Ice-related Strandings of Killer Whales at Shore in the Southern Okhotsk Sea

Introduction Killer whales (Orcinus orca) have a cosmopolitan distribution, are commonly found in pack ice, and may extend their range into ice-covered water in the Antarctic. In the southern Okhotsk Sea, which contains the most southern offshore formation sea ice in the northern hemisphere, killer whales are often seen close to the sea ice during winter and early spring (Sato et al. 2006). On February 7, 2005, an ice-related mass stranding of killer whales occurred at Aidomari in Rausu town in the Shiretoko Peninsula (Eastern Hokkaido, Japan) and attracted great interest worldwide. Before this event, only a few ice-related mass strandings had been reported in the research community, e.g., 19 whales were stranded in Trinity Bay, Canada, in April 1957, and 2 whales were stranded off southwest Newfoundland in March 1975 (Mitchell and Reeves 1988). After the Aidomari event, based on a literature search, Uni et al. (2005) compiled seven icerelated strandings in the southern Okhotsk Sea off Hokkaido and Etorofu Island. Furthermore, Uni et al. (2017) included certain concrete evidence and made a small correction to the previous study. To consider future responses to stranding events, Westdal et al. (2016) reviewed 17 ice-related strandings both onshore and offshore ice entrapment in and before 2013 in the northern hemisphere. This study compiled ice-related strandings at the shore of killer whales in the southern Okhotsk Sea and considered criteria such as weather, tidal range, sea ice behavior, and coastal topography with reviews of responses by locals. Materials and Methods This study is based on a poster presentation for the 14th Biennial Conference on the Biology of Marine Mammals in San Diego in 2005 (Uni et al. 2005). The study area was the southernmost Okhotsk Sea, along the coast of southern Sakhalin, Hokkaido and Etorofu Island, between 44 ° and 50 °N and 142 ° and 148 °E. Information was collected for each event using the following materials: the Marine Mammal Stranding Database created by the National Museum of Nature and Science in Tokyo, which is the most comprehensive database of marine mammal stranding events in Japan (https://www.kahaku.go.jp/english/research/db/zoolo gy/marmam/index.php); official history books of the coastal municipals along the Okhotsk coast of Hokkaido; and related literature concerning sea ice in the Okhotsk Sea. When we found a particular description, extensive searches were made to seek evidence of the event (e.g., newspaper articles, images, and specimens). If there was an observer of the event, we arranged an interview with them. In addition to the abovementioned materials, we used direct observations and video analysis for the recent events. For each event, causes of ice-related stranding regarding weather, sea ice behavior, tide height, and coastal configuration were discussed. To evaluate stranding causes using these criteria, we used tide height data archived by the Japan Meteorological Agency (http://www.data.jma.go.jp/gmd/kaiyou/db/tide/sea_l ev_var/index_hourly.php) and Google Maps. Furthermore, for recent events, satellite imagery of sea ice that occurred in and after 2005 in the Okhotsk Sea was obtained. Visible and microwave satellite imagery were observed by the MODIS mounted on the NASA launched satellite “Aqua” and the AMSR2 mounted on the JAXA-launched satellite “GCOM-W.” The satellite imagery was provided by the Earth Observation Research Center of Japan Aerospace Exploration Agency. No survey was conducted for offshore ice entrapment in the Okhotsk Sea.