Beyond the Meal: Trophic Controls by Pelagic and Demersal Chondrichthyes in Two Different Mediterranean Marine Food Webs

Pelagic and demersal Chondrichthyes can assume different patterns of trophic controls on marine food webs, sustaining the functioning of marine ecosystems. These species are impacted by fisheries requiring conservation measures to mitigate the loss of their ecological roles. Amass-balanced modelling approach based on the Ecopath routine was adopted to investigate the trophic roles exhibited by Chondrichthyes through a comparative analysis of two food webs (Calabrian and Salento) within the Northern Ionian Sea (Central Mediterranean Sea). A total of 10 functional groups (FGs) of pelagic (3) and demersal (7) Chondrichthyes were represented in the models. Five ecological indicators were adopted in the analysis of Chondrichthyes: fractional trophic levels (TL) and their variance expressed by the Omnivory index; the importance of FGs as keystone species through the keystoneness indices and their trophic controls played on discrete TLs through the Mixed Trophic Impact analysis; the exploitation rates of 7 fishing gears and their direct and indirect impacts on target groups. Pelagic sharks showed high TLs and a generalist trophic spectrum, except for the planktivorous basking shark (TL = 3.2). Changes in their importance as keystone species between demersal and pelagic sharks were observed between the two investigated areas affected by different ecosystem traits. The blue shark exerted direct top-down controls on their prey located in the fourth TL, while the kitefin shark (demersal apex predator) showed positive top-down cascading impacts with indirect effects on the FGs of the lower trophic levels. Demersal elasmobranchs played the role of meso-predators, exhibiting negative and positive effects in both food webs. Bottom trawl and drifting longline showed the most negative direct and indirect impacts on demersal and pelagic elasmobranchs, respectively, and fishing overexploitation was estimated for pelagic and bathyal demersal sharks Conservation measures are required to protect these species and their ecological roles in marine ecosystems.