Aims: Eutrophication impacts water quality and limits fish growth and body condition in large reservoirs. This study examined the effects of key physico-chemical factors and their north-south eutrophication gradients in Lake Nasser, Egypt, on Nile perch (Lates niloticus) growth across this transect.
Materials & Methods: Water physicochemical factors (i.e., Temperature, pH, Dissolved oxygen, turbidity, Total suspended solids, Total dissolved solids, Total Nitrogen, and Total Phosphorus) were assessed during the growth period for 1,992 specimens collected from three sites (Aswan, Garf Hussian, and Abou Simbel) from Jan 2022 to Dec 2023.
Findings: A clear downstream increase in overall turbidity, total Nitrogen, and total Phosphorus, coupled with a decrease in dissolved oxygen, correlated with the length-weight relationship of the fish caught in the various sites that demonstrated negative allometric growth (b < 3). The ELEFAN_SA model determined Von Bertalanffy growth factors showing positive growth toward lower asymptotic lengths at Aswan (K = 0.095 y⁻¹; L∞ = 168.6 cm), maximum size potential at Garf Hussian (K = 0.088 y⁻¹; L∞ = 205.2 cm), and reduced growth rate and asymptotic length at Abou Simbel (K = 0.080 y⁻¹; L∞ = 167.96 cm). At the upstream oligotrophic site, DO and, to a lesser extent, TN were significant predictors of condition factor (K), whereas downstream, the relationships were weak.
Conclusion: These results show that, along a eutrophication gradient, spatial heterogeneity in water quality constrains the growth and condition of fish populations in reservoirs, and that variability in water quality is a critical factor that must be integrated into the ecosystem-based fisheries management of Lake Nasser.