Sponsored in part by

World's Heaviest Bony Fish Discovered?

Get National Geographic — And a FREE World Map!

National Geographic magazine cover

Subscribe now for a year of animal adventures, dinosaur discoveries, and more.

• Save 43 percent
• Get up to five wall maps
• Only $34 (U.S. rate)

Easy one-click ordering >>

In fishing circles, talk of the big ones always garners an audience. The bigger the fish, the more awe it inspires. A crowd may soon gather around Tierney Thys, a marine biologist with the Monterey, California-based Sea Studios Foundation.

As part of Thys's study of the little-known ocean sunfish (Mola mola), a behemoth that can grow more than 4.2 meters (14 feet) long from dorsal fin tip to anal fin tip and 3 meters (10 feet) in horizontal length, she has uncovered a fish that may be a new record setter for the world's heaviest bony fish.

The fish in question is an ocean sunfish that was caught off the coast of Kamogawa, Japan, in 1996 in set nets owned and operated by the Kamogawa Fisheries Cooperative Association. Members of Kamogawa SeaWorld measured the fish to be 2.7 meters (8.9 feet) long and say it weighed 2.3 metric tons (5,071 pounds).

ocean sunfish

The ocean sunfish are giants of the fish community. A behemoth caught off the coast of Japan in 1996 may set the record as the world's heaviest bony fish.

Photograph courtesy National Oceanic and Atmospheric Administration/Department of Commerce

In-depth articles about the Earth and its inhabitants, every month of the year. Click here to get one year of National Geographic magazine and a free gift.

"It is not the longest, but it may be the heaviest sunfish that's actually been measured on a reliable scale," said Thys, who is studying the ocean sunfish in a satellite tagging project supported in part by the National Geographic Society's Committee for Research and Exploration.

The project aims to gather the first-ever baseline biological data on the ocean sunfish, offering insights to how they migrate, whether regional populations interact, and where they spawn.

Thys and her colleagues are concerned that the big fish are declining due to their accidental catch by drift net and longline fisheries. The tagging program, they hope, will lead to conservation and appreciation of the fish before they are lost forever.

Thys and her colleagues are currently analyzing the data they have received from satellite tags attached to nine ocean sunfish and plan to publish their findings within a few months in a scientific journal. Meanwhile, they are tracking down just how big the biggest of these fish can get.

Current Record

The current record for the heaviest sunfish in The Guinness Book of Animal Records belongs to a Mola mola that was struck on September 18, 1908 by the Australian steamship SS Fiona about 40 miles (65 kilometers) from Sydney. The fish was towed to port where it was measured and apparently weighed.

According to accounts of the incident published in the March 1928 issue of Scientific Monthly and the December 10, 1910 issue of The Wide World Magazine, the fish measured 3.1 meters (10 feet) in horizontal length, 4.26 meters (14 feet) in vertical length, and weighed 2 tons 4 hundredweight, which converts to 4,927 pounds.

But there is some doubt as to how this particular fish was weighed. Among the skeptics is Julian Pepperell, a marine biologist and fisheries historian with Pepperell Research and Consulting in Noosaville, Australia.

Pepperell said that according to an account by David Stead, the local scientist at the time who would likely have been the most familiar with SS Fiona incident, the mola was never officially weighed, only measured. The weight, which he says is plausible, was probably a guesstimate.

"Sizes of large fish are often cited in various levels of literature, but on closer examination or investigation it is often very difficult to verify that a fish was actually measured, or weighed," he said. "Often someone at the time will make a guess, or guesstimate, that will enter the lexicon."

Pepperell does not doubt the length measurements of the SS Fiona specimen as they were quoted by Stead. But he notes that the weight measurements only come from The Wide World Magazine article, which reports the fish as having been weighed on the company weigh bridge.

"Now it may well be that Stead was unaware of this, but as noted, this seems unlikely," said Pepperell. "Magazine articles are not scientific accounts, whereas Stead was a scientist."

According to Thys, the uncertainty on the true weight of the SS Fiona mola gives the specimen from Kamogawa a shot at being the heaviest.

"We know how and when and who weighed it, so there is less ambiguity in terms of its recorded [size]," said Thys. "It is certainly at least as heavy if not heavier than the Fiona mola."

Big Ocean Fish

Thys suspects that neither of these two specimens in question is the largest ocean sunfish caught or to be caught in the future. In her research, she has found citations for several mola that were quite large in length but not weighed in a reliable manner.

For example, she cites a specimen reported by Keiichi Matsuura at the Fish Division of the National Science Museum in Tokyo caught on August 8, 1999. It measured 3.25 meters (10.7 feet) long.

"It weighed more than two tons (1.8 metric tons). But the exact weight is not clear because the specimen was too heavy to be measured at the fishing port and was skinned immediately after it was landed on the port," said Thys.

Pepperell does not think the ocean is teeming with huge sunfish for the simple reason that the really big ones are likely quite old and therefore there are not as many of them compared to younger sunfish.

"It's a bit like looking at how many people there are in the world who are older than, say, 105 or 110. Not many, even though the world's population is over six billion," he said.

Regardless, Pepperell says that humans are and probably will forever be fascinated by big animals, including big fish. The fact that ocean sunfish can grow so large is yet another reason to try and learn more about them and work to conserve them, he said. "We share the ocean with giants and may it always be so."

Get National Geographic — And a FREE World Map!

National Geographic magazine cover Subscribe now for a year of animal adventures, dinosaur discoveries, and more.

• Save 43 percent
• Get up to five wall maps
• Only $34 (U.S. rate)

Easy one-click ordering >>

 Related Stories

 Related Websites

More Information
Our Top News

Get the latest news of exploration, discovery, science, animals, nature, travel, adventure, and more—updated every weekday by the National Geographic online newsroom:

National Geographic News: The latest top stories.
Go to News>>

Birds: BirdWatcher, National Geographic's online news service about birds and birding.
Go to BirdWatcher>>

Dinosaurs: Latest finds
Go to Dinosaurs>>

Snakes: Science and stories
Go to Snakes>>

Sharks: Science and stories
Go to Sharks>>

Oceans: Science, geography, and creatures of the sea.
Go to Oceans>>

Everest: Adventures, drama, science, and geography about the world's highest mountain.
Go to Everest>>

Iraq: News and features about the geography, nature, and people impacted by the war.
Go to Iraq>>

More Information
National Geographic News Alerts
Register to receive e-mail headlines from National Geographic News. Click here to sign up. (We will not share your personal information, and there is no charge for this service. View our privacy policy.)