Interactions between cage fish farms and fishing in Brazilian semiarid reservoirs: An ecosystemic approach

Knowledge of the trophic web structure and control mechanisms in reservoirs of the semiarid tropical regions can enable a better understanding and proper management of these aquatic environments. This paper used an ecosystem approach to two Brazilian semiarid reservoirs and evaluated the interactions between cage fish farms and fishing activity. The trophic web models of the reservoirs were elaborated using Ecopath with Ecosim (EwE) software. Over two years, the data were locally obtained, including information on fish farm, fishing activity, and functional groups (phytoplankton, aquatic macrophytes, benthic fauna, and fish). The modeling results indicated that most of the system energy is derived from the detritivory chain, which was more critical than the herbivory chain, with a Detritivory/Herbivory ratio of approximately 3.0 in both reservoirs. The key group of the ecosystem was the piscivorous fish, with Hoplias malabaricus and Cichla monoculus as the species with the greatest trophic impact, especially on fish at the intermediate trophic level. The benthic community and detritus had positive impacts on almost all other components of the trophic web. The reservoirs were considered immature, with low nutrient cycling and a high Total Primary Production/Total Respiration ratio. Systemic stability was deemed low for both reservoirs, which was indicated by the low overhead values. In both reservoirs, the presence of a well-developed detritus chain supports the trophic web and represents a source of stored energy in aquatic systems. The simulations demonstrated a positive relationship between fish farming and fishing activity, with all communities benefiting from the nutrient input from cage fish farms, with a biomass increase of aquatic organisms, either by direct or indirect effects.