Evaluation of the comprehensive feeding strategy and trophic role of overexploited mesopredator species in the Sea of Marmara (northeastern Mediterranean)

Abstract

Mesopredator species play an essential role in balancing and sustaining ecosystems through their interactions with various species and with their habitats. In ecosystems that have been subjected to overfishing, it is essential to obtain information on the trophic ecology of such predators. Based on this motivation, this study aimed to investigate the feeding strategies and trophic roles of four demersal sharks (e.g., Mustelus mustelus, Mustelus asterias, Squalus acanthias, and Scyliorhinus canicula) by analyzing their stomach contents and using stable isotope (δ13C and δ15N) analysis. These shark species are subject to overfishing, and their landings have decreased by 98% over the last 18 years in the Sea of Marmara. The results demonstrated that interspecific differences in the main prey resources exist among those sharks. Mustelus mustelus and Mustelus asterias feed mainly on crustaceans, and their diets are very similar. However, Mustelus asterias utilizes a specialist feeding strategy while Mustelus mustelus utilizes a medium-state generalist strategy. Squalus acanthias occupies a high trophic level and mainly feeds on teleosts over both short and long time scales. The probability of isotopic niche overlap of Scyliorhinus canicula with the isotopic niches of S. acanthias was the highest (70.7%), which indicates that both species exploit similar food resources within the same environment. These results contribute to the knowledge of the ecological role of these highly vulnerable cartilaginous species and may help to improve conservation programs in the Sea of Marmara, a lesser-known part of the northeast Mediterranean.