Northwest Fisheries Science Center

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Document Type: Journal Article
Center: NWFSC
Document ID: 8689
Title: Estimates of Chinook salmon consumption in Puget Sound area waters by four marine mammal predators from 1970  2015
Author: Brandon Chasco, I. C. Kaplan, E. J. Ward, Austen Thomas, Alejandro Acevedo-Gutierrez, D. P. Noren, Michael J. Ford, M. B. Hanson, J. Scordino, Steven J. Jeffries, Scott F. Pearson, Kristin N. Marshall
Publication Year: 2017
Journal: Canadian Journal of Fisheries and Aquatic Sciences
DOI: 10.1139/cjfas-2016-0203
Keywords: Chinook salmon,predation,harbor seal,Steller sea lion,California sea lion,killer whale,

Conflicts can arise when the recovery of one protected species limits the recovery of another through competition or predation.  Many of the marine mammal populations on the west coast of the United States have increased since implementation of the Marine Mammal Protection Act (MMPA) and the Endangered Species Act (ESA).   However, within Puget Sound in northwest Washington State, USA, the increased abundance of three protected pinniped species (harbor seals Phoca vitulina, California sea lions Zalophus californianus and Steller sea lions Eumetopias jubatus), as well as a population of resident killer whales Orcinus orca, may be adversely affecting the recovery of threatened Chinook salmon Oncorhynchus tshawytscha in the region.  To address the potential impacts of these marine mammals on the recovery of Chinook salmon in the Puget Sound region, we combine estimates of annual predator abundance and diet data with bioenergetics models to estimate the annual consumption of Chinook salmon.  Between 1970 and 2015, the annual biomass of Chinook salmon consumed has remained relatively constant for killer whales, ranging from 576 to 567 metric tons per year, but the consumption by pinnipeds has increased from 68 to 625 metric tons.  Consumption of individual Chinook salmon by pinnipeds has increased from 1.0 to 8.9 million (largely juveniles), while consumption by killer whales during that same period remained stable at ~84,000 individuals (almost exclusively adults).  However, uncertainty in the pinniped abundance exists, and one recent analysis suggests a 4% annual decline in harbor seal abundance since 1999 which would result in them consuming half as many Chinook salmon in 2015.  Because the predators differ in the age of salmon consumed, we transformed the smolt consumption from pinnipeds into adult equivalents, accounting for the survival and variable age at maturity between juvenile and adult stages.  We found that potential consumption of adult equivalents by pinnipeds in 2015 to be roughly double that of resident killer whales.  Our results suggest that increases in pinniped abundance could be a large potential source of predation that is limiting the recovery of threatened Chinook salmon.  Additionally, because the pinnipeds focus on consuming juvenile Chinook salmon before they return as adults, these predators may also be limiting the recovery of endangered resident killer whales through competition.  As more protected species respond positively to recovery efforts, managers will be required to evaluate the trade-offs from these efforts with the unintended ecosystem consequences of predation and competition on other protected species. 

Theme: Ecosystem approach to improve management of marine resources
Foci: Provide scientific support for the implementation of ecosystem-based management
Characterize ecological interactions (e.g. predation, competition, parasitism, disease, etc.) within and among species.