Age Structure, Growth Rate, and Mortality Estimation of Synodontis schall in the Lower Benue River Using Otolith Microchemistry

This study estimates age structure, individual growth rates, and mortality parameters of Synodontis schall in the lower Benue River using otolith microstructure analysis, addressing a gap in the biological knowledge needed to manage this commercially important catfish species in Kogi and Benue States, North Central Nigeria. Synodontis schall is among the top five most commercially important species in the lower Benue artisanal fishery, yet age-based stock assessment data are absent from the Nigerian literature. Sagittal otoliths were extracted from 240 specimens collected across five months and prepared by sectioning and polishing for annulus counting. Daily increment counts were validated using marginal increment analysis and chemical marking in a captive cohort. Von Bertalanffy growth parameters were estimated from age-at-length data. Total mortality was estimated by catch curve analysis. Otolith microchemistry (Sr:Ca and Ba:Ca ratios) was analysed by electron probe microanalysis to detect migration between fresh and brackish water. Maximum age observed was 9 years, with von Bertalanffy parameters L-infinity of 42.1 cm and k of 0.26 per year. Total mortality coefficient was 0.88, with natural mortality estimated at 0.41 and fishing mortality at 0.47, giving an exploitation rate of 0.53, suggesting moderate overfishing. Otolith microchemistry profiles confirmed exclusively freshwater life histories for all analysed specimens. The study provides age-based parameters essential for stock assessment and recommends exploitation rate reduction through mesh size regulation in the lower Benue Synodontis fishery. Keywords: Synodontis schall, otolith microchemistry, age structure, growth rate, Benue River.