Tracing time's footprints: exploring feeding ecology and historical changes of Mediterranean common dolphin (Delphinus delphis) over two centuries

The Mediterranean population of common dolphin (Delphinus delphis) presents an enigmatic natural history in the last centuries. Once widely prevalent in the 19th and early 20th, it faced a significant decline across most of the basin by the late 1960s. The causes and mechanisms behind this decline remain largely unknown and poorly understood. The IUCN has designated the Inner Mediterranean population of common dolphins as Endangered. Here, we analyzed stable isotopes of nitrogen and carbon (δ15N and δ13C) on skeletal remains collected in 22 Mediterranean natural history museums to investigate spatio-temporal trophic shifts and niche partitioning of the Mediterranean common dolphin over the past 200 years. We compared the isotope composition of historical common dolphins with those of the contemporary populations of Stenella coeruleoalba and Tursiops truncatus which co-occur in the Mediterranean occupying different habitats and trophic niches. We found that common dolphin individuals inhabiting the Mediterranean before 1970 occupied a significantly higher trophic level than the contemporary ones. The comparative analysis between dolphin species revealed that historically common dolphins occupied a well-defined isotopic niche separate from other species, while the contemporary populations shifted to overlap those of the striped dolphin. We cannot exclude that this significant ecological shift could be driven by habitat and trophic competition with human activities as prolonged and intense fishery stock exploitation has occurred in the basin. However, it appears that a key factor in the common dolphin's decline was competition with the striped dolphin, a widespread species in the region, that has effectively "squeezed" the common dolphin's habitat. This is the first long-term evidence of an ecological and/or trophic shift in the common dolphin Mediterranean population, which could be the starting point for understanding the causes and impacts of the species’ near-collapse in the Mediterranean.