Implications of Mark‐Selective Fishing for Ocean Harvests and Escapements of Sacramento River Fall Chinook Salmon Populations

Brian J. Pyper,Steven P. Cramer,Randolph P. Ericksen,Richard M. Sitts

doi:10.1080/19425120.2012.679575

Abstract

AbstractThe need to protect imperiled salmon stocks along the Pacific coast of North America has led to an increasing use of mark‐selective fisheries (MSFs) as a management strategy to reduce harvest mortality of wild salmon while allowing harvest of abundant hatchery salmon. However, MSFs remain untested in ocean fisheries for Chinook salmon Oncorhynchus tshawytscha off the coasts of California and Oregon, where hatchery fish have been estimated to compose the majority of Chinook salmon but where harvests have been restricted to protect several imperiled stocks. We developed a quantitative framework based on conventional cohort models to examine how aggregate ocean harvest and in‐river escapement of Sacramento River fall‐run Chinook salmon, the numerically dominant stock in the region, would have differed under MSF scenarios compared with the historic, traditional fishery. At historic contact rates (fishing effort) for 1988–2007, we estimated that annual in‐river escapement of natural‐origin fish would have increased by 119% on average under MSF scenarios, while reductions in harvest would have been inversely proportional to the fraction of hatchery‐origin fish. During the more recent period of constrained fishing (2001–2007), we estimated MSF outcomes for a range of plausible contact rates (40–60% of age‐4 fish) and hatchery fractions (40–80% of Chinook salmon). The combination of these factors determined the magnitude of estimated harvest reductions or gains under MSFs, with total MSF harvest (2001–2007) ranging from 46% lower to 48% higher than historic harvest. Increases in total escapement of natural‐origin fish (2001–2007) under MSFs ranged from 24% to 48% depending on the contact rate. Comparisons between the traditional fishery and simulated MSF outcomes were robust to a wide range of cohort parameter values, suggesting that our aggregate results provide useful insights into potential MSF outcomes and the effects of key uncertainties.Received November 1, 2010; accepted December 19, 2011

Highlights

The need to protect imperiled salmon stocks along the Pacific coast of North America has led to an increasing use of mark-selective fisheries (MSFs) as a management strategy to reduce harvest mortality of wild salmon while allowing harvest of abundant hatchery salmon
We found that simulated MSF harvest and escapement were reasonably robust to all but a few key parameters, suggesting that the general outcomes of comparisons between the traditional fishery and MSFs follow straightforward and intuitive relationships that should be applicable to SRFC populations
Relative changes in harvest will be proportional to the number of marked hatchery adults that are available to the MSF and the relative fishing effort directed at those fish (Hoffman and Patillo 2008), while changes in escapement of natural fish will depend on the indirect MSF mortality on unmarked fish—their relative contact rates and subsequent release mortality (Lawson and Sampson 1996)

Summary

Introduction

The need to protect imperiled salmon stocks along the Pacific coast of North America has led to an increasing use of mark-selective fisheries (MSFs) as a management strategy to reduce harvest mortality of wild salmon while allowing harvest of abundant hatchery salmon. The need to protect imperiled salmon stocks along the Pacific coast of North America (Good et al 2007) has led to an increasing use of mark-selective fisheries (MSFs) as a management strategy for reducing harvest mortality on threatened or endangered wild salmon runs while allowing for harvest of abundant hatchery salmon (Hoffman and Patillo 2008; PSC 2009). The use of mass marking in combination with MSFs has not been tested or formally proposed for Chinook salmon in the extensive ocean fisheries of California and southern Oregon (Figure 1), several factors suggest that the use of MSFs would be beneficial These mixed-stock fisheries are dominated by Sacramento River fall-run Chinook salmon (SRFC) of California’s Central Valley (CV; Winans et al 2001; O’Farrell et al 2008). Beginning in 2007, SRFC experienced dramatic declines in abundance (O’Farrell et al 2008), raising concerns over the long-term viability of SRFC populations and the ocean fisheries they support

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