Relating otolith to water [SR/CA] ratios: experimental validation for sand goby <i>Pomatoschistus minutus</i>

The chemical composition of fish otoliths reflects to a certain extent that of the ambient water at the time of deposition. This premise led to new applications in fisheries science such as fish stock discrimination, assessment of estuarine dependency or reconstruction of movement patterns into estuaries. The latter require prior knowledge of the variations in ambient elemental concentrations along the estuarine gradient and an accurate description of the relationship between aqueous and otolith [element/Ca] ratios. This study explored the applicability of otolith [Sr/Ca] to reconstruct sand goby Pomatoschistus minutus (Pallas 1770) migrations in the Scheldt estuary (Belgium - The Netherlands). Firstly, the concentration ratio of Sr/Ca over the entire salinity gradient was determined. Secondly, an experiment was conducted towards Sr incorporation in sand goby otoliths at varying ambient [Sr/Ca] levels. To this aim five experimental water mixtures of different salinity levels (3, 7, 12, 20 and 30) were prepared from seawater and water from the fresh water reaches of the estuary. The experiment was performed at two temperatures (13 and 18°C). [Sr] and [Ca] acted conservatively in the estuary resulting in a positive but nonlinear relationship between salinity and ambient [Sr/Ca], which was highly similar in the Scheldt and the experiment. Experimental results revealed positive linearity between aqueous and otolith [Sr/Ca] ([Sr/Ca]otolith = 0.31[Sr/Ca]water + 0.44). The otolith [Sr/Ca] was significantly different between each salinity level but there was no difference in otolith [Sr/Ca] between both temperature conditions. The partition coefficient (DSr) averaged around 0.38 and was slightly higher at the lowest salinity, which might indicate different incorporation mechanisms of Sr over the salinity range. Nevertheless, whether salinity per se has an effect on otolith [Sr/Ca] in sand goby or not, our results support the use of otolith [Sr/Ca] in terms of broad scale movements between marine and estuarine habitats in general, and between the North Sea and the Scheldt estuary in particular