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.