New climate regime started and further shaped the historic Yellowstone Lake cutthroat trout population decline commonly attributed entirely to nonnative lake trout predation

Abstract

The storied Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri; YCT) population of Yellowstone Lake, Yellowstone National Park, Wyoming, USA, has shown a profound and mostly continuous decline since the 1970s despite pristine habitat and vanishing angler harvests. An age-structured Leslie model that had a broad climate index as its only driver (regulating age-0 YCT survival) was Bayesian-fitted to data for a key YCT spawning stock from 1977 to 1992, arguably before predation from an introduced lake trout (Salvelinus namaycush) population (1994 discovery) measurably affected YCT population dynamics. The middle 50% of model fits encompassed 12 of 14 observed spawning populations and nominally excluded the others. Forecasting to 2007 (the final empirical data year) showed the new climate regime largely explained declining YCT numbers, but a growing predation effect became evident soon after lake trout discovery and quickly became the major YCT population driver. The climate effect may have involved the fatal blockage of out-emigrating YCT fry by natural alluvial deposits at spawning tributary mouths in warm, dry years. The previously reported Yellowstone Lake ecosystem shift suggested by the YCT population decline actually began with the 1977 North Pacific Basin climate shift and was only later enhanced by lake trout predation. This study showed identifying and accounting for the overarching population drivers is important to structuring statistical models intended to detect and assess the effects of new and emerging population impacts across historic data sets. Management actions intended to protect these YCT must consider climate effects and probable future climates.