The Living World

Yoko Mitani

Marine Mammals and the Seas They Inhabit

Yoko Mitani , Associate Professor

Field Science Center for Northern Biosphere (Department of Marine Biology, School of Fisheries Sciences)

High school : Oin High School (Tokyo)

Academic background : School of Mathematical and Physical Science, Graduate University for Advanced Studies

Research areas
Marine mammal biology
Research keywords
Ecology, environment, behavioral ecology, marine ecology, marine mammals, bio-logging

What prompted your current research?

A female Weddell seal and newborn cub

When I was a high school student, I learned that humpback whales migrate huge distances between the equator and polar regions, and that got me interested in cetacean ecology.The Internet still barely existed at the time I was applying for university, so I looked through the list of translators of the Japanese edition of Whales and Dolphins (ed. Anthony R. Martin, Japanese ed. Toshio Kasuya, pub. Heibonsha) to find universities in Japan that would enable me to study whales, and as a result, I enrolled in the Department of Fisheries in Kyoto University’s Faculty of Agriculture. While I was there, there were no faculty members studying whales, but I kept on insisting that I wanted to study dolphins, and for my graduation thesis, I analyzed the trace elements in the teeth of cetaceans and northern fur seals. For my master’s thesis, I analyzed stable isotopes in the baleen plates, liver and muscle of minke whales, and found out what they had been eating over time. I went on to the Graduate University for Advanced Studies to carry out research for my doctoral thesis on Weddell seals that live in Antarctica. With this research, I finally found myself studying actual living creatures. I ended up studying seals instead of whales basically because whales don’t spend any of their lives on land. For my research, I used a small recording instrument known as a data logger. This kind of data collection is called “bio-logging”. It’s a great technology that enables you to track the behavior of marine creatures swimming freely in the sea, but you need to be able to attach these data loggers to the bodies of the creatures you’re studying, and to recover them eventually. Recently methods have been developed for attaching loggers to whales from boats, and in my postdoctoral years I used this method to study blue whales in American waters.

A Steller sea lion fitted with a satellite transmitter

I’m currently studying marine mammals that inhabit waters around Hokkaido, which is surrounded by the Sea of Japan, Pacific Ocean, and the Sea of Okhotsk—three bodies of water that differ from each other in terms of the marine mammals that can be seen according to season. Hokkaido’s seas are very fertile, but in recent years, earless seals, Steller sea lions, fur seals and other marine mammals have come to be regarded as nuisance animals for the way their presence conflicts with fishing industry interests. As I conduct my research, I’m constantly thinking about what can be done about the competition between marine mammals and the fishing industry that provides us with tasty fish to eat, and what can be done to ensure coexistence between people and marine mammals.


What kind of research are you carrying out?

To explore how people and marine mammals might be able to coexist, we need to know (1) where those marine mammals have come from and where they are going (migration), (2) how many are present in different locations (distribution), and (3) the quantity of fish they are eating (feeding).

For (1) migration, Steller sea lions, northern fur seals, and spotted seals visit Japanese waters in winter, and head north to Russian waters in summer. We attach satellite transmitters to individuals caught alive as bycatch in fishing nets or rescued for some reason, and release them to track their movements and find out where they go to breed and the routes they take. Data sent via satellite from the transmitter tells us about the migration routes and the various environmental parameters of the waters through which the animals travel.

A spotted seal fitted with a satellite transmitter

Spotted seal migration route

Making observations from a ship

To learn about (2) distribution, we conduct sighting surveys with binoculars from the upper deck of a boat to search for marine mammals. At the same time, we also take continuous measurements of surface water temperature and other data on the physical environment, use quantitative echo sounders to gather information on prey availability, and make CTD (Conductivity-Temperature-Depth profiler) casts to collect oceanographic data. This kind of data enables us to look at the relationship between marine mammal distribution and marine environment.
For (3) feeding, we collect any corpses from bycatch or strandings to examine their stomach contents and learn what the animals had eaten just before their death, and analyze stable isotopes to develop a picture of what they might have been eating over the medium to long term.
Marine mammal research cannot stand alone. It’s only thanks to the cooperation of fishermen and other people in local communities, the crew who operate our boats, the people managing rearing facilities and many others that we’re able to do our work. I feel that I can do my part in enabling us to coexist with marine mammals by collecting scientific data as a researcher, and passing on my findings to policymakers and society at large. I want to continue my research in the hope that I can move things in the right direction, even if only a step at a time.



(1) Japan Society of Bio-Logging Science (Eds.), Bio-Logging - Using the Latest Science to Study Animal Ecology (bio-logging – saishin kagaku de kaimei suru dobutsu seitaigaku: WAKUWAKU Tokimeki Science Series)” Information Design Associates Kyoto (2009)