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

Otolith shape as a valuable tool to evaluate the stock structure of swordfish Xiphias gladius in the Indian Ocean

Swordfish Xiphias gladius is an oceanic-pelagic species. Its population structure in the Western Indian Ocean was studied from the shape of the sagittal otoliths of 391 individuals collected from 2009 to 2014. Normalised elliptical Fourier descriptors (EFDs) were extracted automatically using TNPC software. Principal components analysis (PCA) conducted on EFDs showed no significant effect of side (i.e. left or right otolith). Consequently, all 391 sagittal otoliths were used to identify stocks among six geographical areas: Reunion Island, Mozambique Channel, Rodrigues, South Africa, Madagascar South and Sri Lanka. To investigate the effects of sex, sampling year, sampling season, lower jaw fork length or geographical area on variations in otolith shape, redundancy analyses (RDAs) with permutation tests were conducted. The first four were non-significant (respectively, p = 0.124, p = 0.721, p = 0.197, p = 0.463), but geographical area appeared to discriminate groups significantly (p < 0.05). Furthermore, linear discriminant analysis (LDA) was performed and overall jackknife classification success reached 30%. Finally, a cluster analysis was conducted using Ward’s hierarchical algorithm, which discriminated three different groups. However, each group consisted of individual samples from all geographical areas. In conclusion, our results were unable to identify a clear geographical separation of swordfish at the Indian Ocean scale, corroborating recent genetic studies in this region