PHOTO-IDENTIFICATION OF ROUGH-TOOTHED DOLPHINS OFF LA GOMERA (CANARY ISLANDS) WITH NEW INSIGHTS INTO SOCIAL ORGANISATION
Off La Gomera (Canary Islands), rough-toothed dolphins are present year round and distributed relatively close to shore. Photo-ID research was conducted from 2000 until 2003 on board of whale-watching vessels of a local operator. Rough-toothed dolphins showed several distinct features suitable for individual identification, such as notch patterns on the fin, global fin shape, pigmentation and distinct scratches. 63 individuals were identified. These were ranked according to the quality of obtained photographs and according to the recognisability of markings, thus measures of reliability for the re-identification of individuals were created. Changes of markings over time occurred, with colour/pigmentation patterns, global fin shape and notch patterns on the dorsal fin being the most reliable ID features. All identified individuals were included into the world-first electronic ID catalogue of rough-toothed dolphins. 65% of identified individuals were seen within more than one year, 37% within three or four years, strongly suggesting a resident population. This has several implications for management and conservation of this species in the Canary Islands. The formation of very tight subgroups is an outstanding behavioural peculiarity of rough-toothed dolphins. Subgroup composition was found to be dynamic, with subgroup sizes of 2-8 animals. The Half Weight Index (HWI) was used to asses non random associations between individuals. HWI values of up to 0.89 indicated the existence of a structured organisation of rough-toothed dolphin groups/populations. Association patterns also showed that this species has both strong social bonds between mother and calf/juvenile and between individuals of different age classes. With this first of its kind study the use of whale watching vessels for photo-ID studies was proven to be feasible with this species. Moreover, the study shed first light social life of a still not well understood species.