Sr Isotope Ratios (87Sr/86Sr) in Water and Fish Otoliths as Estuarine Salinity Tracers: Case Studies from Three NW African Rivers

Variations of strontium isotope ratios (Sr-87/Sr-86) in river systems are increasingly utilised to geochemically trace origin and movement patterns of migratory fish species. Accretionary calcified structures, such as otoliths, preserve Sr-87/Sr-86 signatures of the surrounding water during a fish's lifetime. In this study, we present Sr-87/Sr-86 measurements of water samples and catfish otoliths collected in the estuaries of the Sine-Saloum (Senegal), the Gambia River (The Gambia), and the Volta River (Ghana) to assess their systematics and relationships with salinity. The three rivers possess distinct hydrological properties resulting in variable degrees of correlations between 87 Sr/ 86 Sr and salinity. The Gambia River (Sr-87/Sr-86 of similar to 0.71209) proved exceptionally preconditioned for the approach due to well-defined geochemical end-members, allowing for quantitative estimates of salinity based on otolith Sr-87/Sr-86 measurements. The Volta River (similar to 0.71392) presents a more complex case due the possible influence of multiple water sources to the main channel, while the inverse salinity gradient and excessive evaporation in the Sine-Saloum estuary (similar to 0.70915) impede any significant correlations between Sr-87/Sr-86 and salinity. Bulk otolith Sr-87/Sr-86 values in the Gambia River and Volta River clearly depicted a mixed influence of seawater and riverine compositions, strongly encouraging the application of this approach for geochemical fingerprinting of critical NW African species.

Automated summary

The study assesses the relationship between salinity and Sr-87/Sr-86 measurements of water and catfish otoliths collected in three estuaries. The hydrological properties of the rivers result in variable correlations between Sr-87/Sr-86 and salinity. The Gambia River proves to be well-suited for the approach, while the Volta River and Sine-Saloum estuary present more complex cases.