Fluctuating salinity levels and an increasing pollution gradient on fish community structure and trophic levels in a small creek in the Niger delta, Nigeria

Abstract

Assessing the ecological status of rivers, creeks, and streams is a fundamental and increasingly important water management issue worldwide. This study examines the variations in fish community structure along an increasing pollution gradient in a small Niger Delta creek in Nigeria in both time and space. Salinity of the area fluctuated between fresh and mesohaline brackish waters with values ranging from 0.4 to 5.2 PPT and electrical conductivity values ranging between 16.9 and 136.0 µS/cm. The site in the upper creek (Site 1) was relatively free from gross pollution, with significantly high dissolved oxygen levels and low nutrient and salinity levels. Site 3 recorded relatively higher values of 5-day biochemical oxygen demand (BOD5) and high nutrient levels, indicating substantial input of organic matter to the creek upstream. Six (6) species, <em>Tilapia guineensis</em> (Cichlidae), <em>Bostrychus africanus</em> (Eleotridae<em>)</em>, <em>Hemichromis fasciatus</em> (Cichlidae), <em>Ethmalosa fimbriata</em> (Clupeidae), <em>Gerres melanopterus</em> (Gerreidae), and <em>Clarias macromystax</em> dominated the sample and accounted for about 70 % of the total catch. <em>Tilapia guineensis</em> constituted the major dominant species and accounted numerically for about 20 % of the total catches and 45 % of the total biomass. Multi-species fisheries dominate the coastal zone with <em>Tilapia guineensis</em>, <em>Gerres melanopterus</em> and <em>Ethmalosa fimbriata</em>, <em>Liza falcipinus</em>, <em>Mugil</em> spp., <em>and Chrysichthys nigrodigitatus</em>, the major species in the commercial catches. Canonical correspondence analysis indicated that dissolved oxygen, electrical conductivity, total nitrogen, BOD5, and temperature were important variables structuring the overall fish assemblages. Overall, salinity was the major environmental variable determining the diversity of fish communities at the various sites. The effect of anthropogenic activities and land use practices around the creek only marginally affected the abundance of the fish species but not the diversity. These findings indicate that the water body is still useful as a good fishing ground for the rural communities and every effort should be put in place to ensure its protection and conservation for the production of healthy fish.