California Sea Grant College Program

The state of the Chinook salmon (Oncorhynchus tshawytscha) fishery in California is imperiled. One of the proposed mechanisms for recent population declines and fisheries closures is environmental variation having deleterious effects on prey availability and the ocean survival of Sacramento River fall Chinook (Lindley et al. 2009). To address this hypothesis, we analyzed collaborative fisheries research data (stomach samples obtained from commercial and recreational fisherman) on Chinook salmon diet in central California during the early-mid 1980s and mid 2000s. We compared this information with historic diet records for this population from 1955. Salmon diet composition changed significantly through time, with a decline in diet diversity. Specifically, previously important prey groups including juvenile rockfish (Sebastes spp.), krill (Euphausiidae), Pacific herring (Clupea pallesi) and market squid (Loligo opalsecens) were either greatly reduced or disappeared from the diet altogether. Conversely, Pacific sardine (Sardinops sagax), conspicuously absent from the diet (and environment) since the 1950s, became the second most important prey item in the 2000s. Northern anchovy (Engraulis mordax) remains an important prey item. The strongest correlation between salmon diet and environmental conditions was for local SST for all years in the study between 1955-2007. In the early 1980s, SST best explained salmon diet composition. While relationships were not significant in the mid 2000s, likely due to the short time period sampled, upwelling or the NPGO were the best descriptors of salmon diet variation. These physical pressure-state indicators have exhibited increasing trends and increasing variability through time and are indicative of the changing climate. Finally, we decomposed multiple prey variables into a univariate annual index and thus compared diet variation with the Sacramento Index (SI) of Chinook ocean abundance. We found a relationship between diet and the SI lagged by 2 years, potentially an indication of prey availability experienced by smolts after ocean entry (Figure 5). Collaborative fisheries research offers a means to track variation in salmonid diet with implications for understanding the connections between environmental variation, salmon survival, and declining fisheries in California.