the time of colonization (Skúlason et al., 2019; Snorrason et al., 2004). Given the novel environment and new ecological opportunities, a newly established population may begin to display among-individual differences in behavior and other phenotypic characteristics (Svanbäck et al., 2007). Phenotypic plasticity, the capacity for one genotype to produce different phenotypes in response to environmental cues, could be a character subject to selection, facilitating the process of diversification (De Jong, 2005). Despite uncertainties of how phenotypic plasticity promotes divergence, plasticity appears to serve as an important element in early phases of diversification (Handelsman et al., 2013; Nonaka et al., 2015; Snorrason et al., 2004). Theory predicts that stable and predictable recently colonized systems would favor foraging and habitat specialization and increase the probability of eco-morphological diversification (Skúlason et al., 1999; Snorrason et al., 2004; Van Kleunen et al., 2005).
Phenotypic plasticity in temporally and spatially variable environments has been demonstrated repeatedly within and among populations (Skúlason et al., 2019). Whether niche expansion of a population is achieved by a general increase in niche widths for all individuals overall or by an increase of among-individual variation (i.e., expression of multiple individual specializations within a population) is a question in evolutionary ecology that remains unanswered (Bolnick et al., 2003; Roughgarden, 1972; Svanbäck et al., 2012). Several apparent generalist populations have been reported to be composed of combinations of specialized individuals using several narrow niches that together yield an overall wide population niche (Araújo et al., 2011; Araújo et al., 2008; Bolnick et al., 2003). Post-glacial lakes and co-inhabiting species offer a wide range of characteristics that may favor or constrain individual specialization. Post-glacial lakes are depauperate ecosystems with low interspecific competition, which provides ecological opportunities that likely favor niche expansion (Bolnick et al., 2010; Costa et al., 2008; Parent et al., 2014). Additionally, the large flexibility within post-glacial colonizing species, with individuals having the potential to exploit a wide range of resources, can facilitate the evolution of individual resource specialization and population divergence. Yet, northern ecosystem food-webs are subjected to strong seasonal and episodic influences of climate and the environment (McMeans et al., 2015). Accordingly, using a broad resource spectrum has been identified as a useful strategy for fishes living in Arctic environments, where food can be patchily distributed and ephemerally available. From all the facets of niche use that are possible in northern lakes, understanding the magnitude and effect of individual specialization in species and trophic positions is necessary to understand the role that variation among individuals can play at the beginning of differentiation processes (Cloyed et al., 2016; De León et al., 2012; Svanbäck et al., 2015).
Great Bear Lake (Northwest Territories, Canada), straddling the Arctic Circle, provides an excellent opportunity to investigate the role of among-individual diet variation in diversification processes in post-glacial lakes. Lake trout, Salvelinus namaycush, in this lake show a high degree of intraspecific diversity within a geologically young system (8,000–10,000 yr BP; Johnson, 1975; Pielou, 2008). Specifically, extensive sympatric divergence has occurred for this species with four ecotypes inhabiting the shallow-water (≤ 30 m) zone of Great Bear Lake (Fig. A1; Chavarie et al., 2016a; Chavarie et al., 2015; Chavarie et al., 2013; Harris et al., 2015). Three of these four shallow-water lake trout ecotypes are described as trophic generalists with differing degrees of omnivory along a weak benthic-pelagic gradient (Chavarie et al., 2016a; Chavarie et al., 2016b). Despite habitat and dietary overlap, significant differences in morphological, genetic, and life-history variation have been reported (Chavarie et al., 2016 ; Chavarie et al., 2013; Harris et al., 2015). The suggested resource use among the three ecotypes could be caused by the combination of individual specialists along a resource continuum (Chavarie et al., 2016b). In other words, although ecotype resource use may appear similar, individuals within an ecotype may differ in their resource use. One of these three generalist ecotypes (Ecotype 2; generalist with a tendency to consume more fish than other ecotypes, referred to here as the piscivorous ecotype; Fig. 1) showed at least two different feeding strategies, benthic cannibalism and interspecific piscivory in the pelagic zone (Chavarie et al., 2016c).
To characterize niche use and individual variation within an ecotype in relation to observed differentiation of feeding strategies, we focused this study solely on the piscivorous lake trout ecotype. Fatty acid analysis assumes that dietary lipids are broken down into their constituent fatty acids and incorporated relatively unchanged into consumer tissues (Howell et al., 2003; Iverson, 2009; Iverson et al., 2004), allowing spatial and temporal diet comparison among individuals (Duerksen et al., 2014; Eloranta et al., 2011; Hoffmann, 2017; Iverson, 2009; Scharnweber et al., 2016). Fatty acids have been assessed to be a robust tool to characterize lake trout diets (Happel et al., 2017; Happel et al., 2016; Iverson, 2009). Thus, fatty acids were used as trophic bio-indicators to better understand dietary patterns of piscivorous lake trout and investigate whether variation occurred among individuals in this ecotype and if individual specialization may be contributing to the trophic breadth of the ecotype. Specifically, our aims were to 1) compare resource use among lake trout individuals within Ecotype 2 (piscivores) by characterizing their fatty acids profiles, 2) determine whether resource-use differences were influenced by life-history traits (e.g., size and age), 3) characterize and compare morphological variation among groups that expressed different feeding strategies, and 4) determine if genetic differences existed among groups. In addition, we examined a sub-set of large lake trout of this ecotype from our collections (> 900 mm in fork length) referred to locally as “Giants” (Fig. 1), to determine if they showed any ecological and genetic differences from others of this ecotype. These exceptionally large individuals comprise < 1% of the lake trout population sampled in Great Bear Lake, and are among the largest lake trout in the world (Chavarie et al., 2016 ). Except for their large body-size, these individuals show no major morphological or spatial and temporal distribution differences relative to other co-occurring piscivorous lake trout.