Stable isotope evaluation of population‐ and individual‐level diet variability in a large, oligotrophic lake with non‐native lake trout

Abstract

Non‐native piscivores can alter food web dynamics; therefore, evaluating interspecific relationships is vital for conservation and management of ecosystems with introduced fishes. Priest Lake, Idaho, supports a number of introduced species, including lake trout Salvelinus namaycush, brook trout S. fontinalis and opossum shrimp Mysis diluviana. In this study, we used stable isotopes (δ13C and δ15N) to describe the food web structure of Priest Lake and to test hypotheses about apparent patterns in lake trout growth. We found that isotopic niches of species using pelagic‐origin carbon did not overlap with those using more littoral‐origin carbon. Species using more littoral‐origin carbon, such as brook trout and westslope cutthroat trout Oncorhynchus clarki lewisi, exhibited a high degree of isotopic niche overlap and high intrapopulation variability in resource use. Although we hypothesised that lake trout would experience an ontogenetic diet shift, no such patterns were apparent in isotopic signatures. Lake trout growth rates were not associated with patterns in δ15N, indicating that variation in adult body composition may not be related to adult diet. Understanding trophic relationships at both the individual and species levels provides a more complete understanding of food webs altered by non‐native species.

Inferred Trophic Characteristics and Ecological Roles of Native and Nonnative Fishes in Odell Lake, Oregon

“…However, there was little overlap in the distributions of δ 13 C values between Bull Trout and Lake Trout, which resulted in moderately low isotopic niche overlap between these species (Table 5; Figures 6 and 7). This pattern of similar δ 15 N values but incomplete overlap among δ 13 C values has been observed in other stable isotope analyses of sympatric Bull Trout and Lake Trout (Clarke et al 2005;Meeuwig et al 2011;Ng et al 2017;Wainright et al 2021). In the present study, size-related variation in δ 13 C and δ 15 N values for Lake Trout and the size composition of sampled Bull Trout and Lake Trout can provide some context for differences in the distributions of δ 13 C. First, stable isotope data were only available for four Bull Trout, which varied in length from 358 to 746 mm; three of these individuals were ≥594 mm.…”

“…However, we assume that both Bull Trout and Lake Trout are top-level predators in Odell Lake and, once they attain sufficient size, prey primarily on fishes. This assumption is based on the well-documented ecology of these species (Scott and Crossman 1973;Behnke 2002), food habit data for these species in sympatry (Vidergar 2000;Clarke et al 2005;Guy et al 2011), and stable isotope data from this study (Figure 6) and others (Clarke et al 2005;Meeuwig et al 2011;Ng et al 2017;Wainright et al 2021) coupled with documented and generally accepted patterns of increased δ 15 N values of consumers relative to their diet (Minagawa and Wada 1984;Post 2002;Martínez del Rio et al 2009).…”

“…where δ 15 N uncorrected is the laboratory-reported δ 15 N value, B 0 is the intercept parameter from the quadratic polynomial model, B 1 is the linear effect coefficient, B 11 is the quadratic effect coefficient, and δ 13 C is the lipidnormalized δ 13 C value. Mountain Whitefish would be an appropriate taxa for evaluating isotopic overlap and interactions within the Odell Lake food web based on recommendations by Post (2002); however, we chose to use zooplankton as a baseline taxa because (1) there was no statistically significant difference between the slopes of the quadratic polynomial models for zooplankton and Mountain Whitefish (F 2, 203 = 0.34, P = 0.71) and (2) use of a putative primary consumer as a baseline is more consistent with previously published studies (e.g., Vander Zanden and Rasmussen 1999; Post 2002;Ng et al 2017). Nevertheless, all stable isotope analyses were conducted with both zooplankton and Mountain Whitefish as baseline taxa and general inferences and conclusions were not affected by the choice of baseline taxa The isotopic niche of each fish species was estimated using bivariate isotope data (δ 13 C and δ 15 N), and it was treated as a proxy for trophic niche in order to infer trophic niche variation and overlap among fish species (e.g., Jackson et al 2011;Swanson et al 2015).…”

“…Spatial variation, or δ 13 C-specific variation, in δ 15 N at the base of food webs in freshwater lakes has been documented Post 2002); consequently, baseline-corrected δ 15 N values are often calculated prior to evaluating isotopic interactions and overlap among consumers of interest Post 2002). A variety of taxa have been used to develop baselinecorrected δ 15 N values, including, but not limited to, a combination of various primary consumers (Vander Zanden and Rasmussen 1999), freshwater snails and mussels (Post 2002), amphipods (Syslo et al 2016), zooplankton (Ng et al 2017), and zooplankton and macrophytes (Heredia and Budy 2018). However, Post (2002) recommends that taxa used to develop an appropriate baseline should (1) integrate isotopic changes at a time scale similar to the consumer of interest, (2) be sampled over the same time period as the consumer of interest, and (3) encompass similar spatial variability as the consumer of interest.…”

Food Web Structure Informs Potential Causes of Bimodal Size Structure in a Top Predator

“…In a lake ecosystem, the δ 13 C is normally used to differentiate between pelagic and littoral carbon sources [23]. Characterization of trophic niches by stable isotope analysis can be used to assess areas of the food web with high levels of competition and energy bottlenecks, and to infer energy flow in the system [24,25]. Fish Lake is located in central Utah and is a popular sportfish destination supporting about 100,000 angler hours per year [26].…”