Stranding, entangling, and sighting records of marine mammals along the beach and offshore regions of Niigata Prefecture, Sea of Japan, were compiled from January 2008 to May 2009, in order to better understanding the relationship between the species characteristics and oceanographic conditions. As a result, 119 instances, comprising 12 stranding, 5 entangling, and 102 sighting instances were noted. These instances consisting of 45 Niigata~Ryoutsu route, 44 Naoetsu~Ogi route, and 3 Teradomari ~ Akadomari route observed by Sado Line (Sado Kisen) car ferry boats, jetfoils and a rapid-transit boat. The species composition was 7 species of cetaceans (minke whale, Baird’s beaked whale, Stejneger’s beaked whale, Dall’s porpoise, Bottlenose dolphin, Pacific white-sided dolphin, and Risso’s dolphin) and 2 species of pinnipeds (northern fur seal and harbor seal). Compared with previous years, an increase in the incidences of sighting of Dall’s porpoises was remarkable. Minke whales represented a single species of entangled animals among the recorded marine mammals. It seems likely that a small number of stranding records in 2008 was related to a few heavy storms occurred in the central region of Sea of Japan in winter season.
A male Risso`s dolphin (Granpus griseus) was found stranded at Kusｈima, Miyazaki, Japan (E:131.20, N: 31.24) on 19 March 2009.
The tympanic bulla was found to be infested bilaterally by about 10 parasites (Crassicauda grampicola). Histo-pathological studies revealed severe damage of the cochlear nerve without inflammatory signs in the bilateral cochlea. In the soft tissue around the periotic bone, many eggs and the round sliced bodies of the nematode and larva were observed.
It was concluded that the hunger and weakening due to parasitogenic loss of the echolocational function may be one of the causes of single stranding-death of dolphins.
From the sighting survey in the Tsugaru Strait, which is located between Hokkaido and Honshu in northern Japan, it is known that Pacific white-sided dolphin (PWS) is the most common cetacean species in this strait, and is mostly observed from April to June. In this study, to show temporal changes of school size of PWS in the strait, the data were analyzed which is obtained from sighting surveys on a ferry between Hakodate, Hokkaido and Aomori, Honshu from May 2003 to September 2009 . The school size of PWS in March to July was analyzed by using smooth spline. Its school size was large in April when PWS came to the strait, small in May to the beginning of June when PWS fed on prey in the strait, and large in mid-June again when PWS went out of the strait. These result supported the past study.
Following a previous paper (Honma et al. 2010), sighting records of whales by Sado Kisen (Sado Line) jetfoils and car ferry boats operating on the Niigata-Ryoutsu-route, Sado Strait, Sea of Japan, were compiled from January to December, 2009. Comparison was made among the instances of previous years investigated 1994－2008. One hundred forty-one instances, comprising 39 car ferry and 101 jetfoil observations were enumerated. These prominent numbers were the greatest than those of previous years. The site of sightings was concentrated between the points 2.0 and 4.5 in the Sado lane, divided into 5 points. The greatest number of sightings was in April, one month earlier than those of ordinary year, and, the decrease in number was remarkable since August. The time zone of higher frequency was concentrated in 10 a.m and 4 p.m , but sighting records were thoroughly obtained in the daytime. It was very usual that only single individual was observed by a sighting. The greatest numbers of one school was 20 to 30 individuals for one occasion. The directions in which the whales were swimming (= migrating route) were noted 84 instaces, the largest than those of previous years, consisting of 34 north, 47 south, and 3 others. The species identified by a crew was the Minke whale,as the largest frequencies, Baird’s beaked whale, Risso’s dolphin, and killer whale, successively. There was no collision with cetaceans and other unknown large subjects in 2009.
Historical and recent records of western population of the gray whale Eschrichtius robustus were reviewed. The range of occurrence of the gray whale is from Bering Sea off Kamchatka and Okhotsk to South China Sea off southern China. The waters of occurrence for the gray whales were almost limited to the shallow seas along the coasts, but gray whales were exceptionally recorded in the deep sea with 4000 to 5000m in depth off Touhoku district. Although original feeding ground of gray whale was reported to be in northern Okhotsk Sea, recent feeding ground seems to be limited to the coast of northeast Sakhalin. According to documents published in Meiji era, it seems that the coasts around Hokkaido were feeding grounds for the gray whale. Two migration routes, a rout along continental coast of the Sea of Japan and along the east coast of Japan of the Pacific side have been known. As for the former route, because the last record was in 1989, it seems that the gray whale hardly uses this road recently. On the other hand, as for the latter route, for there have been sightings, catches by set nets and strandings since 2000, the gray whales recently tended to use this route. In addition, as four records were recognized along the west coast of Japan since the 1950’s, it seems that there is a third migration route along the west coast of Japan. As the records on the third route are fewer than those on the route along the east coast of Japan of the Pacific side, it is suggested that main rout of recent migration of the western gray whale population is along the east coast of Japan of the Pacific side. The calving area of the western population of the gray whale is likely to be between off south end of Korea and off southern China. However, as the records of the gray wale breeding in this water were scarce, further investigation is necessary.