Food web model to assess the fishing impacts and ecological role of elasmobranchs in a coastal ecosystem of Southern Brazil

Large sharks generally occupy the highest trophic levels and are important links in the structuring of trophic networks. However, the decline of these predators has been observed worldwide, with population reduction rates reaching over 90% for some species. Through these declines, it is believed that the effect of the loss of predators could result in the disturbances in ecosystems. Based on this, the present study aimed to test the role of elasmobranchs in a coastal ecosystem in southern Brazil. Also, we tested if the changes in fishing effort cause changes in trophic structure. For this, a trophodynamic model was performed, based on reliable data on biomass and species feeding in the region. The mass balance modeling software Ecopath with Ecosim (EwE) was used to assess the trophic position of sharks and rays, and to understand what their interactions are with other groups in the ecosystem. Among the parameters of the model, the Paraná coast showed to be an immature system, with high connectivity and medium complexity. The trophic levels of the coast varied from 1 to 4.4, with level > 4 composed of sharks, and rays belonging to the third trophic level. Sharks were strong impacted by fishing. According to changes in fishing effort simulations, we observed changes of intermediate levels due to the decline of large sharks. However, no cascade effects were observed from the simulations. The high diversity of the system, feeding overlap, and the redundancy between mesopredator and other teleosts can decrease the chances of a cascade effect occurring.