Oil spill impact on the finfish of Azhiwari swamp, Joinkrama in the Niger delta of southern Nigeria
The impact of an oil spill on surface water, sediment and finfish assemblage characteristics in a freshwater swamp forest was assessed by dividing the wetland into four zones on the basis of spilled oil cover on the water surface – High Impact Zone (HIZ >60%), Medium Impact Zone (MIZ 30 – 60%), Low Impact Zone - (LIZ 5 – 30%) and No Impact Zone (NIZ <5%) which served as control. Samples of water, sediment and fish were collected over a period of 6 months and analyzed using standard methods. Species richness determined by Fischer’s α; similarity in assemblage by Ppos, and percentage incidence of lesions, finrot and empty stomachs evaluated by physical examination of specimens were carried out. Dissolved oxygen followed the trend - HIZ < MIZ < LIZ < NIZ while BOD5 exhibited a reverse trend (NIZ < LIZ < MIZ < HIZ). Other physicochemical parameters did not show any trend. Species loss declined from HIZ (100%) >MIZ (70%)> LIZ (10%) > NIZ (0.0%) while richness increased from HIZ (0) < MIZ (1.3) < LIZ (3.6) < NIZ (5.1). Species similarity was NIZ vs LIZ (0.86); NIZ vs MIZ (0.43); LIZ vs MIZ (0.46). Stress indicated by empty stomachs and finrot were (MIZ (65.7-72.3) > LIZ (26.0 37.5) > NIZ (17.5-21.1) and MIZ (20-25) > LIZ (0-5) > NIZ (0 – 0) percent respectively. The results indicate that the impact of oil spills on finfish goes beyond the immediate fish mortality observed during oil spills and includes stress on surviving finfish resulting from changes in water and sediment quality. Loss in finfish species richness, reduction in diversity, and abundance are other impacts of oil spills in freshwater swamps.
Key Words: Oil spill, fish diversity, richness, stress, Niger Delta.