Beurteilung des Zustandes und der Auswirkungen der handwerklichen Rifffischerei und ihrer Bewirtschaftungsimplikationen in der kenianischen Südküste

Artisanal fisheries of tropical waters are estimated to harvest about 25% of the world s fisheries catch. Despite this importance, a majority of tropical fish stocks remain unassessed and poorly managed. Reasons include a severe under-reporting of catches or the lack of reliable information of the fishery. With the growing concern over overexploitation and the challenge to assess fisheries status in these data-limited situations, a suit of assessment approaches have been proposed. In this study, we explore the usefulness of these data-limited approaches for the multi-species and multi-gear fishery of the Kenyan coast. The primary objective was to evaluate the current level and impacts of the fishery at the species and ecosystem level and to revise current management measures. In a first step, we used the Schaefer and Fox production models to estimate the sustainable catch and effort limits of the pooled catches for the entire coastal fishery and also explored possible changes in the mean trophic level of the catch by analysing officially reported time series data over sixty years. The results indicate that the current fish extraction and effort surpass sustainable limits (MSY) and that the mean trophic level of the catch has continuously declined over the years. In a second step, the size structure of currently obtained catches from the multi-gear fishery was studied based on a case study area of the Kenyan South coast. Results reveal that the multi-species fisheriesa catches are dominated by small to medium-sized species and individuals. While these finding may indicate an unsustainable fishery, where older and larger fish have been serially depleted from the stock leading to a truncation of the size structure of the aggregated catches and a critical removal of large spawners, it is also possible that the observed pattern has emerged because of a fishers shift towards the smaller, more abundant and productive elements of the fished community. In this context, it is important to mention that catches from different gears overlap in species and sizes but also differ due to gear selectivity and spatial differences in gear use (inshore shallow lagoon versus more offshore waters). In a third step, the exploitation rates of the four commercially most important target species of the fishery were determined using lengtha based single-species stock assessment approaches. Results suggest moderate to high mean exploitation rates for all species with low spawning potential ratios, supporting the results of the above analysis of an unsustainable fishery, with some species experiencing both growth and recruitment overfishing. In a fourth step, results from the single-species stock assessment were compared to those obtained from a holistic trophic model constructed for the study area. The results from the latter suggest that the system is in a perturbed (immature) state, likely due to the very intense resource exploitation. Overall catch volumes are relatively low (4.6 t Km-2 year-1), and comparable to other intensively exploited coastal and coral reef ecosystems of the world. Our findings reveal that it may not be sufficient to rely on the current single-species management approaches such as gear restrictions and size limits for sustaining this multi-species fishery. Instead, control and reduction of the fishing effort and the establishment of specific areas closed to some fisheries may be needed if sustainable, ecosystem-based management is to be achieved. This should be done while considering the fishing impacts, the economic and social benefits within the ecosystem context