A mysterious fish that lives deep below the ocean's surface has been popping up in the waters of Toyama Bay, causing marine experts to scratch their heads.

A deep-sea slender oarfish was caught in a fixed net about 1 kilometre off Imizu on Monday and delivered to the Uozu Aquarium in Uozu in the prefecture. That makes three fish caught in Toyama Bay just this month, as two slender oarfish were also found on January 19 in waters off Imizu and Namerikawa.

A deep-sea slender oarfish is examined at the Uozu Aquarium in Uozu, Japan. Photo / Japan News-Yomiuri

Slender oarfish live at a depth of 200 to 300 meters, and are characterised by long silvery-white bodies and red fins. The fish is called "Ryugu no tsukai" in Japanese, meaning "messenger from the palace of the dragon king."

Monday's fish is 394.8 centimetres long, the fourth longest to be found in Toyama Prefecture. According to the aquarium, 20 slender oarfish have been found in Toyama Bay since the first confirmed sighting there in 2009. Four fish were found in 2015, the most in a single year.

"[Finding several in a row] is said to be the forerunner of an earthquake or to be influenced by ocean temperatures, but research is scarce and we don't know the cause," said Satoshi Kusama, 35, a caretaker at Uozu Aquarium.

According to the prefecture's Fisheries Research Institute, the water temperature on the surface of Toyama Bay this month is several degrees higher than usual, while temperatures at a depth of about 200 to 300 meters are slightly below average.

The fish's carcass is in good condition and will be exhibited on February 2 and 3 at the aquarium.