Pacific sardine ( Sardinops sagax ), a commercially valuable species, have a broad distribution along the North American coast that spans Canada, the United States, and Mexico. The goal of this research was to evaluate water temperature history of the fish inferred from stable oxygen isotopes in otoliths in order to differentiate and connect stocks across regions, and between juveniles and adults. Local seawater composition in the Pacific Northwest affected major north–south trends in δ 18 O composition of juvenile otoliths. Inferred temperature correlated inversely with the age (otolith weight) of juveniles within a region, possibly resulting from changes in depth preferences as the fish grew. The correlations between δ 13 C and δ 18 O in juvenile otoliths were relatively weak in the northernmost and southernmost samples, but comparisons of the sample means indicated significant differences between some regions. Otoliths from adult sardine captured between California and Canada recorded δ 18 O values reflecting cooler temperatures than otoliths from juveniles, and without regional differentiation. These results are consistent with a northern stock that mixes during annual migrations.
