Structure and seasonal variability offish food webs in an estuarine tropical marine protected area (Senegal): Evidence from stable isotope analysis

摘要

West African tropical estuaries play an important role in the growth and survival of many commercially exploited fish species and enable the sustainability of considerable artisanal fishery yields. However, their trophic functioning remains poorly understood. In the Sine Saloum estuary (Senegal), a marine protected area (MPAs) was designated in 2003 to manage fisheries resources. The present study aimed to determine the structure, trophic functioning and seasonal patterns of the fish assemblages in this MPA, Throughout the study, 28 fish species were collected, with higher values of biomass (3826 kg km~(-2)) recorded during the wet season and lower values during the dry season (1228 kg km"2). Fish assem blages in both seasons were dominated by species with marine affinity, which accounted for 87% of the total biomass in the wet season and 70% in the dry season, with their abundance varying from 83% to 57%, respectively. Based on stable isotopic composition (δ(13)C and δ(15)N), species were combined into trophic groups. Primary consumers were partitioned into suspensivores (pelagic copepods, oysters and mussels), which fed mainly on paniculate organic matter, and intermediate consumers, feeding on freshly deposited organic matter and benthic microalgae {Sarotherodon melanotheron and Area senilis). Secondary consumers were divided into three groups. The first group included mullet, which fed by grazing on benthic microalgae (benthic affinity feeders). The second group, pelagic affinity feeders, was the most heterogeneous and fed mainly on pelagic components. The last secondary consumer group termed the intermediate group, included piscivores and benthic and pelagic invertebrate feeders, which dominated the top of the food web. The food chain in October was lengthened due to the occurrence of tertiary consumers. Food webs were dominated by secondary consumers, which constituted 89% of total biomass in the dry season and 71% in the wet season. The fish food web varied largely with season in faunal composition and food chain length. This study highlighted the need to take into account the seasonal dynamic of the food web both in terms of composition and structure to assess carbon fluxes in tropical estuaries.