Ocean harvest of Central Valley Chinook salmon: a threat to viability of California’s wild salmon?

Off the coast of California and southern Oregon, Chinook salmon are subjected to a uniquely intense commercial and recreational fishery. The fishery relies upon the availability of abundant, hatchery-produced Central Valley fall-run Chinook. However, as currently managed, the fishery provides minimal protection for natural-origin (wild) fall-run Chinook and for winter-run and spring-run Chinook that are listed under the federal Endangered Species Act (ESA), especially fish maturing at age-4 or older. Variation in age-at-maturity is essential for the viability of all runs of Chinook salmon because it buffers populations from short-term environmental changes in freshwater habitats and from catastrophic events. The age-structure of Central Valley Chinook salmon populations is severely truncated. Though multiple factors may have contributed to this age-structure, California’s ongoing and intensive ‘mixed-stock’ ocean fishery almost certainly exerts strong selective pressure against delayed maturity among Chinook salmon. Modernizing California’s harvest and hatchery management to reduce those impacts would substantially enhance viability of winter-run and spring-run Chinook, and help to avoid a potential listing of Central Valley fall-run Chinook. 


A comparison of ocean Chinook salmon harvest north of Cape Falcon, Oregon, as reported by the Pacific Salmon Commission (PSC), to harvest off the coast of California, as reported by the Pacific Fisheries Management Council (PFMC) yields the following findings:

• Annual commercial troll harvest of Chinook salmon off the coast of California (1996-2007 and 2010-2019) has averaged approximately 250,000 fish.   For roughly the same period of years, California commercial harvest of Chinook salmon is comparable to commercial troll harvest of Chinook salmon in southeast Alaska and is greater than reported commercial troll harvest in British Columbia, Washington, or Oregon.

• Over the same years, annual recreational harvest of Chinook salmon off the coast of California has averaged 99,000 fish. This harvest is consistently larger than recreational Chinook harvest in southeast Alaska, British Columbia, Washington, or Oregon.

• Ocean fishery exploitation rates, the fraction of adults that would have spawned but instead suffer mortality due to commercial or recreational ocean fisheries, for Sacramento River fall-run Chinook averaged 0.50 — for every fish that survives and returns to spawn, one is lost to fishery-related mortality. This ocean fishery exploitation rate is higher than values reported for Klamath basin fall-run Chinook, higher than the ocean fishery exploitation rate for the Alaska spring-run indicator stock, and higher than for nearly all index fisheries reported for Alaska, British Columbia, Washington, and Oregon.  Only one indexed population exhibited a consistently higher ocean exploitation rate than Sacramento River basin fall-run Chinook salmon; however, that stock is produced by a hatchery specifically to provide an isolated, intensive recreational fishery within Puget Sound.
• Total exploitation rates (including both ocean and inland harvest) for Chinook salmon in Alaska, British Colombia, and Washington are in many cases comparable to those reported for Sacramento basin fall-run Chinook.  But the balance between ‘mixed-stock’ ocean fisheries and stock-specific terminal harvest is a key factor that distinguishes California’s management of Chinook salmon harvest from practices in the Pacific Northwest.

The  PFMC  works closely with the National Marine Fisheries Service (NMFS) to manage ocean harvest of Central Valley-origin Chinook salmon.  Although reasons are unclear, NMFS allows ocean harvest of Central Valley Chinook salmon in a manner that appears out-of-step with others that it manages on the West Coast. California’s ocean salmon harvest imposes a high cost on the viability of Central Valley salmon and threatens the continuing viability of the ESA-listed salmon stocks.  While harvest can be compatible with protection of the ESA stocks, the prevailing management of California’s ocean salmon harvest appears to be detrimental to natural-origin and ESA-listed Central Valley Chinook salmon in at least four ways.

Ocean harvest impacts on age-at-maturity

 

Life-history diversity is a key indicator of population viability for Chinook salmon.  Juvenile salmon rearing in different freshwater habitats for variable periods of time will enter the ocean at a variety of sizes and times, providing for greater resiliency to variable environmental conditions.  The concept of spreading risk among different juvenile rearing and migration strategies has been termed the ‘portfolio effect’.

  

Chinook salmon maturing at age-4 and age-5 predominate in rivers of British Columbia, Washington, and Oregon (see the figure below). Though a decline in age-at-maturity and size has been reported for Alaska Chinook salmon, Chinook salmon encountered in ocean fisheries off the coast of California have by far the lowest age diversity on the West Coast of the United States, with 70 to 80% of fish taken age-3 or younger. In California ocean fisheries, age-4 fish are uncommon and age-5 or older Chinook salmon are exceedingly rare.

 

Available evidence strongly suggests Central Valley Chinook salmon historically expressed diversity in age-at-maturity comparable to what is currently observed in Washington, British Columbia, and Alaska.  Chinook salmon maturation schedules are heritable (that is, are passed genetically from parents to offspring); therefore, if later-maturing salmon are reduced or eliminated by harvest (or marine mammal predation), then the proportion of early maturing salmon will increase over time.  California’s intensive ocean fishery is high relative to what other west coast Chinook populations experience.  Thus, ocean harvest of Central Valley Chinook likely continues to impose substantial selective pressure for early maturity, and against extended residency in the ocean environment.

