Temperature record in the oxygen stable isotopes of Pacific sardine otoliths: Experimental vs. wild stocks from the Southern California Bight

Pacific sardines (Sardinops sagax) are commercially fished in Canada, USA, and Mexico along approximately 5000 km of coastal waters that experience a wide range of temperatures. Trinational management of the species can be problematic because the connectivity between spawning, recruitment, stock residency, and migration in some years may not be well predicted. Oxygen isotopic value of otoliths (δ18Ootolith) has been used to infer stock residency and movement of fish populations within regions, but few studies have used laboratory data to establish a predictive temperature model to validate δ18Ootolith values of wild fish. We conducted a growth experiment with juveniles at different temperatures using Southern California Bight (SCB) seawater to test the assumption that Pacific sardine otoliths accurately record environmental water temperature in the presence of constant salinity. Sardine δ18Ootolith values were significantly and negatively correlated with temperature according to the linear model:δ18Ootolith(‰)−δ18Owater(‰)=−0.132(±0.003 SE)×Temperature(°C)+2.455(±0.043 SE)δ18Ootolith(‰)−δ18Owater(‰)=−0.132(±0.003 SE)×Temperature(°C)+2.455(±0.043 SE)We used this model to predict the temperature history of wild age-1 sardine collected in the SCB from 1995 to 2003. The field samples generally fit the model of SCB residency when both the average coastal sea surface temperatures and temperatures at 30 m were used to bracket the range of calculated otolith temperatures.

Research Highlights
► Pacific sardine recorded seawater temperatures as δ18O values in their otoliths.
► A δ18O-temperature model was developed based on the growth of juveniles.
► The model predicted temperature history of age-1 sardine in Southern California.
► Otolith δ18O values likely reflected average depth distributions of 0–30 m.
► Sardine otolith δ18O values reflected El Niño and La Niña conditions in some years.