Investigation of Spatial and Temporal Patterns of Fish Mortality in the Forcados River System, Delta State

Episodic fish mortality events in the Forcados River system, Delta State, South South Nigeria, have been reported with increasing frequency in recent years, causing significant economic losses to artisanal fishing communities and raising public health concerns about contamination of the food supply. This study investigates the spatial and temporal patterns of fish mortality events in the Forcados River system over a three-year period through systematic documentation and chemical analysis of mortality events. Data were collected through a network of trained community informants at 18 riverine settlements, complemented by four intensive sampling campaigns conducted during reported mortality events. Water and dead fish tissue samples were collected for physicochemical and toxicological analysis. Results documented 27 discrete mortality events, with highest frequency between April and June, corresponding to the onset of rainfall and flushing of dry-season organic accumulations. Spatial clustering of mortality events was identified near two operational hydrocarbon flow stations and one artisanal refining area. Total petroleum hydrocarbons in water at mortality event stations were 8 to 23 times above DPR guidelines. Fish tissue analysis confirmed accumulation of phenanthrene, pyrene, and fluoranthene in muscle tissue. The study provides the first systematic spatial and temporal analysis of fish mortality events in the Forcados system and attributes the majority of events to hydrocarbon pollution rather than natural hypoxia, with direct implications for regulatory enforcement and NDDC remediation programme prioritisation.

Keywords: fish mortality, Forcados River, hydrocarbon pollution, Delta State, water quality