Male sperm whales (Physeter macrocephalus) leave their natal units before sexual maturity and form the all-male groups. Although there are several reports about　cohesive all-male group of sperm whales called “bachelor schools” during the modern whaling period, present-day studies revealed that males feed dispersing over a few kilometers and rarely form the cluster. On 16 September 2019, a large group of male sperm whales was observed in Nemuro Strait, Hokkaido, Japan. Approximately 30 individuals, within a range of about 1 nautical mile, were observed moving, diving and surfacing in a coordinated fashion. The sperm whales formed clusters of various sizes (from 2 to 18 individuals) when they came to the surface. The cluster of 18 animals is the largest cluster recorded in Nemuro Strait since the start of the survey in 2006. The body length of all of the individuals found near the whale-watching boat was about 10 m long, which is smaller than minimum individuals estimated the body size in this area, and there were no calves. During this encounter, males made a formation similar to the “Marguerite formation” twice, which female groups form against predators. The observation of a cohesive and large scale group of male sperm whales and their behavior is extremely rare, and these provided important information to understand the social structure of male sperm whales.

We received the whale stranding records from Matsumae-cho, Hokkaido, for the period 1992-2002 in 2013. Species and sex were identified as far as possible from the photographs provided. As a result of the identification, 11 individuals (SNH13901 to SNH13911) were obtained in 11 cases: nine individuals of Stejneger's beaked whales, one Baird's beaked whale, and one minke whale.

Killer whales are extensively distributed throughout the world’s oceans and migrate to areas where there is seasonal and year-round sea ice. Even at the southern tip of the Okhotsk Sea, the lowest latitudes where the offshore formation sea ice exists, killer whales are commonly observed during the winter months. At least eight ice-related strandings at the shore of killer whales occurred in Etorofu Island, Hokkaido, and southern Sakhalin between 1925 and 2016. These strandings were observed roughly once per decade. All stranding events occurred with drift ice floe, and not in fast ice. The thickness and density of sea ice floes were different among the events, e.g., from a primary ice floe to a separated front mass or small ice floe fragment in geological scale or from a large ice cake to brash ice debris. Until the 1970s, the response of the locals was food consumption of whale meat; however, rescue efforts were attempted for three cases in the 21st century.