Evaluating Estuarine Habitat Connectivity for Diadromous Fish in the Sombreiro River, Rivers State

Diadromous fish species that migrate between freshwater and estuarine or marine environments depend on unobstructed habitat connectivity for the completion of their life cycles, yet connectivity in Nigerian coastal river systems is increasingly compromised by culverts, weirs, and land reclamation blocking estuarine access. This study evaluates estuarine habitat connectivity for diadromous fish in the Sombreiro River system, Rivers State, South South Nigeria, which supports populations of mullet (Mugil cephalus), tarpon (Megalops atlanticus), and the estuarine catfish Chrysichthys nigrodigitatus. A connectivity assessment framework integrating physical barrier surveys, acoustic telemetry, and habitat suitability modelling is applied across a 75-kilometre river reach from the Sombreiro River mouth to the Ahoada confluence. Physical barrier surveys identify and characterise 18 man-made structures in terms of passability, water velocity, and jump height criteria. Acoustic telemetry using V9 coded tags implanted in 30 Mugil cephalus individuals tracks individual movement within the river system over a nine-month monitoring period spanning both wet and dry seasons. Habitat suitability index models for each species are parameterised using salinity, temperature, dissolved oxygen, depth, and substrate variables collected at 40 habitat assessment stations. Model-predicted habitat availability is compared between current (barrier-present) and counterfactual (barrier-free) connectivity scenarios. Findings are expected to demonstrate 40 percent reduction in accessible high-quality habitat attributable to connectivity barriers, with dry season low-flow conditions further restricting migration through shallow barrier zones. Keywords: estuarine connectivity, diadromous fish, acoustic telemetry, Sombreiro River, habitat suitability