Study area and field sampling
Great Bear Lake is an oligotrophic Arctic freshwater system, 250 km
south of the Arctic Ocean, in Northwest Territories, Canada (N66° 06’
W120° 35’) (Johnson, 1975). As the world’s ninth largest and
19th deepest lake, the lake has a complex, multi-armed
surface area of 31,790 km2 and a maximum depth of 446
m (mean depth = 90 m). Great Bear Lake was formed by scouring from the
Laurentide ice-sheet during the Pleistocene and was originally part of
glacial Lake McConnell 8,000–10,000 yr BP (Johnson, 1975; Pielou,
2008). The lake has characteristics typical of an arctic lake:
ultra-oligotrophic, short ice-free season, and a simple food web
supporting only 15 fish species (Alfonso, 2004; Johnson, 1975; MacDonald
et al., 2004). Great Bear Lake lacks a commercial fishery but plays an
important role in the local economy, supporting a fly-in sport fishery
for tourists and a subsistence fishery for the small Sahtu community of
Déline. Great Bear Lake has considerable intraspecific diversity within
lake trout, lake whitefish (Coregonus clupeaformis ), and cisco
(C. artedi ) (Chavarie et al., 2013; Howland et al., 2013).
Piscivorous lake trout were caught at depths ≤ 30 m using paired bottom
sets (ca. 24 h) of 140-mm and multi-mesh (38–140 mm) stretched-mesh
gill nets from late-July through August over multiple years (2002–2011)
among all five arms of the lake (Chavarie et al., 2016b; Chavarie et
al., 2015; Chavarie et al., 2013). During 2012-2014, multi-mesh gill
nets (38 to 140 mm), with a typical soak time of 24 hours, were
distributed across random depth-stratified sites (0–150 m) among Keith,
McVicar, and McTavish arms (Table A1). Compared to the other ecotypes,
piscivores have a streamlined body, large gape, and high growth rates
throughout life, similar to other piscivores (Chavarie et al., 2016 ;
Chavarie et al., 2013). The piscivorous ecotype also displayed a modest
level of genetic differentiation from the three other ecotypes (Harris
et al., 2015).
We focused on adult trout due to the difficulty of classifying juveniles
into ecotypes (Chavarie et al., 2013; Zimmerman et al., 2006; Zimmerman
et al., 2007) and to avoid the confounding effects of ontogenetic shifts
in morphology and diet. Of 79 fish analyzed herein, 35 piscivourous lake
trout (Ecotype 2) were previously analyzed for fatty acids by Chavarie
et al. (2016b) and 44 fish were new additions to the diet analyses
presented here. Fish were selected from collections analyzed
morphologically by Chavarie et al. (2015) to include a range of sizes
and ages within the piscivorous ecotype. For analyses invloving giant
individuals, we selected lake trout with fork lengths> 900 mm.
A left lateral full-body digital image was taken for each lake trout
caught according to the procedures in Muir et al. (2012). Measurements,
tissues, and structures were sampled to determine biological
characteristics related to life-history, including otoliths, fork
length, round weight, sex, and stage of maturity (i.e., immature,
current year spawner, or resting) (Chavarie et al., 2016 ; Chavarie et
al., 2013). A dorsal muscle sample was collected and frozen at −20ºC for
fatty acid analysis (Budge et al., 2006; Kavanagh et al., 2010; Loseto
et al., 2009) and tissue from pectoral fins was collected and preserved
in 95% ethanol for genetic analyses.