Mystery of Tuna Covered in Strange Round Holes Explained

A tuna has been caught with bizarre, perfectly round scoops taken out of its body.

The fish was caught on October 29 off the coast of Australia by professional fisherman and tour guide Jason Moyce, who goes by Trapman Bermagui. He shared a picture of the bizarre sight to Facebook.

"Serious cookie cutter bites on this tuna..." said Bermagui in the caption of the post.

Bermagui refers to the cookie-cutter shark, which is a small species of shark known to take precise bites out of larger prey.

"It is highly likely these were from cookie-cutter sharks," Ingrid Visser, a marine biologist and founder and principal scientist of the New Zealand-based Orca Research Trust, told Newsweek.

Cookie-cutters grow to only around 17 to 22 inches long at most, and typically live in deep water and at night come closer to the surface to feed. They migrate vertically in the water column each day, swimming around 2 miles up to feed before descending again. They have also been observed to bioluminesce, glowing in the dark.

"Unlike bite wounds made by squid or lampreys, cookie-cutter bite wounds often have very smooth bite wound peripheries that are circular to oval shaped, and are further characterized by having a relatively deep and smooth depression," Mark Grace, a fisheries' scientist at NOAA, told Newsweek.

According to the Australian Museum, cookie-cutters feed by attaching their mouths to their large prey using suction, then spinning to use its razor-like rows of rigid teeth to cut a plug of flesh out of the animal. Their bite wounds are generally around 2 inches wide and 2.75 inches long.

The tuna pictured by Bermagui had multiple wounds along its body.

"The tuna was [likely] bitten by multiple cookie-cutter sharks at the same time (and likely it did indeed flee as to any animal with a nervous system, wounds to this extent will be painful!)," Visser said.

Fish are made easy pickings for cookie-cutter sharks when they are on fishing lines, Aaron B. Carlisle, a shark biologist at the University of Delaware, told Newsweek.

"Likely the tuna was on the line when it was attacked, so it can't really get away. You see this all the time in fish markets, as fish caught on longlines and brought to market are often snacked on by [cookie-cutter] sharks while they are vulnerable and can't escape," Carlisle said.

These small sharks' bite marks have been found on a huge variety of species, including porpoises, dolphins, sperm whales, fur seals, dugongs, great white sharks, stingrays and bony fish like tunas. Cookie-cutters are known as facultative ectoparasites, as while they take chunks out of some of their victims, they also eat prey whole when they can, including smaller squid and fish.

"It is not known how often cookie-cutter sharks feed but there has been an instance where the stomach content of a cookie-cutter shark had prey flesh from two separate bites. Known cookie-cutter shark prey include fishes, squid, marine mammals, and very infrequently humans," said Grace.

There have been attacks on humans before: in 2019, a 58-year-old swimmer was bitten by a cookie-cutter off the coast of Hawaii as he was swimming during the night. The man was bitten in the abdomen, and was taken to hospital, but survived. Later the same year, two further swimmers were bitten by cookie-cutters, despite using electrical shark deterrents. There have also been several reports of bodies lost at sea being found with cookie-cutter bite wounds made post-mortem.