Relationship among adult body length, abundance, and ocean temperature forBritish Columbia and Alaska sockeye salmon (Oncorhynchus nerka), 1967-1997

Body length of adult Pacific sockeye salmon (Oncorhynchus nerka) has decrea sed significantly in recent years. We used 69 time series of age-specific b ody-length data (1967-1997) for 30 sockeye salmon stocks from southern Brit ish Columbia to western Alaska to test hypotheses about the effects of ocea nographic conditions and competition on growth rate of sockeye salmon. Usin g principal components analysis (PCA), we constructed a single time series (PC1) that represented the dominant pattern of variability in length-at-age shared among these stocks. Taking into account time trends and autocorrela tion in residuals, we found that increases in total Gulf of Alaska sockeye abundance and increases in sea-surface temperature (SST) across the Gulf of Alaska were significantly associated with reduced adult body length. Abund ance and SST together accounted for 71% of the variability in PC1. Although researchers have documented increases in both abundance of sockeye salmon and their food in the northeastern Pacific Ocean over the last few decades, it is possible that increased food was more than offset by increased socke ye abundance, leading to greater competition and reduced body size.