North Pacific Anadromous Fish Commission

Technical Report 23

Table of Contents

Trends in Adult Return Abundance of Idaho Salmon and Steelhead Relative to Pink Salmon Abundance the Year of Ocean Entry

Authors:
John Cassinelli and Josh McCormick

Abstract:
The Snake River basin contains four runs of ESA listed anadromous salmonids—spring/summer Chinook salmon (Oncorhynchus tshawytscha), fall Chinook salmon, sockeye salmon (O. nerka), and steelhead trout (O. mykiss)—as well as several large hatchery mitigation programs. Within a given juvenile outmigration year, millions of juvenile salmon and steelhead from the Snake River basin enter the Pacific Ocean, where they will typically spend one to three years before returning to freshwater and migrating back to their natal waters in Idaho. While year- and species-specific adult return abundance of Idaho’s anadromous salmonids is highly variable, in the early 2010s managers began to notice some distinct every-other-year patterns in adult returns across many of Idaho’s stocks. Concurrently, commercial pink salmon (O. gorbuscha) harvest in the North Pacific Ocean surpassed 400 M fish in 2009 and odd-year abundance and commercial harvest has remained high since that time. We developed a state-space model to estimate abundance of Idaho’s returning salmon and steelhead and embedded a regression model to estimate the effect of pink salmon abundance (using Alaska commercial harvest as a surrogate) during the year of ocean entry, on the intrinsic growth rate. For all species of pacific salmon and steelhead returning to Idaho, pink salmon abundance had a strong negative effect on intrinsic growth rate and the negative effect has increased through time. Additional work understanding the mechanism for increasing odd-year pink salmon abundance effects on Columbia basin anadromous salmonid ocean survival is needed, especially given the increasing impacts in a changing ocean climate.

DOI:
https://doi.org/10.23849/npafctr23/lk99-0p

Citation

Cassinelli, J., and J. McCormick.  2024.  Trends in adult return abundance of Idaho salmon and steelhead relative to Pink salmon abundance the year of ocean entry.  N. Pac. Anadr. Fish Comm. Tech. Rep. 23: 13–15.  https://doi.org/10.23849/npafctr23/lk99-0p