Temporal Trends in Hatchery Releases of California Central Valley Fall-Run Chinook Salmon

Fall-run Chinook salmon (Oncorhynchus tshawytscha) from California’s Central Valley collapsed in 2008, resulting in a moratorium on fishing in 2008 and 2009, and only limited openings in 2010. The closure was the first in the fishery’s 157 years and was declared a federal emergency disaster. While the proximate cause of the collapse is thought to be poor ocean conditions that resulted in low juvenile survival when fish first arrived at the ocean, the strong influence of hatcheries in this system also warrants inspection. Recent research suggests that roughly 90% of ocean fishery in this system are fall-run salmon produced in one of five hatcheries (4 state, 1 federal) suggesting that hatchery practices are critical to the survival of the fishery. Here we compiled information on the temporal trends in the number of juveniles released and their sizes and ages at release across all five hatcheries for a period of roughly five decades. For the four state hatcheries, this involved reviewing over 170 annual reports, many of which were still in draft form (33% of total). For the single federal hatchery, this involved requesting a preexisting electronic database that included these same data. Our preliminary results for the state hatcheries suggest an increased number of fish stocked from the mid-20^th century to a peak in the mid 1990’s followed by a slight decline which has remained relatively constant until now. In the past, some hatcheries released a mixture of age-0 and age-1 smolts whereas most have now shifted to releasing age-0 fish exclusively. Additionally, in the last decade, there has been a shift towards stocking larger age-0 fish. Together these results indicate a decrease in interannual variance of total amount stocked and the average age and size at release. We suggest that a thorough review of previous hatchery release practices will lead to a better understanding of the influence of hatcheries on this stock complex and perhaps lend some insight into its recent collapse.