A mackerel sturgeon, which is the largest species of tuna, broke into Norwegian salmon farmer Masoval's site at Bjorndalen in southern Norway.

"The incident was discovered quickly and divers were notified so they could plug the hole in the netpen," Masoval Community Liaison Officer Berit Flamo told local newspaper Hitra-Froya.

There were salmon in the facility but no escapes were registered.

The mackerel sturgeon, which is also known as bluefin tuna, is the largest tuna species and can live up to 40 years. They migrate across all oceans and can dive below 3,000 feet.

"It was a big beast weighing 350 kilos," Flamo said.

Sturgeon feed on crustaceans, squid and fish, including herring, pollock, cod and flounder.

"It was probably looking for food. It doesn't take a big hole for it to get in as it goes with the pointed snout first, and then pushes in," Flamo said.

The mackerel sturgeon stayed inside the pen for almost a day, before it was taken out and euthanized.

Flamo praised the staff for handling the situation well.

"The authorities were immediately notified, and the incident was handled according to the book. It was later checked more carefully whether there had been any escapes, but we believe the situation is under control."

While it is rare that sturgeon enter salmon farms, it is not unheard of.

"It has happened once before during my time with the company."