Predator-prey interactions between harbor seals and migrating steelhead trout smolts revealed by acoustic telemetry

Abstract

Changes in the Puget Sound ecosystem over the past 3 decades include increases in harbor seal (Phoca vitulina) abundance and declines in many of their preferred prey species. Harbor seals were outfitted with acoustic telemetry receivers and GPS tags to investigate spatial and temporal interactions with steelhead trout Oncorhynchus mykiss smolts implanted with acoustic transmitters. A total of 6846 tag detections from 44 different steelhead trout smolts (from an initial group of 246 smolts released into 2 rivers) were recorded by the 11 recovered sealmounted receivers. Central Puget Sound seal receivers detected a greater proportion of smolts surviving to the vicinity of the haul-out locations (29 of 51; 58%) than Admiralty Inlet seal receivers (7 of 50; 14%; p < 0.001). Detection data suggest that none of the tagged smolts were consumed by the 11 monitored seals. Nine smolts were likely consumed by non-tagged harbor seals based partly on detections of stationary tags at the seal capture haul-outs, although tag deposition by other predators cannot be ruled out. Smolts implanted with continuously pinging tags and smolts implanted with tags that were silent for the first 10 d after release were detected in similar proportions leaving Puget Sound (95% CI for the difference between proportions: −0.105 to 0.077) and stationary at harbor seal haul-outs (95% CI: −0.073 to 0.080). This study suggests that harbor seals contribute to mortality of migrating steelhead smolts, and we hypothesize that documented changes in the Puget Sound ecosystem may currently put steelhead smolts at greater risk of predation by harbor seals and possibly other predators.

Declining patterns of Pacific Northwest steelhead trout (Oncorhynchus mykiss) adult abundance and smolt survival in the ocean

“…Recent research in inland seas, including Puget Sound and Georgia Basin, suggests that pinnipeds such as harbor seals (Phoca vitulina) contribute to the mortality of migrating smolts, including steelhead (Berejikian et al 2016) and other salmon (Thomas 2015). Puget Sound steelhead experience high levels of mortality in the Sound during their migration to the ocean despite relatively short migration distances, mortality rates that could not be sustained and have any adults return to spawn (Moore et al 2010b(Moore et al , 2015.…”

“…However, steelhead populations appear to have declined in abundance over time throughout much of the southern part of their range (Busby et al 1996), and there are specific concerns about the viability of populations in various regions, especially Puget Sound (NMFS 2006(NMFS , 2007Berejikian et al 2016;Ford 2011;Gayeski et al 2011;Moore et al 2015).…”

A mathematical model of predator-prey interaction between seal-herring and steelhead trout

“…The population of Steelhead Trout has decreased during migrating through Puget Sound to the Pacific Ocean [6] . The mortality rate of steelhead is high at this time of migration [8], while the abundance of seals increased significantly [9].…”

“…There are various studies related to marine mammals and fish [10,11]. Some studies only discuss the overview of predator-prey interactions between seals and fisheries [12,6]. The study of the predator-prey interactions between marine mammals and fisheries with a mathematical model is limited.…”

“…At this time, seals eat other prey [3]. It can be seen in the Salish Sea when the abundance of demersal decreased, predation of salmon is done by seals [6,7]. Salmon predation occurs when salmon migrate from freshwater to the sea, and this salmon is steelhead trout species.…”