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.