ここまでわかった大阪湾のスナメリ

摘要

To evaluate distribution, abundance and habitat use of finless porpoise in Osaka Bay, interview surveys to fishermen, analyses of encounter records to the porpoise by a research ship of Osaka pref., and visual observations from small ships and a ferry were conducted. Interview surveys on the sightings to 77 fishermen were carried out at 22 ports of this area in 2010. 57 fishermen have seen the porpoises in the past. 25 people (43.9%) sighted the porpoises within two years from the hearings. There were more sightings by fishermen of eastern and southeastern areas than western and southern areas in the bay. The encounter records between April, 2004 and December, 2007 were used for the analyses. The research ship encountered 25 groups for 263 days in this period. Although the ship ran in a large area of Osaka Bay. the encounters were limited to the waters off the coast from Kishiwada-shi to Misaki-cho. The encounter rates tended to increase in April to June mainly. But it increased in July to September in 2005. The visual observations with small ships were conducted in 2005 to 2013 on the wide area of the bay. Ships ran total 2,210 km, and we observed 22 groups of the porpoise. The sighting positions were distributed off the coast from Izumisano-shi to Misaki-cho. The feeding aggregations were observed in August and September, 2005. Line transect sighting surveys were carried out on the track line of a ferry between Izumisano port and Tuna port from October, 2005 to January, 2007. We surveyed 4,403 km and detected 72 porpoises. Most sightings were in the eastern part of the track line. The density of the porpoises increased from March to June. The highest density (0.238 individuals/km2) was recorded in April. These results suggest that the main distributional area of finless porpoises is in the eastern-southeastern part of Osaka Bay, and the abundance is higher in mainly spring-early summer (mating and parturition season). However, feeding aggregations are occasionally formed in mainly on summer and autumn, and the porpoises may increase in these seasons, depending on a year.