Distribution of the Rough - Toothed Dolphin ( Steno bredanensis ) around the Windward Islands ( French Polynesia ) 1

The rough-toothed dolphin (Steno bredanensis) has been described as a primarily pelagic cetacean species that is found in tropical and subtropical oceans throughout the world. Information on distributional patterns or habitat preference in most regions where S. bredanensis has been reported is limited. This study reports on the distribution of S. bredanensis around the Windward Islands of French Polynesia. Data were obtained from vessel surveys between 1996 and 2000, where rough-toothed dolphins were sighted 38 times. Group sizes of rough-toothed dolphins ranged between 1 and 35 individuals, with an average size of 10.8 individuals. When corrected for effort, results indicated that in French Polynesia S. bredanensis is found over a wide area but is more commonly distributed inshore than offshore. Rough-toothed dolphins were usually sighted 1.8 to 5.5 km from the barrier reef, in water depths between 1,000 and 2,000 m. Our results also demonstrate the year-round presence of this species around Tahiti and Moorea. Steno bredanensis has been reported in many oceanic archipelagos, and our findings may provide insight into preferred habitat and small-scale oceanographic conditions associated with regions where this cetacean species is relatively abundant. Information on rough-toothed dolphin (Steno bredanensis) distributions is limited, but published reports that include this species are quite widespread (Miyazaki and Perrin 1994). Steno bredanensis is rarely the focus of field studies but is often mentioned from a few sightings. Although it is present in almost all tropical and subtropical regions, cetacean sighting data suggest that S. bredanensis is not a particularly common species in any area (Au and Perryman 1985, Hewitt 1985, Wade and Gerrodette 1993, Miyazaki and Perrin 1994, Mullin et al. 1994). Previous reports on the distribution of rough-toothed dolphins are primarily from the eastern tropical Pacific, where very large regions of open ocean waters were surveyed. These studies have extended over 50 degrees of latitude, and for this open-ocean environment a large-scale distribution map for S. bredanensis has been generated from approximately 40 sightings (Wade and Gerrodette 1993). Other large-scale surveys in the eastern tropical Pacific indicate the presence of S. bredanensis in relatively low abundances compared with other delphinid species in that area (Au and Perryman 1985, Hewitt 1985). Cetacean surveys covering large areas of the western tropical Indian Ocean have also mapped S. bredanensis sightings; there the percentage of cetacean abundance represented by this species appears similar to that Pacific Science (2005), vol. 59, no. 1:17–24 : 2005 by University of Hawai‘i Press All rights reserved 1 Funding support for this project was provided by the Laboratoire Terre-Océan at the Université de la Polynésie française, Delegation a la Recherche of the Territoire de Polynésie française, Sea World, and donations by Hawaiian Airlines. Dolphin Quest Inc. supported K.L.W.’s Ph.D. dissertation, of which this work is a part. Manuscript accepted 20 February 2004. 2 Laboratoire Terre-Océan, U.P.F., BP6570 Faaa Aeroport, Tahiti, French Polynesia. 3 Centre de Recherche sur les Cétacés, Marineland, 306 avenue Mozart, 06600 Antibes, France. 4 University of Hawai‘i, Department of Physiology, John A. Burns School of Medicine, 1960 East-West Road, Biomed T-608, Honolulu, Hawai‘i 96822 (phone: 808-230-8976; fax: 808-956-9722; e-mail: kristiwest@ earthlink.net). 5 Corresponding author. in the eastern tropical Pacific and the Gulf of Mexico, where this species is also present (Ballance and Pitman 1998). An aerial cetacean survey in the Gulf of Mexico documented only a single S. bredanensis sighting but in a water depth very similar to that of the inshore areas of French Polynesian islands (@1,000 m) (Mullin et al. 1994, Gannier 2000). Although S. bredanensis distributions have been assessed over the wide eastern tropical Pacific and this may allow for insight into preferred habitat associated with largescale oceanographic conditions, smaller-scale studies on the distribution of this species have yet to be undertaken. In French Polynesia, cetacean surveys have indicated that S. bredanensis is the second most commonly observed species after Stenella longirostris (Gannier 2000). This provides an opportunity to investigate the habitat preference of this species around small oceanic islands. Based on multiple sightings of rough-toothed dolphins and on sampling effort spread over 4 years, this study reports on seasonal and temporal variation in distribution around the islands of Tahiti and Moorea. materials and methods