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Behaviour

In 2001, an oarfish was filmed alive in the wild. The 1.5-metre (4.9-foot) fish was spotted by a group of U.S. Navy personnel during the inspection of a buoy in the Bahamas. The oarfish was observed to propel itself by an amiiform mode of swimming; that is, rhythmically undulating the dorsal fin while keeping the body itself straight. Perhaps indicating a feeding posture, oarfish have been observed swimming in a vertical orientation, with their long axis perpendicular to the ocean surface. In this posture, the downstreaming light would silhouette the oarfishes' prey, making them easier to spot.

In July 2008, scientists captured footage of an oarfish swimming in its natural habitat in the mesopelagic zone in the Gulf of Mexico. It is the first ever confirmed sighting of an oarfish at depth, as most specimens are discovered dying at the sea surface or washed ashore. The fish was estimated to be between five and ten metres (16 and 33 ft) in length.

As part of the SERPENT Project, five observations of apparently healthy oarfish Regalecus glesne by remotely operated vehicles were reported from the northern Gulf of Mexico between 2008 and 2011 at depths within the epipelagic and mesopelagic zones. These observations include the deepest verified record of R. glesne (463–492 m or 1,519–1,614 ft). In the 2011 sighting, an oarfish has been observed to switch from swimming with a vertical posture to swimming laterally, using lateral undulations of its entire body. Oarfish were found to have late or slow flight responses towards approaching remotely operated vehicles, supporting the hypothesis that oarfish have few natural predators.

From December 2009 to March 2010, unusual numbers of the slender oarfish Regalecus russelii (竜 宮の使い "Ryūgū-No-Tsukai"), known in Japanese folklore as the Messenger from the Sea God's Palace, appeared in the waters and on the beaches of Japan, the appearance of which is said to portend earthquakes. After the 2011 Tōhoku earthquake and tsunami which killed over 20,000 people, many pointed to the oarfish from 2009–2010 to build up this myth.

In 2016, Animal Planet aired an episode of the television series River Monsters named "Deep Sea Demon" in which Jeremy Wade was filmed during an encounter with a live oarfish while diving. The oarfish at this location seemed to be using a buoy anchor chain as a guide to ascend to the surface. On his second diving attempt, he was able to film two live oarfish as they ascended relatively close to the surface. Wade was even able to touch one of the oarfish with his hand. The oarfish were propelling themselves by an amiiform mode of swimming as noted by other sightings.

In January 2019 two oarfish were found alive in the nets of fishermen on the Japanese island of Okinawa.

The fish is often nicknamed the "doomsday fish" because, historically, appearances of the fish were linked with subsequent natural disasters, namely earthquakes or tsunamis.