Skin Marks in Capitoline Dolphins Shed Light on Threats to the Population at the Tiber River Estuary (Mediterranean Sea)

Abstract

The analysis of skin marks and lesions is used for many cetacean species to assess the general health status of the populations with the purpose of developing effective conservation strategies. This study applies mark analysis to characterise common bottlenose dolphin (Tursiops truncatus) skin marks and lesions, and their variation at the individual level across years, in the Tiber River Estuary area (Mediterranean Sea, Italy), to test their efficacy as proxies of individual or population health. Using high-quality photographic data collected between 2016 and 2023 during 205 sightings, marks were identified, classified, counted and measured on 39 individuals photographed on multiple occasions. Marks related to intraspecific interactions (e.g. tooth-rakes), anthropogenic activities (e.g. signs of interaction with fishery) and health conditions (e.g. skin diseases and emaciation) were selected as indicators, and five indices were applied to estimate their extension and progression through time. Prevalent marks in all individuals were of social origin and aggressive nature. Marks related to skin diseases and emaciation were present in 97% and 70% of individuals, respectively. Almost half of the individuals showed physical signs of interaction with fishing gears. No significant temporal trends were observed. These results highlight that the local population is under the pressure of multiple stressors, mostly related to human activities, both directly (fishing) and indirectly (malnutrition, aggression). The consequences of stressor interactions may be complex to predict and raise challenges for the conservation of this protected species—and of the ecosystem it belongs to—in a highly anthropised and currently unmanaged area.