Seasonal occurrence of the killer whale (Orcinus orca : morphotype A) off New Caledonia
Présence saisonnière de l'orque (Orcinus orca : morphotype A) au large de la Nouvelle-Calédonie
Résumé
Little is known about the population of killer whales Orcinus orca that occurs in the Coral Sea. Only three sightings made during the decade 1991-2000 have been reported in the scientific literature; no stranding has been recorded up to 2006; no killer whale has been sighted during extensive aerial surveys all around New Caledonia from October to December 2014. However, several images and video records are available from the Internet. The objectives of the present work are to compile observations of killer whales made over the last three decades off New Caledonia in the eastern Coral Sea, to examine the temporal pattern of killer whale occurrence, and to identify their morphotype.
Accounts of killer whale sightings were obtained from interviews with yachtsmen and women, from searching the local daily newspapers Nouvelles Calédoniennes, and from searching the Internet.
Nine unpublished sightings of killer whales were recorded between 1993 and 2017. Six of these observations were supported by photographs or videographic records, making it possible to characterize individual morphotypes. All sightings but one were outside the barrier reef off the western coast of New Caledonia; one observation was made inside the southern lagoon of New Caledonia in a channel between two passes. The period of killer whale occurrence was from July to December with a peak in August, that is, mostly during the austral winter season, with an extension into the first month of the austral summer. All individuals for which there was sufficient photographic evidence of pigmentation patterns had the following characteristics: supra-ocular white patch elongated and roughly elliptical, with long axis horizontal or quasi-horizontal; its length was ~0.85 to ~1.1 times that of the basis of dorsal fin (N = 6) and ~0.22 to ~0.24 times the distance of snout to abscissa of dorsal fin origin (N = 6); long axis was ~2.8 to ~3.8 times the small axis (N = 2); grey saddle patch posterior to dorsal fin moderately large and generally moderately contrasted (N = 6); no dorsal cape visible (N = 5).
Pigmentation patterns as well as the shape and length of the supra-ocular white spot designated morphotype A previously described in Antarctic waters as the sole killer whale morphotype identified off New Caledonia. Killer whales of this lineage are presumed to mainly prey upon marine mammals, of which several species (New Zealand fur seal Arctocephalus forsteri, minke whale Balaenoptera acutorostrata, humpback whale Megaptera novaeangliae, and great sperm whale Physeter macrocephalus) occur in the Coral Sea during approximately the same period as the killer whale. Although they do not formally validate it, these observations are compatible with the hypothesis of an annual migration of morphotype-A killer whales from more-southerly waters of the Southern Hemisphere up to the Coral Sea.
La population d'orques fréquentant la mer de Corail est mal connue. Les objectifs de ce travail sont de présenter les observations d'orques faites ces trois dernières décennies au large de la Nouvelle-Calédonie, d'examiner le pattern temporel d'occurrence et de déterminer les morphotypes présents. Neuf observations d'orques, non publiées, ont été rassemblées entre 1993 et 2017, à partir d'interviews de plaisanciers, de recherches dans le quotidien local et d'une recherche sur Internet. La période d'occurrence des orques s'étalait de juillet à décembre avec un pic marqué en août. Six des observations sont documentées par des photographies ou des enregistrements vidéographiques, permettant de caractériser le morphotype. Les patterns de pigmentation ainsi que la forme et la longueur de la tache blanche supra-oculaire désignent, à chaque fois, le morphotype A décrit précédemment dans l'océan Antarctique. Cette lignée est réputée être prédatrice de mammifères marins dont plusieurs espèces (l'otarie à fourrure de Nouvelle-Zélande Arctocephalus forsteri, le petit rorqual Balaenoptera acutorostrata, la baleine à bosse Megaptera novaeangliae, le grand cachalot Physeter macrocephalus) ont une présence saisonnière concomitante en mer de Corail. Sans la valider de façon formelle, ces observations sont compatibles avec l'hypothèse d'une migration annuelle des orques de morphotype A depuis les latitudes plus élevées de l'hémisphère Sud vers la mer de Corail.
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