Kyoto, Japan -- Humpback whales are one of the most popular species for whale watching. Since they are active close to the water's surface, groups of whale watchers can often see them breaching and diving during breeding periods. One popular whale watching destination is the Ogasawara Islands -- also known as the Bonin Islands -- a remote archipelago about 1,000 kilometers south of Tokyo. These isles serve as a key breeding ground for humpback whales, which migrate in large numbers every December to May to these coastal waters.
Scientists have identified one of the main habitats of humpback whales in the Ogasawaras to be the Chichijima Island group, also home to the archipelago's main human settlement. Yet exactly which areas around Chichijima are critical for these whales has remained a mystery. This motivated a team of researchers at Kyoto University, the Ogasawara Whale Watching Association, and Tokyo University of Marine Science and Technology to model the whales' distribution using sighting survey data.
The team examined visual survey data collected from vessels used by the Ogasawara Whale Watching Association in 2013 and also from 2015 to 2018. This data includes the locations of 160 groups of whales, totaling 234 individuals, that were confirmed during the five-year survey. The team then used two species distribution models to combine this data with topographic factors such as water depth, seabed slope, and distance from the coastline in order to predict the most suitable habitats for humpback whales.
This analysis led to the creation of a habitat suitability map, the first ever visualization of habitats for humpback whales around Chichijima. The results revealed that water depth and seabed slope are connected to whale distribution, with water depth having a particularly strong influence. The researchers observed the most suitable habitats at shallow water depths with gentle slopes, conditions were more prevalent off the west coast of the island.
This map is expected to enrich understanding of how humpback whales utilize habitats in the Ogasawara waters, providing important scientific insights that will contribute to marine conservation.
"Whale watching focused on humpback whales is a thriving activity in the Ogasawara waters, so this species holds significant economic value not only as a component of marine biodiversity but also as a local tourism resource," says first author Koki Tsujii. "We aim to scientifically and visually identify suitable habitats in hopes of contributing to the effective conservation of this species."
There is more work to be done. In other regions it has become clear that differences in marine habitat use depend on pod composition: for example, mother and calf pairs tend to prefer shallow waters. Moving forward, the team hopes to elucidate usage patterns that account for pod characteristics, and develop a distribution model that covers the entire Ogasawara archipelago.
