Pacific white-sided dolphin (PWS) Lagenorhynchus obliquidens is the most common cetacean species found in the Tsugaru Strait, Japan. It is suggested that PWS use this strait as a feeding ground and Japanese anchovy is one of the major prey species. PWS also feed Walleye Pollock, Squids and Salmonidae. The population of PWS migrated in the Tsugaru Strait fluctuate every year. In this study, the PWS population migrated in the Tsugaru Strait from 2004 to 2009 were estimated. More over, the relationship between the population of PWS and an abundance of Japanese anchovies was examined. The estimated population fluctuated between 2,000 and 8,000 every year. A significant correlation was found between the PWS population and the abundance of Japanese anchovy Pacific stock (r²=0.75, P=0.0249). Regression analysis between the PWS population and catches of other prey species, such as Walleye Pollock, Cherry salmon, Japanese common squid and Japanese anchovies in the Tsugaru Strait, and the surface water temperature were also conducted. As a result, no significant correlations were found. The fluctuation of PWS population migrated in the Tsugaru Strait was affected by the population of Japanese anchovies in the Pacific Ocean. It suggested that the majority of PWS in the Tsugaru Strait are migrated from Pacific Ocean.
Following the previous paper (Honma and Iwashita 2010), sighting records of whales by Sado Kisen (Sado Line) jetfoils and car ferry boats operating in the Niigata-Ryoutsu-route, Sea of Japan, were compiled from January to December in 2010 and 2011. Comparison was made with the incidences of previous investigated years, 1994-2009. 32 incidences comprising 8 car ferries and 24 jetfoil, and 81 incidences comprising 29 car ferries and 52 jetfoil observations were enumerated in 2010 and 2011 respectively. 2011 ranks the second in terms of number of incidences since 1994. The sites of sightings were concentrated between points 2.0 and 4.5, particularly between point 3.0 and 4.5 of the Sado Lane which was divided into 5 points. The greatest number of sightings was concentrated in May, similar to those in ordinary year, whereas a drastic decrease in number was remarkable since July. In 2010, high frequency was concentrated at 10 a.m. and 2 p.m., but sighting records were thoroughly obtained in the daytime. Single whale observation was common in a sighting. The greatest number of one school was ten and odd individuals for one occasion. 26 incidences were noted in the directions which the whales were swimming in 2010, consisting of the same number of north and south directions, whereas 30 in 2011, 16 north and 14 south. Two species of whale were identified by a crew, namely Minke whale and Risso’ s dolphin. There was no collision with cetaceans and other unknown large objects in both 2010 and 2011.
Dalls’s porpoise, Phocoenoides dalli stranded in Hokkaido were studied for their stomach contents. Seventeen individuals were reported from May to July in 2008. This is biggest number to have been reported within such a short period of time in the history of stranding recorded in the area. Nine and ten individuals were reported in 1997 and 2005 respectively but neither clear details nor stomach contents were recorded. The paper reported the stomach contents of Dall’s porpoise and clarified the species of their prey. The stomachs were taken from thirteen individuals. Six individuals had empty stomachs. These individuals were small in size compared to individuals with partly filled or filled stomachs. To investigate the quantitative composition of the stomach contents, frequency of occurrences and composition of the prey species were determined. The stomach contents revealed that the porpoises fed mainly on Gonatidae species. This was different from the previous studies in which walleye pollock Theragra chalcogramma was the main prey. In this study, stomach contents showed that walleye pollock was not a prey in any Dall’s porpoise. This suggested that the main prey species have changed probably due to decrease in population of walleye pollock in recent years.
A hands-on exhibit specimen was made using a whole skeleton of Stejeneger’s beaked whale, 4.9m in body length. The purposes of the study were to create a pleasure of the experience, a touch-and-feel recognition of the skeleton, appreciation of the size of the whale and understanding the texture of the whale bones. These purposes were accomplished, judged by the answers given to the questionnaire for the visitors, who joined the events of this hands-on exhibit. The results show that visitors recognized the hands-on exhibit as an excellent one for understanding the structure of the skeleton.