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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.

We examined the role of individual size and origin (wild versus hatchery) to predation risk and marine survival for threatened juvenile steelhead (Oncorhynchus mykiss) in a coastal California watershed. In this study, we found that individual size and origin were strongly associated with increased predation risk of steelhead by a generalist avian predator (western gull, Larus occidentalis) and associated with survival to reproduction by tracking the fate of juvenile steelhead tagged with passive integrated transponder (PIT) tags. Across six cohorts (2005)(2006)(2007)(2008)(2009)(2010), larger steelhead (>170 mm fork length (FL)) experienced marine survival rates at least 60 times higher than the smallest individuals. Predation risk by western gulls was highest for intermediate-sized fish , which was at least ten times higher than the predation risk of the smallest individuals and four times higher than the predation risk of the largest individuals. Wild steelhead experienced both higher predation risk and higher survival rates than hatchery fish of the same size. Although gulls disproportionately remove intermediate-sized wild steelhead from the population, they also remove large wild individuals that may otherwise experience the highest adult return rates. Instead of focusing on population size alone, conservation measures could also be guided towards the recovery of larger and wild individuals, whose survival is paramount for population recovery.Résumé : Nous avons examiné le lien entre la taille et l'origine (sauvage ou d'écloserie) des individus, d'une part, et le risque de prédation et la survie en mer, d'autre part, pour des truites arc-en-ciel (Oncorhynchus mykiss) juvéniles menacées dans un bassin versant de la côte californienne. En suivant des truites arc-en-ciel juvéniles dotées de radioétiquettes passives intégrées (PIT), nous avons constaté que la taille et l'origine des individus étaient fortement associées à un risque de prédation accru des truites arc-en-ciel par un prédateur aviaire généraliste (le goéland d'Audubon, Larus occidentalis) et étaient associées à la survie jusqu'à la reproduction. À l'échelle de six cohortes (2005)(2006)(2007)(2008)(2009)(2010), les plus grandes truites (longueur à la fourche (LF) >170 mm) présentaient des taux de survie en mer au moins 60 fois plus élevés que les truites les plus petites. Le risque de prédation par des goélands d'Audubon était le plus élevé pour les individus de taille intermédiaire , ce risque étant au moins 10 fois plus élevé que pour les individus les plus petits et quatre fois plus élevé que pour les individus les plus grands. Les truites arc-en-ciel sauvages présentaient un risque de prédation et des taux de survie plus élevés que les poissons issus d'écloserie de même taille. Si les goélands extraient une quantité disproportionnée de truites arc-en-ciel sauvages de taille intermédiaire de la population, ils en retirent également de grands individus sauvages qui, autrement, pourraient présenter les taux de retour d'adultes les plus élevés. Plut...