Evaluating Target Levels of American White Pelican Abundance to Achieve Management Objectives for Adfluvial Yellowstone Cutthroat Trout

Recent declines in abundance of Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri in the upper Blackfoot River drainage in Idaho coincided with establishment of a nearby nesting colony and increased abundance of American white pelicans Pelecanus erythrorhynchos (hereafter, pelicans). Published pelican predation rates have been estimated to be as high as 70% on Yellowstone Cutthroat Trout in the Blackfoot River. Consequently, the Idaho Department of Fish and Game established management objectives for pelicans of less than 350 breeding pairs or 700 adults. However, a thorough assessment has not been conducted to evaluate if the pelican abundance objective would reduce predation to an extent that would allow Yellowstone Cutthroat Trout to recover to levels that meet minimum objectives for abundance. A Bayesian state-space model that integrated survival and abundance data was used in this study to evaluate the effects of local pelican abundance on survival, abundance, and population growth rate of Yellowstone Cutthroat Trout. Survival of both juvenile and adfluvial adult Yellowstone Cutthroat Trout decreased as pelican abundance increased, and there was a positive relationship between survival of both life stages and population growth rate. If the current management objective for abundance of 700 breeding pelicans was met, the probability of meeting or exceeding the Yellowstone Cutthroat Trout objective was 0.44 and declined substantially as simulated pelican abundance increased beyond 700. Results of this study suggest that if pelican abundance can be maintained at or below the current management objective, there is a reasonable chance of meeting the management objective for Yellowstone Cutthroat Trout.

Automated summary

The recent decline in Yellowstone Cutthroat Trout abundance in the upper Blackfoot River drainage in Idaho is linked to the increase in American white pelican population, with high predation rates observed. A study using a Bayesian state-space model showed that survival of the trout decreased with increasing pelican abundance, and meeting the current pelican management objective could lead to a reasonable chance of achieving the management goal for the trout population.