Sunfish Weighing 6,000lbs Is the World's Heaviest Ever Bony Fish

A behemoth sunfish found dead in the Azores has broken the record for the heaviest bony fish ever recorded.

According to a paper published in the Journal of Fish Biology, the enormous bump-head sunfish (Mola alexandrini), was discovered on December 9, 2021 in the waters off the Portuguese Azore islands in the North Atlantic Ocean, and measured 12 feet high, 11 feet long, and weighing 6,049 pounds. For comparison, that's about the same weight as a white rhino.

This smashes the record of the previous largest bony fish, which was that of another giant sunfish from Japan in 1996, which weighed around 5,070 pounds.

Giant sunfish are the largest species of sunfish, large plate-like fish which are occasionally seen floating on their side at the surface in order to warm themselves in the sun after diving into deeper waters. Because they have no tail, sunfish use their top and bottom fins to swim up to 16 miles per day, despite initial assumptions that they cannot move independently.

"Sunfish are actually related to toadfish, pufferfish, filefish and leatherjackets. They seem to have strong physiologies so drifting to unsuitable habitat is unlikely," David J. Booth, a Professor of Marine Ecology, at the University of Technology Sydney in Australia, told Newsweek.

Giant sunfish are only rivaled in size by their sister species the ocean sunfish, as well as large species of cartilaginous fish like whale sharks and basking sharks.

"Why [they are] so large [is] unclear, but they produce many offspring (100s of thousands) so possibly selection for more eggs has driven body size," Booth said.

Their characteristic round body and unusually enormous size has made giant sunfish a popular sight for seafarers, but they are rare. Until 2017, they were thought to be the same species as the ocean sunfish, which are listed as "vulnerable" on the International Union for the Conservation of Nature (IUCN) Red list. While the giant sunfish was defined as a separate species in 2017, they are not yet distinguished in terms of their vulnerability.

"They are widespread but rare in any particular location. We have seen them at 400 meters eating off oil and gas infrastructure," Booth said.

The rarity of sunfish species may be made worse by human actions, as they are a delicacy in areas including Taiwan and Japan. They are also frequently killed as bycatch in other fisheries, and deliberately caught and de-finned, as some fisheries view them as bait thieves. Exact population numbers of both species of sunfish are unknown.