PRELIMINARY ASSESSMENT OF WILD FISH POPULATIONS AND MORPHOLOGICAL DEFORMITIES ALONG AN ANTHROPOGENIC STRESS GRADIENT IN RIVER ETHIOPE, DELTA STATE.

The study of ecological changes in wild fish populations is one of the tools to monitor environmental degradation and potential risks to food security. This survey evaluated fish abundance and external morphological deformities as biological indicators along a pollution gradient in River Ethiope, Nigeria. Wild fish (n=97) were collected from three stations of differing human impact (Obiaruku, Abraka, Sapele) from March to June 2025 and examined by gross external examination. Environmental parameters revealed a distinct degradation gradient at Sapele with a dissolved oxygen of 5.00 mg/L and maxima of sedimentary copper (1.092 mg/kg), lead (0.488 mg/kg), zinc (2.062 mg/kg) and surface ammonia (0.60 mg/L). Fish abundance decreased from 61 (Obiaruku) to 12 specimens (Sapele) under equal effort (Kruskal-Wallis H=9.95, p=0.007). The abnormality ratio increased from 0.049 to 0.833, with true deformity prevalence rates of 4.92%, 16.67% and 58.33% respectively. An omnibus chi-square test revealed a significant spatial association between station and prevalence of deformity (χ²=23.30, p<0.001; Cramér's V=0.49), which was driven by the Obiaruku–Sapele contrast (Fisher's Exact Test, p<0.001). External anomalies varied from mild gill changes at Obiaruku to severe eye clouding, skin lesions and fin erosion at Sapele. Spearman correlations (ρ=±1.000) are descriptive only, reflecting the spatial constraint n=3. Though limited in spatial extent, sample sizes, taxonomic resolution and lacking residue analyses of tissues, the co-occurrence of deformities with elevated pollutants establishes gross morphological screening as an accessible, low-cost bio-monitoring tool to identify areas that require further toxicological investigation. Preliminary data are not sufficient to establish causal relationship.