Abstract:
My PhD Thesis assesses the past, present, and future states and the main drivers of ecosystem change in the Gulf of Nicoya, a highly productive estuary located on the Pacific coast of Costa Rica. The 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 was already overexploited 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. 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. 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. 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. 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. 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.