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

Benjamin W Nelson,William A Walker,Austen C Thomas,Amelia Louden,Steven J Jeffries,Scott F Pearson,Madelyn R Voelker,Alejandro Acevedo-Gutiérrez,Iris M Kemp,Monique M Lance,Joseph H Anderson

doi:10.1139/cjfas-2020-0300

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 are often limited. We used new predator diet data and simulation modeling to update our understanding of seal predation on juvenile Chinook (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch) 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.

Highlights

Management conflicts between protected marine species are increasing, and successful resolution requires identification of competing objectives and evaluating the ecological, social, and economic trade-offs associated with various management scenarios (Marshall et al 2016; Pinkerton et al 2019)
We show estimates of ocean age-0 fish consumed by harbor seals during 2016–2018, where monthly prey size is calculated from (1) data from nearshore scientific surveys and (2) estimates of prey length from from otoliths extracted from seal scats
The estimated percentage of juvenile Chinook salmon in the harbor seal diet during the spring and summer (April–August) was 2.2%, and 1.8% for coho salmon

Summary

Introduction

Management conflicts between protected marine species are increasing, and successful resolution requires identification of competing objectives and evaluating the ecological, social, and economic trade-offs associated with various management scenarios (Marshall et al 2016; Pinkerton et al 2019). These conflicts are made considerably more difficult when both predator and their prey are protected species (Marshall et al 2016; Samhouri et al 2017).

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Discussion

Conclusion