Mean Chinook salmon “ocean-age” by state/province. Data based upon scales and coded wire tags collected from Chinook salmon harvested in the ocean fisheries, 1977-2015.  “Ocean-age” indicates the number of winters a fish has spent in the marine environment, not standard age which is calculated as the difference between the year the fish was sampled and the year the fish was born. If smolts entered the marine environment within their first year of life, then standard age will be “ocean-age” plus 1.  If smolts entered the marine environment after 1 year of freshwater rearing, then total age is the “ocean-age” plus 2.  California fish represented in this figure are entirely hatchery origin. Since all California hatchery salmon released as sub-yearlings “ocean-1”, “ocean-2”, and “ocean-3” represent standard ages of age-2, age-3, and age-4 fish, respectively. Source: Ohlberger et al. 2018. Demographic changes in Chinook salmon across the northeast Pacific Ocean. Fish and Fisheries 19:533-546.

Continued reliance on a non-selective, ‘mixed-stock’ ocean fishery

 

The ‘mixed-stock’ Chinook salmon fishery off the coast of California includes relatively abundant runs (hatchery-produced Central Valley fall run),  endangered runs ( winter-run, spring-run, and California Coastal Chinook), and natural-origin stocks of unknown abundance and productivity (Central Valley fall-run Chinook).  A major difficulty with ‘mixed-stock’ ocean fisheries is that both mature and immature salmon are subject to harvest.  In contrast, a fishery that targets salmon as they return to spawn does not impact immature fish that have remained in the ocean. In a “mixed-stock” ocean fishery, later-maturing fish are subjected to more years of harvest than early maturing fish.  Although all stocks and runs are exposed to harvest in the ocean, differences in the spatial distribution, age-at-maturity, and size-at-date can affect their vulnerability to harvest. Harvest managers can use this information to direct harvest toward more abundant stocks, and away from sensitive stocks, but data and management tools that might provide better protections to sensitive stocks in a “mixed-stock” fishery are lacking.

 

Allowable ocean harvest is driven primarily by abundance of hatchery fall-run Chinook

 

Sacramento River Fall Chinook salmon (SRFC) are almost always the most abundant stock available for harvest off the coast of California and in southern Oregon. The predicted abundance of SRFC each year determines the maximum allowable exploitation rate.  Current PFMC regulations allow exploitation rates as high as 70 percent, but in practice exploitation rates have ranged between 42 and 68 percent since 2011. Poor abundance of Klamath Chinook, California Coastal Chinook, and winter-run Chinook can lead to fishery constraints (fewer fishing days, shorter seasons), but predicted abundance of SRFC appears to be the most important driver.  For example, exploitation rates for SRFC have dropped below 42% only because of the collapse of the SRFC stock that occurred in 2007-08. 

 

Importantly, abundance of the SRFC is strongly influenced by hatchery production.  Latest available data suggest 85% of Central Valley fall-run Chinook harvested off the coast of California are hatchery produced. The California Hatchery Scientific Review Group in 2012 called attention to PFMC (and NMFS)  relying on abundance of what they feared were predominately hatchery-produced Chinook salmon to set harvest limits impacting co-occurring natural-origin Chinook salmon.  According to the review group:

 

“Fishery harvests that are sustained at high levels by targeting abundant hatchery-origin fish may over-exploit naturally reproducing salmonids… fishery exploitation rates must be in alignment with the productivity of naturally reproducing salmon stocks for the recommendations in this report to be successful at conserving natural salmonid populations.”

 

Ocean harvest impairs viability of winter-run and spring-run Chinook

 

Winter-run and spring-run Chinook are the two Central Valley runs listed under the ESA. Winter-run Chinook spawners are currently dominated by age-3 fish, and among the four Central Valley salmon runs, they have the least diversity in age-at-maturity. Age structure among Central Valley spring-run Chinook is less-well understood because age is not regularly reported  for key populations, for example, in Deer, Mill, and Butte creeks. However, tagging data available from Feather River Hatchery indicate spring-run salmon, like winter-run, predominantly mature at age-3.

 

Status reviews for these ESA-listed stocks acknowledge the lack of diversity in age-at-maturity. Ironically, the scarcity of age-4 or older winter-run or spring-run Chinook have provided a rationale for allowing California’s uniquely aggressive ‘mixed-stock’ ocean fishery to continue.  Both winter-run salmon and spring-run are large enough to be harvested in the recreational ocean fishery at age-2, though minimum size limits are applied in an effort to reduce harvest mortality at this age.   

 

While PFMC and NMFS harvest strategies have been somewhat effective in protecting winter-run, and presumably spring-run, maturing at age-3 from ocean harvest, existing management provides no protections for endangered Central Valley Chinook salmon maturing at age-4 or older.  To the extent that age-4 or older winter-run or spring-run are present, they are subjected to harvest impacts equivalent to that directed at hatchery-supplemented Sacramento River fall-run Chinook. As discussed previously, this level of harvest exerts a very strong selective pressure against diversity in age-at-maturity and counteracts viability benefits that would otherwise be provided by the ‘portfolio effect’.

 

Conclusion

Ongoing impacts to Central Valley Chinook salmon from California’s ocean harvest are unparalleled among salmon stocks on the West Coast. The viability of Central Valley fall-run, winter-run, and spring-run Chinook populations are threatened by multiple factors.  However, ocean harvest appears to be contributing much more substantially than has been recognized by biologists, resource managers, and policy makers who have tended to focus on inland habitat conditions.  Strategies to reduce harvest impacts on sensitive Chinook stocks, and to improve diversity in age-at-maturity are available.  Those strategies should be implemented with as much urgency as management efforts that address California salmon runs in their freshwater environments.