Suitable habitats for humpback whales around Chichijima Islands, Ogasawara Islands visualized — Preference for shallow waters with gentle slopes

A research group consisting of Chief Researcher Koki Tsujii and Dr. Ryosuke Okamoto (at the time of the research) of the Ogasawara Whale Watching Association (OWA), Professor Hiroto Murase of the Department of Ocean Sciences at Tokyo University of Marine Science and Technology, and Professor Yoko Mitani of the Wildlife Research Center at Kyoto University has successfully visualized for the first time the suitable habitats of humpback whales that migrate for breeding to the waters around the Chichijima Island Group, a part of Tokyo's Ogasawara Islands. The findings were published in Mammal Study.

The Ogasawara Islands represent a critical breeding ground for humpback whales in the western North Pacific, with many whales migrating to coastal areas every year from winter to spring (December to May). However, the areas vital for humpback whales around the Chichijima Islands, which is the primary distribution area within the Ogasawara Islands, had not been scientifically clarified until now.

To scientifically elucidate the spatial distribution of humpback whales in relation to topographical factors around the Chichijima Islands, the research group conducted habitat suitability predictions using species distribution models.

They utilized vessel-based visual survey data collected by the OWA in January of 2013 and from 2015 to 2018. Combining the sighting locations of 234 whales across 160 groups confirmed during the five-year survey with topographical factors such as water depth, seafloor slope, and distance from the shore, they predicted suitable habitats using two species distribution models, GAM and Maxent.

The visual survey data showed that almost all locations where humpback whales were confirmed around the Chichijima Islands were at water depths of less than 200 meters, and the points confirmed in deeper waters were concentrated near the continental shelf. The species distribution modeling revealed that humpback whale distribution is associated with water depth and seafloor slope, with water depth exerting a particularly large influence. Specifically, the results from both models predicted that habitat suitability reaches its maximum in shallow, extremely gentle areas with a water depth of 50 to 60 meters and a seafloor slope of 0.5 to 0.8 degrees.

Furthermore, it was scientifically demonstrated that suitable habitats exist extensively in the waters on the western side of Chichijima, where a shallow continental shelf extends widely, compared to the eastern waters. Past studies have noted that humpback whale distribution is concentrated in waters shallower than 200 meters. This analysis, which covered a wide area including deeper offshore waters and the eastern side of the Islands where data was previously scarce, confirmed that humpback whales prefer shallow waters with gentle slopes. This clarified their specific preferences regarding seabed topography in the waters around Chichijima.

The generated habitat suitability maps provide a deeper understanding of habitat utilization by humpback whales in the Ogasawara region and are expected to serve as crucial scientific knowledge contributing to balancing between marine utilization and conservation.

In the future, the team intends to understand utilization patterns by taking group composition such as mother-calf pairs into account and to expand the habitat suitability maps to encompass the entire Ogasawara Islands region.

Tsujii stated: "The Ogasawara Islands are one of the premier breeding grounds for humpback whales in Japan, but until now, there was no data that scientifically visualized their suitable habitats. Through this study, we were able to clarify 'which locations are favorable for humpback whales' in the waters around the Chichijima Islands, which is their primary distribution area. We believe the results of this research hold significant value as baseline information for balancing marine use and conservation in Ogasawara. On the other hand, research in other regions has shown that marine use differs depending on group composition. For example, mother-calf pairs tend to prefer even shallower waters. But our current study did not investigate these differences. Moving forward, we want to focus on elucidating utilization patterns that account for group composition and creating distribution models expanded to the entire Ogasawara Islands."

Journal Information
Publication: Mammal Study
Title: Predicting Habitat Suitability for Humpback Whales Megaptera novaeangliae Around the Chichijima Islands, Ogasawara Islands, Japan
DOI: 10.3106/ms2025-0036