Notes on two cetacean surveys in the Gulf of Tonkin, Vietnam

Abstract

During 17-26 October 1999, we searched for cetaceans along 665 km of trackline in coastal and offshore waters of the Gulf of Tonkin. We had five cetacean sightings: two of Indo-Pacific humpbacked dolphins Sousa chinensis, one of finless porpoises Neophocaena phocaenoides, one of a mixed school of pantropical spotted dolphins Stenella attenuata and probable spinner dolphins S. longirostris (subspecies unknown), and one of a probable bottlenose dolphin Tursiops sp. During 2-11 April 2000, we searched along 1,146 km of trackline in the same waters. We had four sightings: one of bottlenose dolphins (Tursiops sp.), one of spinner dolphins (probable dwarf form – S.l. roseiventris) and two of Indo-Pacific humpbacked dolphins. Although sighting conditions were often poor, our results indicate that cetaceans occur in low (but un-quantified) densities in Vietnamese waters of the Gulf. Possible reasons for this include historically sparse populations due to natural ecological conditions in the Gulf or population declines caused by anthropogenic impacts, including accidental entanglement in gillnets, reduced prey availability from overfishing, and mortality caused by fishing with explosives. During the surveys we observed push-net fishing vessels using explosives near the mouth of Halong Bay and the partial carcass of a finless porpoise that appeared to have been cut in half to extract it from a gillnet.