Duration of scratches in Guiana dolphin, Sotalia guianensis (Cetartiodactyla: Delphinidae): Supplementary marks to improve abundance estimates

Scratches on small marine cetaceans can be produced naturally from contact with substrate, but most of them are caused by intra- or interspecific interactions. This study provides information on the duration of scratches on Guiana dolphins (Sotalia guianensis) and their potential use in photo-identification studies. The duration of scratches was estimated for twenty individuals in a resident population of Guiana dolphins in Babitonga Bay, South Brazil. The scratches were classified considering their degree of penetration in the animal’s body (1 - superficial scratches, that do not penetrate the dermis; 2 - deep scratches, which usually penetrate deeper into the dermis) and the number of lines present (1 - multiple scratches, with two or more parallel lines; 2 - single scratches, consisting of only one line). For the multiple scratches category the shortest duration was estimated at 11 days and the longest 733 days (188 ± 160). For the single scratches category, the shortest duration was 51 and the longest 369 days (173 ± 111.8). Considering that the species has a small dorsal fin with discrete permanent marks, and that a relevant proportion of the population does not have marks like nicks, we propose that scratches can be used to identify Guiana dolphins in order to estimate their abundance in specific short-term studies. This type of mark could also be used as a secondary mark for the identification of individuals, reducing the bias present in the mark-recapture method.