Interchange and movements of humpback whales (Megaptera novaeangliae) between western North pacific winter breeding grounds in Northern Luzon, Philippines and Okinawa, Japan

摘要

Humpback whales wintering in the northern Philippines and Okinawa, Japan, belong to the western North Pacific subpopulation, now recognised as a distinct population segment. The numbers for this subpopulation are low hence it is being proposed to be listed as endangered. Although previous research revealed movements between breeding grounds, the relationship between these areas remain little understood. Recent survey efforts in the Philippines resulted in a total of 230 photo-identified individuals from 1999-2016 while Okinawa had 1,445 from 1991-2014. Between the seasons a total of 100 matches of 100 individual whales were found between the two breeding grounds including 41 males and 26 females while the sex of the other whales was unknown. Eight individuals were recorded moving within sea- sons, including two individuals moving across seasons multiple times. The two whales were both males, where one was recorded in both breeding grounds in three different seasons (2005, 2008 and 2012). The shortest duration of recorded sightings between breeding grounds was one whale seen in the Philippines in late March 2012 and in Okinawa 13 days later. The high withinre-gion return index values in the Philippines may indicate a small population utilising this breeding ground with a high rate of return, while the lower return index values in Okinawa may indicate a relatively larger population utilising the Okinawa breeding ground. The interchange index value between Okinawa and the Philippines (0.30) was much lower compared with the average value of withinregion return index in the Philippines (3.41). However, 100 out of 230 individuals (43.48%) identified in the Philippines were also observed in Okinawa and the interchange index was only marginally lower than the value of withinregion return index in Okinawa (0.60). These results may indicate that the two breeding grounds are likely sharing the same subpop-ulation and yet, at some level, indicates fidelity of the whales to their respective regions. The low survey and sampling effort in the Philippines presented limitations in assessing the extent of linkage with the Okinawa breeding ground. Renewed and strengthened international collaborations are needed, especially among the Asian breeding grounds in order to unravel the complexities of the population structure and migratory movements of this small western North Pacific subpopulation.