Population structure of Dall's porpoises
Masao Amano
Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo

Although this number is not large compared with other taxa in the marine environment, cetaceans are placed at the higher trophic level in a marine food chain, and many various organisms are required for the existence of an individual cetacean. This means that we have to conserve a whole ecosystem when we would like to conserve cetaceans. We call such kind of organisms 'umbrella species'. Cetaceans are also regarded as 'flagship species', since they have popularity that attracts public attention to the conservation. Therefore, cetaceans are very important for conservation of marine ecosystem and biodiversity. Each organism plays an important role in its an local habitat as a part of local ecosystem, and we have to think conservation very locally, or in a smallest unit, i.e. population. Thus, to know population structure is the basis of the conservation. I here look over some results of the studies on the population structure of the Dall's porpoise.

Dall's porpoises are small toothed whales belong to the family Phocoenidae and distributed widely in the northern North Pacific. About fifteen thousand porpoises are hunted by the hand harpoon fishery in Japan, and an unknown number is incidentally taken by the Japanese salmon drift nets fishery operated in the Russian exclusive economic zone. The conservation and management of the species is a matter of global concern. Two distinct color morphs, dalli-and truei-types are known. The former has a small white flank patch and the latter has a larger patch. Sighting surveys found that the mother and calf pairs were found in particular areas after the calving season. It was suggested that these areas are calving grounds and different population uses each area. To test this, scientists examined morphological and genetic markers that may indicate inter-population differences. Skull morphology of the Sea of Japan porpoises was found to be different from that of other localities. Canonical discriminant analysis on external morphology clearly discriminated the Sea of Japan specimens from others. This was caused by the difference of size of the white patch, which is smaller in the porpoises from the Sea of Japan. This means that the third color morph exists in the Dall's porpoise. Although the results of genetic studies are various, they showed larger differentiation between the Sea of Japan specimens and others. The different color morphs were not discriminated by the genetic markers. These morphological and genetic results all suggested the uniqueness of the Dall's porpoises from the Sea of Japan. However, population differentiation in the dalli-type porpoises in the North Pacific and Bering Sea was not clear. The studies on the Dall's porpoise suggested that we should examine all possible markers, not depend on a small number of markers to determine the population structure. We should also consider that genetic markers are not always effective and ecological information is very important.