High predation on small populations: avian predation on imperiled salmonids

Ann-Marie K Osterback,Andrew O Shelton,Danielle M Frechette,Morgan H Bond,Jonathan W Moore,Scott A Shaffer,Sean A Hayes

doi:10.1890/es13-00100.1

Abstract

Generalist predators can contribute to extinction risk of imperiled prey populations even through incidental predation. Quantifying predation on small populations is important to manage their recovery, however predation is often challenging to observe directly. Recovery of prey tags at predator colonies can indirectly provide minimum estimates of predation, however overall predation rates often remain unquantifiable because an unknown proportion of tags are deposited off‐colony. Here, we estimated overall predation rates on threatened wild juvenile steelhead (Oncorhynchus mykiss) by generalist adult Western Gulls (Larus occidentalis) in six central California (USA) watersheds. We estimated predation rates by gulls from the recapture of PIT (passive integrated transponder) tags that were originally inserted into steelhead and were subsequently deposited at a Western Gull breeding colony, Año Nuevo Island (ANI). We combined three independent datasets to isolate different processes: (1) the probability a tagged steelhead was consumed during predation, (2) the probability a consumed tag was transported to ANI, and (3) the probability a transported tag was detected at ANI. Together, these datasets parameterized a hierarchical Bayesian model to quantify overall predation rates while accounting for tag loss between when prey were tagged and subsequent tag detection at ANI. Results from the model suggest that low recovery rates of PIT tags from steelhead at ANI were mostly driven by low probabilities of transportation (≤0.167) of consumed tags to ANI. Low transportation probabilities equate to high per‐capita probabilities of predation (≥0.306/yr) at the three watersheds in closest proximity to ANI, whereas predation rates were uncertain at watersheds farther from ANI due to very low transportation rates. This study provides the first overall estimate of Western Gull predation rates on threatened wild juvenile steelhead and suggests gull predation on salmonids is a larger source of mortality than was previously estimated from minimum predation rates. This study thus represents an important example of high rates of incidental predation by a generalist consumer on an imperiled prey and provides a quantitative framework to inform robust estimates of predation rates on small populations that can be applied to other systems where direct observation of predation is not feasible.

Highlights

Predation on imperiled populations may contribute to extinction risk or limit their recovery
Because previous research estimated detection probabilities for passive integrated transponder (PIT) tags at Ano Nuevo Island (ANI) (Frechette et al 2012), we used a strongly informative independent beta prior distribution for w, p(w) ; Beta(21.05, 11.65); this is the methods-of-moments estimator for w derived from the detection probabilities reported in Frechette et al (2012)
We developed a robust quantitative framework to better understand the impact of predation by an abundant generalist predator, adult Western Gulls, on populations of threatened wild juvenile steelhead in central California watersheds

Summary

Introduction

Predation on imperiled populations may contribute to extinction risk or limit their recovery. Small prey populations may be relatively buffered from predation when refuges are available or if predators switch to target more abundant prey (Holling 1959, Ackerman 2002, Twardochleb et al 2012). If generalist predators continue to prey on small populations, these prey populations may suffer local extinction (Spiller and Schoener 1998) or may experience further population decline even when predation is incidental (Vickery et al 1992). In these scenarios, there may be few prey individuals relative to the number of predators, and even incidental predation can result in high predation rates (Wittmer et al 2005). The role of predation by generalist predators can be an important factor in the dynamics of small prey populations

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Results

Conclusion