Seasonal and Spatial Distribution of Ichthyofauna of Lake Toho after 2018’s Mass Fish Mortality Events

Abstract

Aims: This study aimed to assess the composition and diversity of the ichthyofauna of Lake Toho across sampling sites and seasons following the 2018 mass fish mortality event. Study Design:  This was a comparative ecological study assessing fish species composition and diversity across different sites and seasons. Place and Duration of Study: Lake Toho, February to December 2024 Methodology: Sampling was carried out bimonthly at two sampling sites. Standard water physicochemical parameters were measured in situ. Fish sampling was conducted using standardised experimental fishing methods, namely fine-mesh seines and gillnets. In addition, catches from local fishermen were examined to complement the information obtained on fish diversity. Samples were preserved in 10% (v/v) formalin solution. In the laboratory, fish specimens were subsequently transferred to 90% ethanol and identified using taxonomic keys. Diversity indices were calculated. A two-way analysis of variance (ANOVA) was performed to evaluate the effects of sampling site and season. Results: Except for dissolved oxygen (range 2.52-3.32 mg/L), all measured water physicochemical parameters were suitable for freshwater aquatic organisms. A total of four orders, seven families, and 14 fish species were sampled. Cichlidae was the dominant family, accounting for 98% of the total abundance and comprising six species. Sarotherodon melanotheron, Oreochromis niloticus, and Coptodon guineensis were ubiquitous species. Species richness varied from 12 species at site Z1 to 13 species at site Z2. It ranged between 8 species during the short dry season and 11 species during the high rainy season. Simpson index (range 0.44-0.62), Shannon index (range 0.82-1.25), and equitability (range 0.44-0.60) showed no significant differences between sites (P > 0.05) and seasons (P > 0.05), indicating low and homogeneous fish diversity. No significant site × season interaction was observed for fish abundance or diversity indices (P > 0.05). Conclusion: These findings suggest that fish diversity may be influenced by multiple anthropogenic pressures and underscore the need for enhanced monitoring, conservation, and sustainable management of Lake Toho.