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

Abstract

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.

Informing Management of Henrys Lake, Idaho, using an IntegratedCatch‐at‐AgeModel

“…Density‐dependent effects on age‐0 survival have also been observed in populations of resident and adfluvial wild Yellowstone Cutthroat Trout populations in the region (McCormick and High 2020; McCormick et al. 2021a). In addition, McCarrick (2021) observed a positive relationship between stocking density and growth of adult Yellowstone Cutthroat Trout in Henrys Lake.…”

“…Consequently, integrated models can result in larger prediction uncertainty (e.g., McCormick et al. 2021a, 2021b) than more traditional methods that may ignore or fail to fully account for uncertainty—particularly process uncertainty (Maunder and Punt 2013). Underestimation of prediction uncertainty can result in misleading low quantification of risk of alternative management or harvest scenarios (Butterworth et al.…”

“…Integrated models are acknowledged as providing more accurate propagation of uncertainty in the future state of nature (Aeberhard et al 2018) compared to deterministic models (which contain no uncertainty) or two-stage simulation models, where parameters and stochasticity are supplied externally (e.g., Leslie matrix projection models) to the model (Maunder and Punt 2013). Consequently, integrated models can result in larger prediction uncertainty (e.g., McCormick et al 2021aMcCormick et al , 2021b than more traditional methods that may ignore or fail to fully account for uncertainty-particularly process uncertainty (Maunder and Punt 2013). Underestimation of prediction uncertainty can result in misleading low quantification of risk of alternative management or harvest scenarios (Butterworth et al 2010).…”

“…2018; McCormick et al. 2021a). Growth and natural mortality are most often influenced by environmental conditions, habitat, or the presence or density of other fish in the population or the assemblage (Pauly 1980; Lorenzen and Enberg 2002; Weisberg et al.…”

“…For instance, managers can directly influence recruitment by stocking fish in some scenarios (Trushenski et al 2010). Recruitment in other fisheries may be regulated by spawner abundance, environmental conditions, and(or) density-dependent factors associated with habitat quality or quantity (Myers et al 1999;Munch et al 2018;McCormick et al 2021a). Growth and natural mortality are most often influenced by environmental conditions, habitat, or the presence or density of other fish in the population or the assemblage (Pauly 1980;Lorenzen and Enberg 2002;Weisberg et al 2010).…”