Geography and ontogeny influence the stable oxygen and carbon isotopes of otoliths of Pacific sardine in the California Current

•Temperature history of Pacific sardine was inferred from δ18O in otoliths.•Seawater in the Pacific Northwest affected north-south trends of δ18O in juveniles.•Inferred temperatures correlated inversely with the age of juveniles within a region.•Adults recorded δ18O values reflecting cooler temperatures than juveniles.•The results are consistent with a northern stock that mixes during annual migrations.

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 δ18O 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 δ13C and δ18O 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 δ18O 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.