Stable isotopes and gut content show diet overlap among native and introduced piscivores in a large oligotrophic lake

Abstract

– In past dietary studies kokanee Oncorhynchus nerka were prominent in the diet of Pend Oreille Lake's large piscivores: native bull trout Salvelinus confluentus, cutthroat trout O. clarki and northern pikeminnow Ptychocheilus oregonensis, and introduced lake trout S. namaycush and Kamloops rainbow trout O. mykiss gairdneri. However, kokanee have declined to 10–20% of their former abundance. We therefore initiated this study to understand current predation demands on kokanee and diet overlap among piscivores, using gut content samples and analysis of stable nitrogen (δ15N) and carbon (δ13C) isotopes from the lake's fish and invertebrate community. In gut content samples, kokanee were the main prey item of large [i.e., ≥400 mm total length (TL)] bull and lake trout; a conclusion that was affirmed by stable isotope analysis. Rainbow trout >500 mm TL consumed mostly kokanee, thus there was a high degree of diet overlap among large bull, lake and rainbow trout. Small (i.e., <400 mm TL) rainbow and cutthroat trout diets overlapped, and were composed mostly of littoral benthic invertebrates. However, gut content and stable isotope analysis did not accord for 400–500 mm TL rainbow trout, small lake trout, and large cutthroat trout. In these instances, a linear mixing model using stable isotope results predicted kokanee consumption for each species, but no kokanee were identified in rainbow or lake trout gut content. Gut content and stable isotope analysis of native northern pikeminnow indicated a diet of mostly littoral benthic invertebrates at smaller (100–150 mm TL) lengths, with kokanee becoming more prominent in the diet of individuals >300 mm TL. Percent of kokanee in the diet of northern pikeminnow has declined from a prior study; otherwise piscivore diets have apparently remained unchanged. In this study, judgments as to the feeding of some piscvores, based on gut content alone, would be tenuous because of small sample sizes, but stable isotope analysis provided an efficient means for confirming diets.

Isotopic niches of sympatric native and exotic fish species in a Neotropical floodplain

“…Moreover, these studies have practical applications when assessing possible impacts of exotic species on native species (Clarke et al 2005, Pound et al 2011. Stable isotope analysis is becoming a very useful tool in studies that investigate the trophic ecology of native and exotic species (Cucherousset et al 2012, Jackson et al 2012.…”

Ontogenetic diet shifts in the reef fish Pseudanthias rubrizonatus from isolated populations on the North-West Shelf of Australia

“…Symbols for P. rubrizonatus size classes as in Fig. 2 trophic interactions (Clarke et al 2005, Paradis et al 2008, Wells et al 2008, Quevedo et al 2009). In the present study, gut analysis provided a description of recently ingested prey items, and the stable isotope analysis quantified diet integrated over longer periods of time.…”

Evaluation of food web and fish dietary niches in oligotrophic Lake Annecy by gut content and stable isotope analysis

“…Fishes have a very complex and key role in aquatic food webs because they may occupy multiple trophic levels, consume diverse food sources, and can feed on both pelagic and littoral food webs (Vander Zanden and Vadeboncoeur 2002, Christensen Gut content analysis (GCA) is a standard tool for identifying fish feeding ecology, trophic interactions, food web structures, and seasonal variations. It provides important taxonomic information about the food items ingested, but only over a short period (Gearing 1991), and may overlook variation between sampling periods (Clarke et al 2005). Consequently, natural dietary variability can be misinterpreted when using traditional GCA alone (Christensen and Moore 2009).…”