Food Web Characteristics of an Alpine Lake in Relation to Cutthroat Trout Reintroduction

Efforts are currently underway to reintroduce Lahontan cutthroat trout Oncorhynchus clarkii henshawi (cutthroat trout) into Fallen Leaf Lake (California). This lake is occupied by native salmonids, suckers, minnows, and zooplankton, as well as nonnative sport fishes, crayfish, and mysids. Previous research has shown that lake trout Salvelinus namaycush consume large numbers of cutthroat trout in Fallen Leaf Lake; however, little is known about interactions between cutthroat trout and other species in Fallen Leaf Lake. Therefore, the aim of this study was to evaluate characteristics of the Fallen Leaf Lake food web to identify potential competitive and predator-prey interactions between cutthroat trout and other native and nonnative species. A combination of stomach content analysis and stable isotope analysis was used to examine dietary and trophic interactions among species using samples collected during 2009 and 2010. The diet of cutthroat trout (N = 14) was most similar to that of mountain whitefish Prosopium williamsoni (N = 38; Shoener’s index = 0.61) and lake trout (N = 189; Shoener’s index = 0.58); which all consumed mysids, insects, and crayfish. The diet of cutthroat trout was less similar to that of brown trout Salmo trutta (N = 18; Shoener’s index = 0.56), which primarily consumed fishes, and rainbow trout Oncorhynchus mykiss (N = 18; Shoener’s index = 0.49), which primarily consumed crayfish. Cutthroat trout were present in the diet of brown trout. Cutthroat trout were identified in the diet of one lake trout. However, the diet of lake trout was comprised of about 19% fish (by dry weight); many of which could not be identified due to stage of digestion. Stable isotope data indicate that lake trout and brown trout are top-level predators in Fallen Leaf Lake. There was a positive relationship between lake trout length and relative trophic position, which is consistent with previous studies on Fallen Leaf Lake. Lake trout, brown trout, and mountain whitefish exhibited a relatively large degree of individual variability in δ¹³C, which, when combined with stomach content analysis, suggests that these species are generalist or opportunistic predators. Overall, these data show that brown trout will prey on reintroduced cutthroat trout in Fallen Leaf Lake; in addition to substantial predation by lake trout shown in this and previous studies. Additionally, the generalist feeding behavior exhibited by lake trout, mountain whitefish, and cutthroat trout may allow these species to partition prey resources, despite their relatively high degree of dietary overlap.