Influence of net cage farming on the diet of associated wild fish in a Neotropical reservoir

Aquaculture in net cages is a well recognized strategy to meet the growing demand for food by the world's population. This type of fish production is increasing in freshwater, especially in large Neotropical reservoirs. However, it may lead to several impacts on freshwater assemblages. Thus, we aimed to determine if the artificial feed may indirectly (the load of nutrients may artificially increase production in the region and fish take advantage of this production) or directly (consumption by wild fish) alter the diet of the fish species living near the cages. We also determined if the intensity of food intake (measured by relative stomach weight) was altered by the inclusion of artificial feed. We gave special attention to the protein levels of the pellets used during the study (32% protein in the first 30 days; 28% protein after), distance from the cages (0 m, 100 m, and 400 m), and strata in the water column (surface and bottom), and to better evaluate the effect of the cage, we sampled before its installation. Overall, the dominant species changed their intake pattern of feeding, and also intensified the consumption of artificial feed. The load of organic matter (feed not consumed by caged fish) led to proliferation of microcrustaceans (which was readily consumed by some fish species), and to abrupt oscillation in the composition of food items of several species. The indirect effects were more conspicuous when the food pellet used presented higher protein level (32%), and directly when protein level was lower. The nature and intensity of the impact of food losses from aquaculture in the aquatic environment and their incorporation into the food chain still require more detailed investigation, at the individual (physiology) and population (autoecology) level, as well as higher levels of organization (communities and ecosystems).

► Natural food intake and diet composition are affected by input of artificial feed. ► Dominant fish species presented alterations in diet along the experiment. ► Direct consumption of artificial feed by wild fish increased along the study. ► High level protein pellet increased plankton density. ► Alteration in plankton availability leads to changes in the diet of local species.