Assessment of Water Physicochemistry in Relation to Fish Diet and Biodiversity across Three Savanna Reservoirs in Katsina State, Nigeria

This study evaluated the effects of water quality on fish community structure and dietary patterns across three inland reservoirs in Katsina State, Nigeria: Zobe (Sudan savanna), Sabke (Sahel savanna), and Mairuwa (Guinea savanna) from October 2023 to July 2024. Seasonal sampling of water and fish was conducted, and statistical analyses were used to assess spatial variation in physicochemical parameters, fish abundance, diversity, and feeding ecology. Analysis of variance (ANOVA) revealed no significant differences in mean temperature (Zobe: 31.00 ± 0.00°C, Sabke: 28.00 ± 0.00°C, Mairuwa: 20.00 ± 0.00°C) and pH (Zobe: 6.40 ± 0.25, Sabke: 6.47 ± 0.25, Mairuwa: 6.60 ± 0.10) across reservoirs (p > 0.05). However, significant differences were found in electrical conductivity (Sabke: 115.86 ± 0.10, Zobe: 101.22 ± 0.05, Mairuwa: 101.65 ± 0.02), dissolved oxygen (Sabke: 10.01 ± 0.03 mg/L), and biological oxygen demand (Sabke: 41.58 ± 0.04 mg/L) (p < 0.05). Fish abundance varied, with Sabke reservoir recording the highest number of Oreochromis niloticus (243) and Lates niloticus (250), while Mairuwa reservoir had the highest biodiversity, reflected by a Margalef Diversity Index of 0.8142. Catch per unit effort (CPUE) highlighted the dominance of Latesniloticus in Sabke (CPUE: 3.71) and Clarias gariepinus in Mairuwa (CPUE: 1.01). Length-weight relationships for key species such as Schilbemystus, Oreochromis niloticus, Lates niloticus, and Clarias gariepinus showed negative allometric growth (b < 3) with high correlation coefficients (R² values ranging from 0.698 to 0.9132), confirming consistent growth trends across sites. Dietary analysis revealed that Schilbemystus fed primarily on insects, with occurrence rates of 56.6% (Zobe), 63.3% (Sabke), and 43.3% (Mairuwa). Clarias gariepinus showed a strong piscivorous tendency, with fish parts constituting 13.3% (Zobe), 70.0% (Sabke), and 46.6% (Mairuwa) of its diet. Latesniloticus in Sabke consumed 90.0% fish parts, confirming its role as an apex predator. In Mairuwa, Bagrusbayad relied heavily on fish (63.3%), while Synodontisclarias showed a mixed diet dominated by insects. Correlation analyses between water quality and fish abundance showed species-specific associations. In Zobe, temperature had a strong positive correlation with O. niloticus abundance, while in Mairuwa, total suspended solids (TSS) correlated positively with Schilbemystus, and chemical oxygen demand (COD) negatively with O. niloticus. Findings showed Latesniloticus as an apical predator in the Sabke reservoir, consuming 90.0% fish parts. Meanwhile, Bagrusbayad in the Mairuwa reservoir prefers fish, making up 63.3% of its diet. Further studies are needed to investigate the interactions among water quality, fish diversity, and habitat conditions in freshwater ecosystems.