Incidence of a solitary bottlenose dolphin, Tursiops truncatus, and interactions with spinner dolphins, Stenella longirostris, in the Fernando de Noronha Archipelago, Brazil

Abstract

The Projeto Golfinho Rotador (PGR, Spinner Dolphin Project) has systematically monitored the distribution, ecology, and behavior of cetaceans in the Fernando de Noronha Archipelago (FNA) since 1990. During this period, the presence of spinner dolphins (Stenella longisrostris) was recorded in 93% of the days of the year, and occasionally another nine species of cetaceans. As of November 2021, the presence of a solitary bottlenose dolphin (Tursiops truncatus), known as Kai, was observed in the FNA. This work aims to describe the frequent presence of the solitary bottlenose dolphin in the FNA, as well as the interactions between that individual and the spinner dolphins. The records were made from two land-based platforms on the main island, and by collecting information and images from collaborators, such as sailors and divers. From 10 November 2021 to 23 July 2022, between the first and last sightings, in 223 days of study and 2,210.22 h of sampling effort, 49 sightings of the bottlenose dolphin were recorded. It was an adult, offshore ecotype, of unidentified sex and apparently healthy. On days when Kai was sighted, the spinner dolphins stayed an average of 5.84 h in the area (SD ± 3.02), while on days without sightings the average permanence was 6.20 h (SD ± 2.65). There was no effect of the presence of Kai on the permanence of the spinner dolphins (Mann-Whitney, U = 1042; p = 0.678), with a predominance of neutral interactions. According to the habituation stages currently adopted, Kai was in stage two, in which it was already known by the local population, followed some boats, but still kept its distance from humans. The existence of legislation to protect cetaceans and the action of the PGR in the community may have helped with the non-progression of the habituation stage.