Modelling two decades of change and future developments in the Gulf of Nicoya (Costa Rica) ecosystem using the Ecopath with Ecosim toolbox

The Gulf of Nicoya is a highly productive estuary located on the Pacific coast of Costa Rica and accounts for the country’s primary fish production. It is increasingly impacted by its fisheries and climate variations, urbanization, tourism developments, the input of pollutants from urban centres, and agriculture activities in the catchment areas of large rivers entering the gulf. Thus, our study intensively assesses the Gulf of Nicoya’s past, present, and future states and the main driver of ecosystem change. The state of the ecosystem, food-web dynamics, detectable patterns in fishing efforts, and the external drivers’ influence on the system were analyzed. Also, the economic impacts of fishing effort regulations were simulated to test and propose possible management plans. A spatial model was also built to test the effect of a temporary fishing closure on the stock recovery of target species in the Gulf of Nicoya and to propose management alternatives.
In the early 1990s, ecological and fishery data from the Gulf of Nicoya were holistically analyzed, and a trophic model was constructed using the Ecopath modelling approach. The results indicated that this tropical estuary, a hotspot for Costa Rican fisheries, was already overexploited by shrimp trawlers and longline fleets, and recent observations suggest further deterioration in this system. To evaluate the ecosystem and socio-economic changes in the Gulf of Nicoya over the last 20 years, the 1993 model was reconstructed with data from 2013 to compare both system states. Although the summary statistics of both states (i.e., 1993 and 2013) suggest that the system maintained its general functionality and even enabled the total harvest to increase by approximately 20%, a more detailed analysis of the levels of the functional groups led to the conclusion that the system was further degraded, causing a tremendous economic loss of approximately 50%. Like other coastal ecosystems, the Gulf of Nicoya is impacted by its fisheries, anthropogenic activities, and pollution from the large rivers entering the gulf. Thus, the gulf is particularly sensitive to short- and long-term changes in the climate, especially during extreme conditions in El Niño periods. For further analysis, we used a holistic approach to examine the impacts of fishing and environmental changes to identify the main drivers in the observed changes in the Gulf of Nicoya ecosystem. While the model simulations indicate that fisheries’ exploitation rates mainly drive variations in the catchment, several species are also substantially affected by climate variations. This factor must be considered alongside implementing fishing regulations to form and develop appropriate management strategies. This dissertation tested the effectiveness of different management scenarios applied to the Gulf of Nicoya, including top-down and participatory fishing policies. Besides testing the current policy (which bans shrimp trawling), we used the automated fishing policy search tool of the EwE software to explore an alternative, optimized management scenario. The analysis indicates that the ban on trawling is important for the ecosystem’s recovery. However, additional measures might be needed, such as reducing fishing efforts by semi-industrial purse seine fleets and artisanal longline fleets. We present two possible alternative management scenarios with different balances of ecosystem conservation and economic losses. We complement our extensive analysis by adding a spatial component using the Ecospace tool, which can simulate species’ spatial distribution and fishing fleet effort over time. One obvious application is the assessment of the efficiency of existing and potential locations of MPAs to explore spatial management options and address various research questions, including the regional consequences of climate change. The approach was used to test the effectiveness of the existing annual seasonal fishing closure in the gulf (known as “Veda” in Spanish) and to tackle further questions. For example, is excluding all fleets from the gulf necessary during fishing closure? Would a seasonal closure covering the entire area of the Gulf of Nicoya be more efficient?
This intensive assessment of the state of the Gulf of Nicoya ecosystem leads to the conclusion that a drastic decrease in fishing effort, especially of semi-industrial fleets, is needed to maintain the ecosystem’s functionality, rebuild the biomass of target species, and strengthen its resilience to environmental change. In this process, the enforcement of the fishing closure will play an important role.