Distribution and density of finless porpoise (Neophocaena phocaenoides) in Osaka Bay, Japan

Abstract

To evaluate the distribution and density of finless porpoise in Osaka Bay, we analyzed encounter records from the research ship “Osaka” and line-transect sighting surveys by a ferry were conducted in the central bay. According to records, the research ship encountered 25 groups for 263 days from April, 2004 to December, 2007. Although the survey area widely covered the bay, encounters were limited to the sea from Misaki-cho to Kishiwada-shi (34°21′–32′N, 135°8′–20′E). Sighting surveys were carried out on the track line of a ferry-boat between Izumisano port and Tuna port from October, 2005 to January, 2007. We surveyed 4,403 km under the conditions of Beaufort wind force scale ≤ 2, and detected 72 porpoises. Most sightings were observed in the eastern part of the track line (34°23′–26′N, 135°8′–13′E). The density of the porpoises increased from March to July, with highest density (0.238 porpoises/km²) in April. These results suggest that the main distributional area of finless porpoises is in the eastern part of Osaka Bay and the abundance is higher in spring through early summer.