An analysis of dorsal edge markings in short-beaked common dolphins (Delphinus delphis) from the Bay of Gibraltar and the Moray Firth

In the present study, short-beaked common dolphins (Delphinus delphis L.) from the Bay of Gibraltar (GIB) and the Moray Firth (MF) were examined to document the relative frequency, distribution and shape of dorsal edge markings (DEMs) in the species and investigate potential causes for their occurrence. A dorsal fin layout system was used to map the relative positions and shapes of presenting DEMs along the anterior/posterior and upper/lower fin margin from 617 animals. A total of 1989 DEMs were extracted from the combined datasets, with individuals exhibiting between one and 11 nicks (median = 3). DEMs (in the form of tears, nicks, notches and indents) were primarily observed along the posterior trailing edges of fins, with the highest concentration being recorded in the upper region of the posterior fin (80.3%). Approximately 80% of all DEMs were round or rectangular in shape. Square (notched) and indented nick shapes were further recorded, but in significantly lower numbers. In contrast to all other nick shapes however, indented DEMs predominantly occurred along the anterior fin margin. Both natural and anthropogenic sources were implicated as causes of DEMs in the dataset. Interactions with fisheries were apparent in both GIB and MF animals and evidently present a threat to both populations. Indeed, the occurrence, prevalence and type of presenting DEM and/or fin injuries (e.g. missing fins or disfigurements) in the species may provide a useful measure of the type and intensity of fisheries interactions affecting different populations.