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

Variations of strontium isotope ratios (Sr/Sr) 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/Sr signatures of the surrounding water during a fish’s lifetime. In this study, we present Sr/Sr 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 Sr/Sr and salinity. The Gambia River (Sr/Sr of ~ 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/Sr measurements. The Volta River (~ 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 (~0.70915) impede any significant correlations between Sr/Sr and salinity. Bulk otolith Sr/Sr 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.

Höpker, S. N., Wu, H. C., Lucassen, F., Sadio, O., Brochier, T., Nuworkpor, I. Y., Kasemann, S. A., Merschel, P., & Westphal, H. (2022). Sr Isotope Ratios (87Sr/86Sr) in Water and Fish Otoliths as Estuarine Salinity Tracers: Case Studies from Three NW African Rivers. Estuaries and Coasts.

We thank the following people for their assistance and support: Sebastian Flotow (ZMT) for sample preparation; Jule Mawick (ZMT) for elemental analysis; Dr. Thomas Mann (ZMT) and Jan Schuster (Kommunale Entwicklungspolitik, Stadt Eschweiler) for assistance with organisation and logistics; Dr. Khady Diouf Goudiaby (Université Cheikh Anta Diop, Institut Fondamental d’Afrique Noir; IFAN) for helpful discussions of otolith collection and interpretation. We acknowledge the AWA project “Ecosystem Approach to the management of fisheries and the marine environment in West African waters” funded by the BMBF and IRD (grant agreement 01DG12073E) for encouraging collaborative research between European and West African countries. We further acknowledge previous joint research efforts between the IRD (UR RAP, Dakar) and the Gambian Fisheries Department for networking. On behalf of all co-authors, we thank Jill Olin, Malte Willmes, and two anonymous reviewers for their constructive feedback and suggestions, which significantly improved our manuscript.

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