Subsurface and nighttime behaviour of pantropical spotted dolphins in Hawai'i

Pantropical spotted dolphins (Stenella attenuata) are found in both pelagic waters and around oceanic is- lands. A variety of differences exist between populations in these types of areas, including average group sizes, extent of movements, and frequency of multi-species associations. Diving and nighttime behaviour of pantropical spotted dolphins were studied near the islands of Maui and Lanai, Hawaii, in 1999. Suction-cup-attached time-depth recorder/VHF-radio tags were deployed on six dolphins for a total of 29 h. Rates of movements of tagged dolphins were substantially lower than reported in pelagic waters. Average diving depths and durations were shallower and shorter than reported for other similar-sized odontocetes but were similar to those reported in a study of pantropical spotted dolphins in the pelagic waters of the eastern tropical Pacific. Dives (defined as >5 m deep) at night were deeper (mean = 57.0 m, SD = 23.5 m, n = 2 individuals, maximum depth 213 m) than during the day (mean = 12.8 m, SD = 2.1 m, n = 4 individuals, maximum depth 122 m), and swim velocity also increased after dark. These results, together with the series of deep dives recorded immediately after sunset, suggest that pantropical spotted dolphins around Hawaii feed primarily at night on organisms associated with the deep-scattering layer as it rises up to the surface after dark. Resume : Les Dauphins tachetes pantropicaux (Stenella attenuata) frequentent aussi bien les eaux des zones pelagiques que les eaux qui bordent les iles oceaniques. Il existe un certain nombre de differences entre les populations de ces types de milieux, notamment dans le nombre moyen de dauphins par groupe, l'importance des deplacements et la frequence des associations multispecifiques. Les plongees et les comportements nocturnes des Dauphins tachetes pantropicaux ont ete etudies au large des iles Maui et Lanai, aH awaii, en 1999. Des chronobathymetres/marqueurs radio VHF ont ete attaches a six dauphins au moyen de ventouses pendant 29 h au total. Les taux de deplacement des dauphins marques sont considerablement plus faibles que ceux signales dans les eaux pelagiques. La profondeur et la duree moyennes des plongees sont moins elevees que celles enregistrees chez des odontocetes de meme taille, mais sont semblables a celles observees au cours d'une etude sur les Dauphins tachetes pantropicaux dans les zones pelagiques du Pacifique tropical oriental. Les plongees (de plus de 5 m) sont plus profondes (en moyenne = 57,0 m, ecart type = 23,5 m, n = 2 indivi- dus, profondeur maximale de 213 m) la nuit que le jour (en moyenne = 12,8 m, ecart type = 2,1 m, n = 4 individus, profondeur maximale de 122 m) et la vitesse de la nage augmente egalement a l'obscurite. Ces resultats, ajoutes a l'observation de plongees profondes immediatement apres le coucher du soleil, indiquent que les Dauphins tachetes pantropicaux se nourrissent surtout la nuit d'organismes associes a la couche diffusante profonde, pendant sa remontee vers la surface la nuit. (Traduit par la Redaction) 996

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