Variation in predator diet and prey size affects perceived impacts to salmon species of high conservation concern

Abstract

Management of protected species is difficult when objectives include the recovery of both predator and its prey. Ideally, identifying trade-offs between competing objectives involves evaluating management alternatives with a quantitative model that integrates information on both species, but data is often limited. We used new predator diet data and simulation modeling to update our understanding of seal predation on juvenile Chinook and coho salmon in the Puget Sound. Under prey size assumptions used by previous studies, estimates suggest predation could be a significant source of mortality for Chinook salmon (mean: 37% of ocean age-0 juveniles), but varied considerably among years (range of median estimates: 22-49%). However, when we estimated prey size from otoliths recovered from seal scats, the impact (numbers of fish consumed) decreased by 71%. Predation on coho salmon was estimated to be relatively low under both scenarios (6-8% of ocean age-0 juveniles) with a 21% decrease in consumption using otolith-derived prey size. Our analysis highlights the importance of updating model inputs and re-evaluating assumptions of multi-species models used for ecosystem-based fisheries management.

Quantifying impacts of harbor seal Phoca vitulina predation on juvenile Coho Salmon in the Strait of Georgia, British Columbia

“…This is somewhat higher than previous estimates of seal predation on Coho Salmon in Puget Sound (Nelson et al. 2021), which may reflect the significantly higher densities of predators in the Strait of Georgia (Nelson et al 2019b). Our analysis estimated an increase in total instantaneous mortality of 2.0 year −1 for juvenile Coho Salmon, which suggests that the increase in the magnitude of seal predation between the 1970s and the present day could explain most of the increase in total marine mortality throughout the same time period (Table A.4).…”

“…Our study complements previous work investigating the impacts of harbor seal predation on Coho Salmon in the Salish Sea (Thomas et al 2017;Nelson et al 2021) and is the first to quantify possible impacts of pinniped predation on juvenile Coho Salmon in the Strait of Georgia at an ecosystem scale. The predation and mortality estimates presented here are potentially useful for quantifying F I G U R E 4 Estimated annual total instantaneous mortality from harbor seal predation during the first year at sea for Coho Salmon (top panel) and the proportion of the juvenile cohort that was lost to seal predation (bottom panel) annually in the Strait of Georgia from 1970 to 2016.…”

“…Even though juvenile Coho Salmon never exceeded 5% of the overall diet in any given month, we estimated that harbor seals consumed an average of 46-59% of the juvenile Coho Salmon entering the ocean during the spring in the first year at sea. This is somewhat higher than previous estimates of seal predation on Coho Salmon in Puget Sound (Nelson et al 2021), which may reflect the significantly higher densities of predators in the Strait of Georgia (Nelson et al 2019b). Our analysis estimated an increase in total instantaneous mortality of 2.0 year −1 for juvenile Coho Salmon, which suggests that the increase in F I G U R E 5 Weekly instantaneous mortality rate (M) of Coho Salmon from harbor seal predation (purple) and from other sources (teal) during the first year at sea.…”

First Diet Description of the Harbor Seal (Phoca Vitulina) in the Northwest Corner of the Olympic Peninsula, Washington State

“…Numerous studies have recently been conducted to understand the potential impacts of abundant Harbor Seals on salmon in the region (for example: Chasco and others 2017; Thomas and others 2017; Nelson and others 2021), all of which rely on knowing in detail the diet of the predator. Harbor Seal diet in Washington State has been described many times in the last century (for example: Scheffer and Sperry 1931; Everitt and others 1981; Thomas and others 2011; Lance and others 2012).…”

“…Unfortunately, the abundance of Harbor Seals has not been estimated for the Olympic Peninsula, but at least a few hundred appear to be present (Akmajian and others 2021). We also have no information on the age class of salmon consumed by Harbor Seals, which is important to determine the effect on prey populations (Nelson and others 2021). Our results underscore the need for further work in the Olympic Peninsula to fully examine the year-around Harbor Seal abundance and consumption of salmon in the region, including age classes consumed.…”