High predation on small populations: avian predation on imperiled salmonids

Abstract

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.

Predation by American White Pelicans and Double‐Crested Cormorants on Catchable‐Sized Hatchery Rainbow Trout in Select Idaho Lentic Waters

“…Our average tag recovery efficiency of 21% is slightly higher than several recent avian predation studies with similar direct-feeding study designs, all of which found that recovery of tags fed directly to birds was less than 10% (Osterback et al 2013;Scoppettone et al 2014;Teuscher et al 2015). Hostetter et al (2015) directly fed PIT-tagged hatchery trout to Caspian terns Hydroprogne caspia, doublecrested cormorants, and California gulls Larus californicus and found tag deposition rates on nearby colonies of 71, 51, and 15%, respectively, but most of their feeding trials were conducted on birds while they were on or immediately adjacent to the colonies, which likely elevated their tag deposition rates greatly.…”

“…Declines in avian predation rates related to distance from colonies have been previously demonstrated (e.g., Fasola and Bogliani 1990;Osterback et al 2013) and would be expected for birds such as adult pelicans that rear chicks with high energy needs and that have high energy demands of their own. The highest observed pelican predation rates in this study were usually at waters within 100 km of the nearest colony except at CJ Strike Reservoir, which was over 200 km from the nearest colony yet still received relatively heavy predation pressure by pelicans in some years.…”

“…Consequently, PIT tag recoveries using this methodology only provide minimum predation estimates since not all tags that are consumed by birds are deposited at and recovered from the colony. We used an updated modification to this approach that incorporates off-colony deposition into the predation estimates (Osterback et al 2013;Scoppettone et al 2014;Hostetter et al 2015;Teuscher et al 2015), thereby producing estimates of total predation (rather than minimum predation) by pelicans. The primary objective of this study was to estimate predation rates by American white pelicans on catchable-sized (i.e.,~250 mm TL) hatchery Rainbow Trout O. mykiss stocked in several southern Idaho reservoirs and community ponds to gauge their general impact on hatchery-supported trout fisheries in southern Idaho.…”

Linking individual size and wild and hatchery ancestry to survival and predation risk of threatened steelhead (Oncorhynchus mykiss)

“…Although previous work in Scott Creek identified or predicted characteristics such as size and origin that were associated with survival of juvenile salmonids Satterthwaite et al 2012;Sogard et al 2012) and documented high rates of western gull predation on these same populations (>30%; Osterback et al 2013), it was unknown whether gulls consumed individuals that would otherwise experience high survival rates. The results from this study provide the first empirical evidence that western gulls are consuming predominately wild steelhead, including many larger wild individuals that are critical to the persistence of the population.…”

Severe mortality of a population of threatened Agassiz’s desert tortoises: the American badger as a potential predator

“…Although populations may occasionally be able to rebound from temporary periods of elevated mortality (Germano & Joyner 1988, Zylstra et al 2013, increasing intensity, frequency, and duration of mortality events, coupled with habitat loss and fragmentation and a more physiologically stressful climate (Zylstra et al 2013, Lovich et al 2014 pose serious threats to the persistence of tortoise populations (Brook et al 2008). Furthermore, as tortoise populations continue to decline, stochastic predation events on small populations may have an increasingly important effect on tortoise persistence (Medica et al 2012, Osterback et al 2013. …”