Water Hyacinth (Eichhornia crassipes) Infestation and Its Impact on Fish Biodiversity in Anambra River, Anambra State

Water hyacinth (Eichhornia crassipes) is an invasive aquatic macrophyte that has severely impacted freshwater ecosystems across sub-Saharan Africa, affecting fisheries productivity, water quality, and navigation. This study assessed the extent of water hyacinth infestation and its influence on fish biodiversity in the Anambra River, Anambra State, South East Nigeria. Coverage mapping of water hyacinth mats was conducted using GPS transects and visual estimation along a 15-kilometre stretch of the river. Fish sampling was performed at infested and non-infested control sites using experimental gill nets and cast nets over a four-month period. Fish species were identified, enumerated, and diversity indices were computed. Water quality parameters including dissolved oxygen, transparency, temperature, pH, and nutrient levels were measured at all stations. Water hyacinth covered approximately 34 percent of the surveyed river stretch, with highest density in slow-flowing backwater areas. Dissolved oxygen was significantly lower beneath hyacinth mats compared to open water sites, averaging 2.1 milligrams per litre versus 6.8 milligrams per litre. Fish species richness was significantly reduced at heavily infested stations. Anoxia-sensitive species including some cichlids and catfishes were absent at the densest infestations. Only tolerant species such as Clarias spp. were abundant beneath mats. Total fish abundance was 43 percent lower at infested compared to control sites. This study demonstrates the severe ecological impact of hyacinth infestation on river fisheries and advocates for integrated biological and mechanical control strategies. Keywords: Eichhornia crassipes, invasive species, fish biodiversity, Anambra River, dissolved oxygen.