Listening In on the Past: What Can Otolith δ18O Values Really Tell Us about the Environmental History of Fishes?

Oxygen isotope ratios from fish otoliths are used to discriminate marine stocks and reconstruct past climate, assuming that variations in otolith δ¹⁸O values closely reflect differences in temperature history of fish when accounting for salinity induced variability in water δ¹⁸O. To investigate this, we exploited the environmental and migratory data gathered from a decade using archival tags to study the behaviour of adult plaice (Pleuronectes platessa L.) in the North Sea. Based on the tag-derived monthly distributions of the fish and corresponding temperature and salinity estimates modelled across three consecutive years, we first predicted annual otolith δ¹⁸O values for three geographically discrete offshore sub-stocks, using three alternative plausible scenarios for otolith growth. Comparison of predicted vs. measured annual δ¹⁸O values demonstrated >96% correct prediction of sub-stock membership, irrespective of the otolith growth scenario. Pronounced inter-stock differences in δ¹⁸O values, notably in summer, provide a robust marker for reconstructing broad-scale plaice distribution in the North Sea. However, although largely congruent, measured and predicted annual δ¹⁸O values of did not fully match. Small, but consistent, offsets were also observed between individual high-resolution otolith δ¹⁸O values measured during tag recording time and corresponding δ¹⁸O predictions using concomitant tag-recorded temperatures and location-specific salinity estimates. The nature of the shifts differed among sub-stocks, suggesting specific vital effects linked to variation in physiological response to temperature. Therefore, although otolith δ¹⁸O in free-ranging fish largely reflects environmental temperature and salinity, we counsel prudence when interpreting otolith δ¹⁸O data for stock discrimination or temperature reconstruction until the mechanisms underpinning otolith δ¹⁸O signature acquisition, and associated variation, are clarified.