ABSTRACT: Salmon are affected by variation in ocean productivity; thus, improved understanding of mechanisms behind variability in ocean survival should help management of these ecologically and economically important populations. Based on a cooperative fisheries research program, we compared central California Chinook salmon Oncorhynchus tshawytscha adult food habits from spring and summer in the mid-2000s with historical records from 1955 and the 1980s. Diet diversity decreased through time, and was particularly low in May and June of the 2000s. Previously important prey, including juvenile rockfish Sebastes spp., krill Euphausiidae, Pacific herring Clupea pallesi and market squid Doryteuthis opalsecens, declined or disappeared from the diet, while Pacific sardine Sardinops sagax became very important prey in the 2000s; anchovy Engraulis mordax remained important throughout the study. Diet composition was correlated with regional mid-water trawls of prey abundance and also with local sea surface temperature (SST). Diet composition was related to the Sacramento Index of fall-run Chinook ocean abundance with a lag of 1 or 2 yr, and reflected the importance of prey availability during the second ocean year and smolt ocean-entry period, respectively. Spring is peak ocean entry for fall-run Chinook smolts, so declining prey diversity (specifically in May and June in the mid-2000s) may be related to recent population crashes. Seasonally, winter and fall diet data further demonstrated the significance of temporal variation in specific prey. This study highlights the importance of marine predator–prey interactions at an appropriate temporal resolution for understanding salmonid population dynamics.