Modelling the trophic roles of the demersal Chondrichthyes in the Northern Ionian Sea (Central Mediterranean Sea)

The maintenance of natural capital and the stability of regulation services of marine ecosystems has proved to be linked to the conservation of key species supporting the ecosystems function. Chondrichthyes are key top-predators and their removal from marine ecosystems due to fishing exploitation could lead to changes in species interactions and biomass that can negatively influence the provision of ecosystems services. The ecological role of the demersal Chondrichthyes living in the marine food web of the Calabrian area in the Northern Ionian Sea (Central Mediterranean Sea) was analysed by means of ecological indicators derived from Ecopath mass-balance food web models estimated in their stationary states during the periods 1995–1997, 2003–2005 and 2013–2015. The trophic web model was described by 57 functional groups (7 of which representing the demersal Chondrichthyes), which were described by their Biomass (t/km2), Production and Consumption rate, Diet, Landings (t/km2) e Discards (t/km2). Remarkable changes in biomass were recorded for shark and rays, as a consequence of changes in fishing patterns and oceanographic regime shift named Bimodal Oscillating Systems (BiOS). Demersal Chondrichthyes occupy high trophic positions in the Calabria food web showing trophic levels higher than 4.1, with the D. licha identified as the main apex predators, while E. spinax and G. melastomus as meso‑predators. The pattern of the consumption flows highlighted a potential sensitivity of E. spinax and G. melastomus to the changes in deep waters circulation caused by the BiOS acting on the entire ecosystem of the Ionian Sea. D. licha was identified as the main keystone predators in all the investigated periods. The analysis of the trophic indirect and direct impacts shows the role of the demersal Chondrichthyes as beneficial predators, with positive effects on several preys. Results also showed that G. melastomus and the Shelf Elasmobranchs group (e.g. Raja asterias, Mustelus mustelus, etc.…) were overexploited in 1995 and that all groups of sharks and rays underwent to a decrease of exploitation rate over time. Chondrichthyes showed medium-low values of overlap with the fishing gears (<0.50), with the more relevant values estimated between the shelf elasmobranch group and passive nets and other gears. According to the findings of this work, the protection of Chondrichthyes could combine biodiversity conservation and fisheries management objectives. Therefore it is envisaged the implementation of appropriate spatial planning and conservation measures for these top-predators to guarantee valuable and sustainable flows of ecosystem goods and services.