They’re the largest fish in the ocean — so big they’ve been mistaken for whales. Scientists admit they don’t have much information about whale sharks, or Rhincodon typus.

But some environmentalists fear whale shark numbers are diminishing.

The group WildEarth Guardians sought to have whale sharks listed as an endangered species in 2013.

But the National Marine Fisheries Service recommended against the request based on the lack of scientific information.

In February the agency’s parent organization, the National Oceanic and Atmospheric Administration, adopted rules to prevent the capture of whale sharks in purse seine nets in the western and central Pacific Ocean.

The action was necessary for the United States to satisfy its obligations under the Western and Central Pacific Fisheries Commission, a group of nations that develops common rules and protocols for fishing in the region.

In instances when whale sharks are captured accidentally, federal marine officials and other members of the commission have established procedures for releasing them.

The commission said it’s not possible to conduct a stock assessment of whale sharks at this time because there is no reliable information on purse seine fisheries bycatch in such nations as Indonesia, the Philippines or Vietnam.

The species is known to range more than a thousand miles of open ocean, filter-feeding on small fish, fish larvae and eggs.

Whale sharks live 60 to 100 years and can grow to more than 60 feet long.

The move slowly and have sometimes become a visitor attraction when they gather in certain warm waters. That includes Isla Mujeres in Mexico, where there is a whale shark festival, and Ninga-loo Reef in Australia.

They remain a novelty in Hawaii, but individuals have been seen off the Na Pali Coast of Kauai and Kona on the Big Island.

“You just have to have the right current and the right plankton in the water,” said Capt. Bob Sylva of Thrill Seeker Sport Fishing.

Sylva, whose vessel is based out of Honokohau Marina in West Hawaii, said he’s seen four individual whale sharks in separate encounters in four years.

He remembers one, about the length of his 32-foot fishing boat, gliding slowly behind his transom using the backwash to gather plankton.

“You could walk as fast as it swims,” he said.