Samoa Research Project – Spinner dolphins (Stenella longirostris) site fidelity, behavior and habitat use along the north‐western coast of Savaii Island, Samoa – Study by Juney Ward

Juney Ward, a postgraduate student at the University of the South Pacific, recently carried out the first dedicated cetacean research in the Samoan waters as part of her postgraduate study. The WDCS funded research was designed as a contribution to the Pacific Cetaceans MoU, and documented the site fidelity, demographic composition, behavior and habitat use of spinner dolphins (Stenella longirostris) in the north‐western coast of Savaii Island, Samoa. Most spinner dolphin sightings were recorded close to the shore and at various sites. However, in comparison with other areas, Falealupo and Sataua areas had much higher encounter rates of dolphins.

Figure 1: Distribution of cetaceans sighted/encountered in the Samoan waters during the spinner dolphin research by Juney Ward.

The field work was carried out over a period of 17 days (23 March – 12 April 2011). Over 13 survey days, Juney recorded a total of 35 cetacean groups – spinner dolphins (28), sperm whales (3), short‐ finned pilot whales (2) and two groups of unidentified cetaceans. From the 94 distinctive spinner dolphin dorsal fins photographed and catalogued, eight were sighted in previous years. Juney found the average group size of spinner dolphins to be 23.8 (CV=19.03). The dominant behavior observed for spinner dolphins were resting and traveling; these were especially prominent early in the morning when the spinner dolphins came close to the shore. 71% of spinner dolphins sighted were in the mornings; most of the encounters and observation were made from 7.00am to 9.30am. In addition, the sperm whale, short‐finned pilot whales and the unidentified cetacean sightings were also made in the morning.

Figure 2: Active and playful behaviour of spinner dolphins in the Samoan waters.

Figure 3: Encounter with short‐finned pilot whale.

Juney reported spinner dolphin encounters at four sites throughout the study: Falealupo – 50%; Sataua – 18%; Vaisala – 14% and Asau – 14%. Only 4% of spinner dolphin encounters were observed at Neiafu. The pod sizes observed in the mornings were larger compared to those observed later in the day. Overall, the pod sizes ranged from 80‐90 individuals to only 2 individuals.

Figure 4: An arcuate leap by a spinner dolphin while bow riding.

Juney’s findings on the spinner dolphins in Samoan waters indicate that the north‐western coast of Savaii Island supports a sufficient density of spinner dolphins. The daily movement and habitat preference of spinner dolphins in Samoan waters were observed to be similar to those observed in Hawaii and French Polynesia; in the mornings, they were observed to be mainly resting and swimming slowly in the same direction for hours and in the afternoons, they were more active – frequently leaping and swimming with the resting behavior to a minimal. Juney’s study also concluded that in addition to the presence of spinner dolphins in Samoan waters, other cetacean diversity and presence (sperm and short‐finned pilot whales observed) can be found year round in the Samoan waters.

This is important information for Samoa as it considers its national conservation activities and as well as those of the Pacific Cetaceans MoU. WDCS and researchers in the Pacific look forward to continuing this supportive work for MoU Signatories.