San Francisco Estuary and Watershed Science

Central Valley spring-run Chinook Salmon (CVSC) are designated threatened by state and federal authorities. Although CVSC are caught in ocean fisheries, their harvest is not actively managed, because it is assumed that measures currently in place to protect endangered Sacramento River winter-run Chinook Salmon (SWRC) will also sufficiently protect CVSC. Recoveries of tags and genetically-identified CVSC suggest these fish have a more northerly distribution than SRWC. Further, escapement data and cohort reconstructions suggest that CVSC mature later than SRWC. Thus, regulations (time/area restrictions and minimum size limits) crafted to protect SRWC alone may not adequately protect CVSC; on the other hand, regulations to constrain impacts on Klamath River and California coastal Chinook Salmon populations may also reduce impacts on CVSC. Trends in CVSC escapement were deemed acceptable in recent status updates, but concerns remain because of the negative effects caused by recent drought and ocean conditions. Should more active management of CVC be desired, current options are limited. The most promising approach is based on estimating age-specific ocean fishing mortality rates by using cohort reconstructions applied to tagged Chinook Salmon that originate from the Feather River Hatchery. At a minimum, ocean fishing mortality rates could be monitored and compared to proxy thresholds. If reference harvest rates were established, harvest models could be developed to predict how CVSC would be affected by fishing regulations, similar to the way fall-run Chinook Salmon fisheries are evaluated. Abundance forecasts would require improved juvenile production data (e.g., from genetic sampling of juvenile emigrants), since sibling-based forecasts commonly used for fall-run Chinook Salmon would not be available in time for pre-season planning. It is unclear if ocean fishing mortality rate estimates derived from hatchery proxies for natural-origin fish are truly representative, but existing data do not demonstrate obvious differences in ocean distribution or size-at-age fish. Substantial new investments in tagging or sampling would be needed to directly estimate ocean fishing mortality rates for natural-origin CVSC. Establishing specific harvest targets or limits for CVSC requires an improved understanding of production throughout their life cycle through juvenile production estimates and long-term information on spawner age structure.