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The Mediterranean subpopulation of common dolphin ( Delphinus delphis) is classified as endangered by the IUCN. Still, information about their diet in the Mediterranean is scarce. Stomach contents of 37 common dolphins stranded in the Alboran Sea and Strait of Gibraltar were analyzed. A total of 13,634 individual prey of 28 different taxa were identified. For fish, Myctophidae was the most important family as indicated by the highest index of relative importance ( IRI = 8,470), followed by the family Sparidae ( IRI = 609). The most important Myctophidae species was Madeira lantern fish ( Ceratoscopelus maderensis) and for Sparids, the bogue ( Boops boops). Cephalopods, instead, were found in low quantities only with 31 prey from the Loliginidae, Ommastrephidae, and Sepiolidae families. Overall, our results indicate that common dolphins are mainly piscivorous (99.77% N, 94.59% O, 99.73% W), feeding mostly on mesopelagic prey. Although common dolphins inhabit mainly coastal waters in the study area, the narrow continental shelf seems to facilitate the availability of Myctophids and other members of the mesopelagic assemblage to dolphins when the assemblage migrates to the surface at night. Our results represent the first attempt at quantifying the diet of this predator in the Alboran Sea and Strait of Gibraltar. [ABSTRACT FROM AUTHOR]

Highlights • Spatial and trophic niche play an important role in niche partitioning in cetaceans. • Cetacean species segregate spatially throughout the bathymetric gradient. • Isotopic niche spaces segregate between small and large-sized cetaceans. • Investigating niche partitioning in different niche axes is desirable/recommended. • Niche partitioning may explain the high abundance of cetaceans in the Alboran Sea. Abstract Co-occurring species are expected to distribute themselves unevenly throughout ecological niche dimensions to avoid competitive exclusion. However, few studies have previously investigated spatial and trophic factors structuring an entire cetacean community. Here, we combined density surface models (DSMs) with two dimension (δ 15N and δ 13C) isotopic niche spaces in order to identify the mechanistic processes underlying niche partitioning for the most abundant cetacean species inhabiting the Alboran Sea: the long-finned pilot whale (Globicephala melas), the Cuvier's beaked whale (Ziphius cavirostris), the Risso's dolphin (Grampus griseus), the bottlenose dolphin (Tursiops truncatus), the striped dolphin (Stenella coeruleoalba), and the short-beaked common dolphin (Delphinus delphis). DSMs provide a spatially-explicit assessment of species distribution through key spatial and environmental gradients, whereas isotopic niches characterize habitat and resource use. Our isotopic niche approach pointed to habitat and/or trophic segregation between the small (striped and short-beaked common dolphins) and large-sized cetacean species (Risso's and bottlenose dolphins, and long-finned pilot whales). Conversely, DSMs suggested a larger degree of spatial segregation among species by depth, with some overlap for offshore species (long-finned pilot, Cuvier's beaked whales and Risso's dolphins) and also between bottlenose and common dolphins. Thus, both components of the ecological niche apparently played an important role in explaining niche partitioning among species, which, in turn, might explain the high abundance and diversity of cetaceans in the Alboran Sea. Further, when both methodologies were applied in isolation, the structure and functioning of this cetacean community was poorly resolved. The combination of both approaches is therefore desirable when investigating niche partitioning among ecologically similar species within communities. [ABSTRACT FROM AUTHOR]