(off Restorff Island, Kimbe Bay, West New Britain, Bismarck Archipelago, PNG)
This is the fifth largest species of the delphinids; both sexes reach 4 m in length and body mass may reach 500 kg (Jefferson et al. 2008). It is a pelagic species, inhabiting deep oceanic and continental slope waters 400-1,000 m deep (Baird, 2002). Resent year-round in most of its range, there may be seasonal onshore-offshore movements in some areas (Carwardine, 1995); prey on a mix of neritic, oceanic, and occasionally bottom dwelling cephalopods. This species appears to be common off the Vogelkop Peninsula (West Papua) as well as in the Kimbe Bay, West New Britain (Bismarck Archipelago); would also appear to be one of the more abundant cetaceans along the coastline of northern New Guinea. Apart from the records mentioned below there is a surprising lack of documented records of G. macrorhynchus, which is likely to reflect the lack of cetacean studies in Papuan waters. West Papua: ca. 20 individuals observed by Mörzer Bruyns off Manokwari, Vogelkop in 1971 (Rudolph, Smeenk & Leatherwood, 1997); deeper waters of the W. Dampier Strait Area as well as between Gebe Island (Maluku Province, Eastern Indonesia) and Palau Kawe, NW Waigeo in Oct-Nov 2006 (Kahn, 2007); two small pods totalling ca. 15 individuals each sighted in deep waters off Sorong on November 22nd 2007 (Borsa & Nugroho, 2010). Papua New Guinea: Miyazaki & Wada (1978a) reported six sightings of small groups (usually <10 individuals), mainly in equatorial waters north of New Guinea. The finding of Risso’s dolphin remains in archaeological material at Motupore Island, Bootless Inlet, south-eastern New Guinea, led Pernetta and Hill (1981) to consider the possibility that small cetaceans were once hunted there opportunistically; it was identified in the Kimbe Bay by Ingrid Visser in April 2003 through direct sightings as well as acoustic data (Visser, 2003) with ca. 40 individuals sighted south of Kimbe Bay on 7 April 2003, another 6-8 individuals north of Kimbe Bay and an single individual north of Cape Huessener on 10 April 2003. Both calf's and juveniles present which implies that the species reproduce in the Bismarck Sea. This species has been observed interacting with the Papua New Guinea purse seine fishery (Nicol et al. 2009).