Abstract

AbstractAccurate assessment of specific mortality factors is vital to prioritize recovery actions for threatened and endangered species. For decades, tag recovery methods have been used to estimate fish mortality due to avian predation. Predation probabilities derived from fish tag recoveries on piscivorous waterbird colonies typically reflect minimum estimates of predation due to an unknown and unaccounted‐for fraction of tags that are consumed but not deposited on‐colony (i.e., deposition probability). We applied an integrated tag recovery modeling approach in a Bayesian context to estimate predation probabilities that accounted for predator‐specific tag detection and deposition probabilities in a multiple‐predator system. Studies of PIT tag deposition were conducted across three bird species nesting at seven different colonies in the Columbia River basin, USA. Tag deposition probabilities differed significantly among predator species (Caspian terns Hydroprogne caspia: deposition probability = 0.71, 95% credible interval [CRI] = 0.51–0.89; double‐crested cormorants Phalacrocorax auritus: 0.51, 95% CRI = 0.34–0.70; California gulls Larus californicus: 0.15, 95% CRI = 0.11–0.21) but showed little variation across trials within a species or across years. Data from a 6‐year study (2008–2013) of PIT‐tagged juvenile Snake River steelhead Oncorhynchus mykiss (listed as threatened under the Endangered Species Act) indicated that colony‐specific predation probabilities ranged from less than 0.01 to 0.17 and varied by predator species, colony location, and year. Integrating the predator‐specific deposition probabilities increased the predation probabilities by a factor of approximately 1.4 for Caspian terns, 2.0 for double‐crested cormorants, and 6.7 for California gulls compared with traditional minimum predation rate methods, which do not account for deposition probabilities. Results supported previous findings on the high predation impacts from strictly piscivorous waterbirds nesting in the Columbia River estuary (i.e., terns and cormorants), but our findings also revealed greater impacts of a generalist predator species (i.e., California gulls) than were previously documented. Approaches used in this study allow for direct comparisons among multiple fish mortality factors and considerably improve the reliability of tag recovery models for estimating predation probabilities in multiple‐predator systems.Received August 1, 2014; accepted November 11, 2014

Highlights

  • U.S Geological Survey, Oregon Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, Oregon 97331, USA

  • We investigated a 6-year data set (2008– 2013) that encompassed the current period of survival standards for Endangered Species Act (ESA)-listed Snake River steelhead in the Federal Columbia River Power System (NOAA 2008)

  • Approaches developed in this study considerably increase the applicability of tag recovery models to the estimation of avian predation probabilities in multiple-predator systems

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Summary

Introduction

We applied an integrated tag recovery modeling approach in a Bayesian context to estimate predation probabilities that accounted for predator-specific tag detection and deposition probabilities in a multiple-predator system. Avian predation on juvenile salmonids Oncorhynchus spp. in the Columbia River basin is considered a limiting factor in the recovery of multiple salmonid populations that are listed as threatened or endangered under the U.S Endangered Species Act (ESA; NOAA 2008). Capture–mark–recovery studies using PIT tags in the Columbia River basin have documented minimum predation rates for numerous piscivorous bird colonies over the past decade (Collis et al 2001; Ryan et al 2003; Antolos et al 2005; Evans et al 2012; Sebring et al 2013). Measurement and integration of predator-specific deposition probabilities are required to produce unbiased estimates of avian predation probabilities and to allow balanced comparisons among multiple sources of fish mortality

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