Organic matter sources and trophic interactions among fish species in pangani estuary-Tanzania

Estuaries are primary habitats that serve as feeding and nursery grounds for most juvenile marine fish. However, estuaries in Tanzania have been affected by anthropogenic activities in the upstream of the river catchments including the use of these areas as fishing grounds by the artisanal fishers. These human activities contribute to determine the abundance and diversity of OM sources that support the nutrition of fish; and also to define the complexity of the estuarine food webs. The four complementary methods; stomach contents, stable isotopes( ),fatty acid (FA) biomarkers and trophic flow modeling approach were used to describe the OM nutritional sources and trophic interactions among the dominant marine fish in the Pangani estuary. Among the fish species investigated (Hilsa kelee, Valamugil buchanani, Arius africanus, Carangoides chrysophrys and Epinephelus malabaricus) the findings indicated significant inter-spesific variations in dietary composition (PERMANOVA, pseudo-F=125.27; P=0.001), Stable isotopes ( PERMANOVA, pseudo-F=300.29; P=0.001), and FA composition (PERMANOVA , pseudo-F=74.75; P=0.001). Moreover, the stable isotope results revealed slight differences in the most important OM sources relied by individual species from depleted in C values to relatively enrich in sources among the estuarine zones. Yet, the combined methods – stable isotopes, FA biomarkers and Eco path modeling emphasized that terrestrially- derives OM supplement the main benthic micro-algae basal resources to sustain the overall estuarine food web, The reliance on benthic diatom-dominated estuarine and marine basal food sources is an indication of low estuarine food web connectivity to the fresh water related food web. This situation is most likely threatening the resilience and stability of the estuarine food web structure .This was also highlighted by Ecopath trophic flow model which indicated that the Pangani estuarine food web structure is less complex, thus less stable and less resilient to any perturbation when compared with the tropical estuarine systems.