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 ($^{87}$Sr/$^{86}$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 $^{87}$Sr/$^{86}$Sr signatures of the surrounding water during a fish’s lifetime. In this study, we present $^{87}$Sr/$^{86}$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 $^{87}$Sr/$^{86}$Sr and salinity. The Gambia River ($^{87}$Sr/$^{86}$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 $^{87}$Sr/$^{86}$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 $^{87}$Sr/$^{86}$Sr and salinity. Bulk otolith $^{87}$Sr/$^{86}$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. ; 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 ...