KOCHI -- An aquarium in the western Japan city of Osaka is set to release its popular whale shark into the Pacific Ocean as part of efforts to understand the species' ecology, while transferring another it keeps at its research facility in Kochi Prefecture to be exhibited at the aquarium.

The 5.5-meter-long male whale shark, "Kai," who is estimated to be 8 to 9 years old, has been kept at Osaka Aquarium Kaiyukan in the city's Minato Ward for three years.

The whale shark is the world's largest species of fish. They inhabit oceans from temperate to tropical areas, and are often spotted in waters around the Japanese archipelago between early summer and autumn. Their gray backs are covered with white dots.

Kaiyukan is conducting joint research on whale sharks' migration routes with Hokkaido University in Sapporo, the capital of the northernmost prefecture of Hokkaido. As part of the project, researchers attach recording devices to the bodies of whale sharks that the aquarium is keeping or ones captured in the sea. They then release them into the Pacific Ocean, where the devices send data on their migratory route to researchers and then automatically detach from the whale sharks' bodies after a set period.

Kai was caught by a fixed fishing net off of the city of Muroto in Kochi Prefecture, western Japan, in June 2015. He has been kept in a tank at Kaiyukan since July 2016 and enjoys a high level of popularity with visitors.

As Kai has grown larger over the past three years, it is feared that his tank will soon be too small for him, so researchers decided to release him into the Pacific Ocean near Cape Muroto on Oct. 19 after attaching a recording device to his body.

The whale shark that will be transferred to Kaiyukan is approximately 4.2 meters long and estimated to be 4 to 5 years old. It has been kept at the Osaka Aquarium Biological Research Institute of Iburi Center (OBIC) in the Kochi Prefecture city of Tosashimizu since it was captured off Muroto in September. It will go on public display from Oct. 19. The new whale shark will also inherit the name "Kai."

"We'd like to conduct research into whale sharks both naturally in the seas and in captivity to clarify the fish's barely understood ecology and contribute to the protection of the species," said an aquarium official.

(Japanese original by Yusuke Kori, Kochi Bureau)