Mangrove degradation and its effects on estuarine fisheries: case study of Tanbi Wetland National Park, The Gambia

Abstract

Climate-induced ecosystem changes were studied in Tanbi Wetland National Park (TWNP) in conjunction with the major land use types i.e. agriculture, settlement, tourism, oyster processing, completely fragmented and partially fragmented zones. To study hydrological changes in TWNP in The Gambia, in-situ analyses of physico-chemical parameters and nutrient build-up were conducted during the peak discharge period of the River Gambia in 2013 and repeated during the peak dry season in 2014. Mangrove vegetation dynamics was assessed by remote sensing, using Landsat images of decadal time series covering 1973 – 2012. To assess seasonal changes in the fish assemblage in relation to the hydrological changes, fish were sampled at all the land use types using a seine net during the rainy and dry seasons. A questionnaire-based assessment of local knowledge on climate-induced ecosystem changes was also conducted. Average salinity in TWNP was 24.5 and 35.8 ppt for the rainy and dry seasons respectively. Mangrove vegetation cover declined by 6% while grassland increased by 56.4% from 1973-2012. 43 fish species from 25 families were recorded from 2013-2014. Fish species richness declined by 16% and abundance by 35% as the seasons changed from rainy to dry. Fish bio-ecological categories were dominated by estuarine species of marine origin (Em) (47%) throughout the year, 2014. Multidimensional Scaling (MDS) of the environmental variables indicated salinity as the main water variable influencing fish species richness in TWNP. Local awareness about the climate change concept was high (70%). However, local adaptation practices to climate-induced ecosystem changes were low (50%). Based on the low percentage loss in vegetation cover, this research concludes that except at the completely fragmented zone, the impacts of human activities on mangrove vegetation in TWNP are negligible. It also demonstrates that the decline in the fish assemblage is due to local extinctions caused by seasonal hyper-salinity.