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Humpback whale parent, calf sighting south of Tokyo suggests breeding grounds expanding

A mature humpback whale and its calf are seen near Hachjojima island. From the fins, it is clear that the whale in the foreground is the younger one. (Photo courtesy of Tokyo University of Marine Science and Technology)

HACHIJO, Tokyo -- Among the humpback whales that have been spotted in recent years in the waters here off this island south of Tokyo, the first parent and calf pair has been located this season between last winter and this spring.

According to the Tokyo University of Marine Science and Technology and others who conducted the survey, humpback whales move southward from northern seas between winter and spring to breed. Based on water temperature, the seas surrounding Hachijo Island, that lies about 300 kilometers south of the capital, were believed to be unsuitable for this purpose, but there is a possibility that breeding grounds themselves are expanding.

Humpback whales are a large species that are seeing a rise in their numbers found swimming in the Pacific Ocean. In the northern hemisphere, their primary breeding grounds were believed to be between at northern latitudes of between 15 to 30 degrees. In Japan, the waters around the Ogasawara Islands at a northern latitude of 27 degrees, Amami-Oshima Island at a northern latitude of 28 degrees, or the Okinawa islands at a northern latitude of around 26 degrees would correspond to the appropriate latitudes. At the same time, Hachijo Island, where whale sightings have been increasing since fiscal 2015, is located at a northern latitude of 33 degrees, which led researchers to believe that the water temperatures and other conditions made it difficult for the huge mammals to breed.

According to visual observations from boats conducted by Gen Nakamura, an assistant professor at Tokyo University of Marine Science and Technology, and the Hachijo Municipal Government, the number of whales rose from 205 in fiscal 2016 to around 400 in fiscal 2017 and the current season. The period of time the whales stay in the waters around Hachijo Island has also grown. This season, not only has a whale been found with its calf, but male whales have frequently been observed emitting mating calls to female whales.

Furthermore, photos provided by the Ogasawara Whale Watching Association and the Amami Whale and Dolphin Association have shown that whales that were seen near the Ogasawara Islands and Amami-Oshima in the past have been visiting Hachijo Island. The number of humpback whales is said to be on an upward trend worldwide in recent years, and their sightings near Hachijo Island is possibly related.

"We're not completely sure why the whales are swimming near Hachijo," said Nakamura, "but it's possible that with the rising number of individual whales, the waters in which they breed have expanded. Going forward, we'd like to continue our survey using various methods, including the use of unmanned submarine robots."

(Japanese original by Suzuko Arai, Science & Environment News Department)

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