Diet and isotopic niche dimensions of predatory fish in two estuaries with different degrees of anthropogenic disturbances: a case study of Wami and Pangani Tanzanian coastal estuaries

Diverse anthropogenic activities including alteration of hydrological regime and agricultural development in the upstream of the river catchments modify the structural components and ecological roles of the species in estuarine ecosystems. The present study compared the diet, carbon (δ13C) and nitrogen (δ15N) isotopic signals, trophic niches and metrics of trophic redundancy of predatory fish and their potential phytodetritivore prey-fish between two estuaries with different degrees of anthropogenic disturbances. The δ13C and δ15N signals, Bayesian mixing models and stomach content analyses were adopted. It was revealed that the diet among the predators Arius africanus, Epinephelus coioides, Sillago sihama and Pomadasys argenteus and their isotopic signals including their phytodetritivore-prey, Valamugil buchanani were significantly different between Wami and Pangani estuaries (PERMANOVA, Pseudo –F ≥ 3.1, p = 0.05). The total isotopic niche area between estuaries accounted by the comparable Bayesian standard ellipse area (SEAb) was significantly smaller in Wami than that of Pangani (t-test, t = 4.3, p < .001). The ANOVA test further confirmed significant variation in SEAb among fish populations in two estuaries (F = 27.84, p < 0.001). The Layman metric indices of trophic diversity and redundancy of Wami estuary were substantially smaller than those observed at Pangani estuary. These findings mean that despite Wami estuary being under conservation status, the extent of disturbances in the upstream is high enough to induce the ecological changes at the base of the food web, subsequently cascading its effect up to the higher-order consumers of the ecosystem. As a result, the ecological redundancy and ecosystem complexity of Wami is somewhat compromised relative to that of Pangani estuary. Therefore, the degree of anthropogenic disturbances in river catchments affect differently the trophic niches of predatory and phytodetritivore fish and hence overall food web structures, trophic redundancy and complexity of estuarine ecosystems.

The Estuarine Ecological Knowledge Network: Future Prospects

Estuaries are profoundly rich, diverse, and complex ecosystems, and crucial to the overall health of Earth's oceans. Estuarine ecological complexity is matched by tremendous human cultural diversity. In the United States, millions of people live in estuarine environments from the Gulf of Mexico to the Arctic—many of whom directly depend on the productivity of marine resources in both commercial and subsistence fishing activities. Yet, estuaries are also among Earth's most threatened landscapes against the backdrop of global warming, sea-level rise, agricultural and industrial pollution, habitat loss, overfishing, and so on. This represents a looming disaster for our oceans at a global scale. The Estuarine Ecological Knowledge Network (EEKN) is based on the idea that fishing communities living within major estuaries are the key to ensuring the health of global oceans. Coastal fishing communities have vast accumulations of ecological knowledge about the functioning of estuarine ecosystems and interact with those ecosystems in intimate ways on a daily basis. This network is designed to connect coastal communities in monitoring the health of estuarine ecosystems and in using traditional ecological knowledge to develop strategies for enhancing ecosystem health and resilience.

Assessment of Ecosystem Services and Their Drivers of Change under Human-Dominated Pressure—The Meghna River Estuary of Bangladesh

The Meghna river estuary has had substantial importance in supporting the coastal community’s livelihood for decades, but the pressure on it is immense due to many anthropogenic drivers. The present study aimed to assess its ecosystem services (ES) according to the framework of the standard international classification of ecosystem services (CICES). This study also identified the drivers, pressure, state, impact, and responses (DPSIR), and the factors responsible for ES changes in the Meghna river estuarine ecosystems. By merging both quantitative and qualitative data, a total of 19 sub-categories of ES were found, among which eight are provisioning, seven are regulatory, and four are cultural services, according to the CICES framework. From the results, it can be concluded that food provisioning from the Hilsha fishery, the national fish as a part of cultural heritage, and nursery habitat functions were the top provisional, regulatory, and cultural services to the society in the study areas and beyond. However, several threats and stressors of both anthropogenic and natural origins were identified as drivers of ES changes, such as overexploitation, destructive and illegal fishing, heavy river bed siltation, and natural hazards such as extreme cyclonic events, floods, and sea levels rising. This study underlines the urgency of research and policy attention to address the challenges, and of transforming management regimes to an ecosystem-based approach, which is part of nature-based solutions according to the International Union for Conservation of Nature (IUCN), which refers to the present case, and particularly to fishery co-management. Creating alternative income sources and raising community awareness regarding the importance of maintaining the healthy condition of the river basins, and comprehensive compliance with the rules and regulations are proposed in order to ensure these estuarine ecosystems’ sustainability.