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

The Mediterranean population of the common dolphin (Delphinus delphis) presents an enigmatic natural history in the last centuries. Once widely abundant in the 19th and early 20th, the species faced a significant decline across most of the basin by the late 1960s. The causes and mechanisms behind this decline remain l poorly understood. The IUCN classifies 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 Mediterranean natural history museums to investigate spatio-temporal trophic shifts and niche partitioning of the Mediterranean common dolphin over the past 200 years. Comparisons with contemporary populations of Stenella coeruleoalba and Tursiops truncatus which co-occur in the Mediterranean, revealed a marked change in the common dolphin's ecological history. We found 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 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 widespread striped dolphin, which has effectively "squeezed" the common dolphin's habitat. This is the first long-term evidence of an ecological and/or trophic shift of the Mediterranean common dolphin, which could be the starting point for understanding the causes and impacts of the species’ near-collapse