Onset of climate-change impact on the renowned Oncorhynchus metapopulation of Yellowstone Lake revealed by Leslie modeling of annual gill-net catches

The native Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri; YCT) of Yellowstone Lake, Yellowstone National Park, Wyoming, USA, comprise a metapopulation that inhabits an ecologically simple and uniquely pristine environment. A recent study Bayesian fit an age-structured Leslie model that had a local climate index (total annual mean-daily air temperatures >0°C at the lake's north shore) as its only driver (regulating age-0 YCT survival) to data for a key YCT spawning stock (median fish age, 5 years) from 1977 to 1992, before predation from an illegally introduced and growing lake trout (Salvelinus namaycush) population (1994 discovery) measurably affected YCT population dynamics. Results showed the new climate regime that began in the late 1970s had an overarching effect and largely explained declining spawning runs, although a growing predation effect became evident near lake trout discovery and quickly became the major YCT population driver. In the present study, that model was likewise fitted to estimated abundances of age-5 YCT (~75% mature) that had been annually (also 1977–1992) gill-netted from Yellowstone Lake and thus characterized the metapopulation. Results showed age-0 YCT survival declined when a climate index of 1440 was reached and exceeded. Metapopulation recovery to historically high levels requires that the climate index decline to and remain near or below 1440 and—as shown by another recent study—that the lake trout population be reduced to its mid-1990s levels. The most recent time of evident YCT metapopulation persistence at historically high levels and broadest spatial extent occurred under those simultaneous conditions.