Sources of Notch and Scar Patterns on the Dorsal Fins of Dusky Dolphins (Lagenorhynchus obscurus)

Few studies have investigated the origins of distinctive marks on cetaceans and quantitatively evaluated the causal factors. We used photo-identification data to categorize the ecological sources of scars and notches on the dorsal fins of free-ranging dusky dolphins (Lagenorhynchus obscurus) off Kaikoura, New Zealand. Dorsal fin photographs of 1,171 individuals that had marks from ecological sources were collected from October 2011 through January 2012. Photographs of scars and notches were compared to marks of known origin, cross-validated by experts, and categorized as derived from conspecifics, killer whales, sharks, vessel strikes, fishing gear, or unknown. A total of 1,019 dusky dolphins had notches, of which 419 (41%) individuals had additional scars. A smaller subset of dolphins (152 individuals) had only scars or pigmentations as marks. The marks on the majority of dusky dolphins were attributed to intraspecific interactions (notches: 84%, n = 983; scars: 30%, n = 355). Indications of predation attempts (sharks: 0.17%, n = 2; killer whales: 0.09%, n = 1; unclassified natural predators: 0.26%, n = 3) and human impact (net/line: 0.43%, n = 4; vessel: 0.17, n = 2; unclassified human impact: 0.34%, n = 4) were comparatively low. These results are consistent with previous studies and indicate that most marks on dusky dolphins are caused by conspecifics and that predation pressure and bycatch rates are low off Kaikoura. We suggest that these data indicate that current management actions regulating commercial and recreational boating activities in the area are sufficient, with no need for immediate modification.