A trophic model for the Ungwana Bay fishery, Kenya : linking fisheries to ecosystem management

The Ungwana Bay shallow water fishery covers an area of 1200.5 km 2. This shallow bay is joined by two major rivers that bring enormous amount of organic sediment yearly. Industrial shrimp trawling take place in this bay and also a great number of artisanal fishers are supported by the fisheries from the Ungwana Bay. However the overtime exploitation of the fisheries resources from this Bay has led to changes in the composition and size of fish landed with high landings being of fish of low commercial and hence thought to be a aresource use conflict To determine if the fishing has had an impact and if its ecologically sustainable or not the mean trophic level of the catch was analysed over 21 years-from 1984 t0 2004.In order to integrate the available information on biomass, fish catches and the dynamics and relationships of the main species in the system, a tropical model was constructed using Ecopath version 5.1 software.The Ungwana Bay ecosystem is largely supported by the benthic community, with largeflows of biomass originating from detritus which forms the benthic domain. The total system throughput was estimated at 6660 t.km-2. yr -1 while the net system production is1343.45 t.km-2. yr -1 and a PP/R ratio of 3.063. The system energy transfer efficiency is11.8%, a Finns recycling index of 0.07% with a mean path length of 4.86. All these information give an indication that Ungwana Bay is an immature ecosystem with very little or no recycling of total system throughput including detritus. The system has low biomass in relation to the high productivity and the results suggest that some degree of instability may occur in this system due to over exploitation. Detritus in this system serves as thesource of food to many commercially important species of Ungwana Bay. Most of theenergy throughput is within the II and III trophic level which is expected for estuarine coastal ecosystem and especially where there is high input of organic matter from the rivers.The fishery operates at the level II and III with most targets on demersal fish species and benthic invertebrates. Total catch was estimated at 3.039 t.km-2.yr -1 and the mean trophic level of the fishery as 2.99.The fishery is a competitor as they target fish that is prey to top predators. The fishing mortality has increased over the years and has led to reduced fish biomass and especially to the highly targeted species by artisanal fishery. This review indicates that there is overexploitation of some target species and the fishery as a whole has changed with decrease in biomass of most fish groups in 2004 compared to 1984. There is a shift in target of the fishover the years. It is also clear that management of the Ungwana bay should focus of specific fishery and integrate the relationship with other fisheries in the system. Also the two fishery need to be separated if enforcement of the regulations and any mangement measure is to be effective