A Trophic Structure Model of the Douala-Edea Reserve Mangrove (Cameroon) with Consideration of Sustainable Utilization of its Resources

The Douala-Edea reserve mangrove is the largest mangrove area in a national park in Cameroon. It has an area of about 272.79 km2 (Findi, 2015) and is subjected to substantial anthropogenic impact through intensive harvest of mangrove wood and fishery. In order to integrate available information on biomass, catches, food spectrum and dynamics of the main species populations of the system, a trophic steady state model of 21 compartments was constructed using the ECOPATH II software (Christensen & Pauly, 1992). About eighty percent of total system biomass is made up by mangroves (Rhizophora spp), which are assumed to cover about 65 % of the total area and contribute about 70 % to the systems primary production. The remaining biomass is distributed between the pelagic and benthic domains in proportions of 10 % and 90 % respectively. Through litter fall, mangroves inject the main primary food source into the system, which is either consumed directly by herbivores (principally land crabs) or, when already metabolized by bacteria, by detritus (principally fiddler crabs, Uca spp. ). According to the model estimates, energy flow through the fish and shrimp compartments is of relatively high importance for the energy cycling within the system, a finding which is similar to the situation in other mangrove estuaries reported in the literature. The dominance of mangrove epibenthos is attributed to the fact that a large part of the systems production remains within the mangrove forest as material export to the estuary is restricted to spring tides, when the forest is completely inundated. Total system throughput (400.54 t/km/year) and mean transfer efficiency between trophic levels (13.8 %) calculated by the model fit well into the range reported for other tropical coastal ecosystems. The very high gross efficiency of the fishery (catch/net primary production) of 17.46 % and its low trophic level (21) is explained by a high harvesting rate of mangroves and the fact that the main animal resource in the system are the mangrove crabs (Ucides cordatus), which feed at the first trophic level. The model was balanced assuming a turnover rate for the land crabs of P/B=044 (P/B production per unit of biomass) which is possibly low and more realistic, suggesting a situation in which more biomass is being harvested than produced, which hints to an overexploitation of this resource A ranking of the various system components in terms of their contribution to the system function (ascendency sensu Ulanowicz, 1997) revealed that detritus and associated bacteria contribute 27 %, mangroves 26 %, fiddler crabs and predatory crabs 13 %, phytoplankton and zooplankton 10 %, mangrove crabs 10 %, and the remaining 14 groups 14 % to the total ascendency. Summary statistics of the model are given and compared with those of other coastal ecosystems.