Assessment of Water Hyacinth (Eichhornia crassipes) Infestation and Impacts on Fish Diversity in Kainji Reservoir, Niger State

Water hyacinth infestations in Nigerian freshwater bodies have expanded significantly over the past decade, threatening fish diversity, navigation, and the livelihoods of fishing communities dependent on inland water bodies. Kainji Reservoir in Niger State, one of Nigeria's largest man-made lakes, has experienced recurrent water hyacinth cover with incompletely understood consequences for fish assemblage diversity. This study quantifies the spatial extent of water hyacinth coverage, evaluates its impact on dissolved oxygen and light penetration, and compares fish species diversity between heavily infested and open water sampling zones. Remote sensing analysis using Sentinel-2 imagery estimated current infestation extent. Water quality parameters including dissolved oxygen, turbidity, pH, and temperature were measured at twenty paired sampling stations. Experimental gill net sampling was used to compare fish assemblage composition, diversity indices, and catch per unit effort between infested and open zones. Results show infestation covering approximately 18% of the reservoir surface, concentrated in the northern bay and delta zones. Dissolved oxygen levels beneath dense mats averaged 1.8 mg/L, significantly below critical thresholds for most fish species. Fish species richness was 34% lower in infested compared to open water zones. Cichlid and catfish assemblages showed the greatest sensitivity to infestation. Mechanical harvesting of mats combined with controlled introduction of Neochetina weevils is recommended as an integrated management approach. Keywords: water hyacinth, invasive aquatic plant, Kainji Reservoir, fish diversity, biological control