A model of size-spectrum dynamics to estimate the effects of improving fisheries selectivity and reducing discards in Mediterranean mixed demersal fisheries

Size-spectrum models are good candidates to examine the effects of fishery management because predation and fishing are largely body-size dependent. We examine the effects of increasing trawl fisheries’ selectivity through the application of a size-spectrum model to a “continental shelf system” in the NW Mediterranean. This system is sustained by detritus, as background resource, and by benthic invertebrates that channel the energy to the fish community. The “continental shelf system” in our model consists of three components: 1) demersal and benthic fish and invertebrate species modelled by size spectrum dynamics, 2) carrion and 3) detritus. The model was able to exemplify the effects of changes in fishing patterns on the different biological components of the shelf system. According to the model outputs, in the short term the main target species, hake and red mullet, would be the main beneficiaries of the increased net selectivity and reduction of fishing effort. Discards reduction would have negligible effects. Despite the positive outcomes, this exercise was not exempt of challenges, mainly due to the data-demanding nature of the approach applied to a system with high diversity of life-histories and feeding strategies of invertebrate and fish species. And yet, our work is a first and crucial step to understand the size-spectrum dynamics of a continental shelf ecosystem subjected to fishery activities