- Scientists have documented the first-ever record of an oarfish (Regalecus russellii) in Sri Lanka, a 2.6-meter (8.5-foot) specimen caught off the country’s western coast.
- The find expands the known distribution of oarfish into the Indian Ocean, offering a new baseline for studying this rarely seen deep-sea species.
- Meanwhile, another oarfish was recorded in India’s Tamil Nadu this year, while within 20 days, three oarfish have been recorded from Australia and New Zealand, puzzling naturalists.
- The importance of promoting citizen science and raising awareness among fishers is needed.
COLOMBO — In the shadowy depths of the ocean lives a ribbon-like giant crowned with a fiery red crest — long mistaken for a sea monster. Rarely glimpsed alive, the oarfish holds the record as the world’s longest bony fish, capable of growing up to 8 meters (26 feet). It usually surfaces only by chance, and one such encounter off Sri Lanka marked the first confirmed record of an oarfish in the Indian Ocean, igniting maritime legend and modern science.
The individual, measuring 2.6 m (8.5 ft), was accidentally caught in a surface tuna gillnet set by a multiday fishing vessel off Sri Lanka’s west coast. Curious fishers, unfamiliar with the strange catch, brought it ashore at Beruwala Fishery Harbour, where it was handed over to the authorities.
Globally, three species of oarfish are recognized, namely giant oarfish Regalecus glesne), Russell’s oarfish (R. russelii) and streamerfish (Agrostichthys parkeri). Detailed examination by Ishara Rathnasuriya of the Ocean University of Sri Lanka confirmed the Sri Lankan specimen as R. russelii. Though collected in 2021, its significance was only recently formalized in a paper published in Acta Ichthyologica et Piscatoria, documenting it as the first oarfish record for the Indian Ocean.
Only a few records of oarfish are scattered over the Indian Ocean and only two specimens have been identified to species level as R. russellii, Rathnasuriya says. The limited observations of R. glesne from the Indian Ocean highlight the importance of the current report, he adds.
Meanwhile, reports of oarfish have surfaced elsewhere recently. In May this year, Indian media described a 9.1 m-long (30-ft) specimen of oarfish off Tamil Nadu in southern India, not too far from Sri Lankan waters. Just weeks later, carcasses appeared in Tasmania and New Zealand, totaling three sightings in just 20 days that puzzled naturalists.
Encounters in the deep
Russell’s oarfish can reach up to 8 m, so the Sri Lankan find represents a juvenile, Rathnasuriya notes. With no external injuries, the reason it rose from the depths of the sea remains a mystery. Oarfish typically inhabit waters below 200 m (656 ft), surfacing only when injured, dying or accidentally netted. Until now, most confirmed records of R. russelii came from the Pacific and Atlantic, so Sri Lanka’s specimen fills a key gap in understanding its distribution and offers a baseline for future research.
Ribbon-like, silvery and marked by a crimson dorsal fin running the length of its body, R. russelii is striking yet harmless, feeding mainly on plankton. Still, its elusive lifestyle has left much of its biology — reproduction, population dynamics and ecology — largely unknown.
In 2011, scientists made history by recording the first live observations of the giant oarfish in its deep-sea habitat, a creature previously known almost entirely from strandings or dead specimens. The encounter took place in the northern Gulf of Mexico during the SERPENT project (Scientific and Environmental ROV Partnership using Existing Industrial Technology), which uses industrial remotely operated vehicles (ROVs) says Mark Benfield of Louisiana State University in the U.S.
The team documented five separate sightings of oarfish at depths between 40 m and 500 m (131 ft and 1,640 ft), Benfield tells Mongabay. The videos revealed the oarfish’s striking vertical swimming posture, powered by undulating movements of its long dorsal fin.
It is important to carry out more deep-sea research in this part of the world and it would definitely reveal much more interesting sightings, says Ranil Nanayakkara, a conservation biologist with Biodiversity Education and Research (BEAR) who is involved in marine research. The advent of submersible unmanned vehicles that can go deep has enabled such research, but due to the cost and other factors, such research is still rare, particularly in the Global South, Nanayakkara tells Mongabay.
The deep-sea creatures that surface can be carried out by oceanic currents long distances on some occasions, hence it is also difficult to judge the exact location or their range only by the carcasses, Nanayakkara says, noting the importance of doing deep-water explorations.
Citizen science to bridge lack of studies
Even though countries like Sri Lanka lack deep-sea explorations, the citizen science on fisheries can reveal some interesting finds, and the oarfish find is such a good example, says Nishan Perera of the Blue Resources Trust (BRT).
In 2015, Perera co-authored a paper on the first record of the megamouth shark (Megachasma pelagios) in Sri Lankan waters based on an individual shark landing at the fisheries harbor in Negombo, off the island’s west coast.
“A fish trader who keeps in touch with a co-author informed about the unusual fish. Then we visited the area to study the carcass,” Perera tells Mongabay.
It’s an extremely rare shark, and this particular specimen, which landed at the Negombo market in 2012, is only the fifth megamouth to be recorded in the Indian Ocean, Perera says.
In the case of oarfish or rare sharks, the fishers identify them due to their abnormal features. However, there would be dozens of smaller fish going unnoticed, and among them would be those never recorded, very rare or even species totally new to science, Perera says, emphasizing the importance of promoting citizen science and having a mechanism to collect data from fisheries landing sites. Most of these unidentified fish would not have a commercial value, hence there is good chance that fishers just throw them as worthless catch, Perera adds.
Oarfish as omens
In Asia, the oarfish is steeped in cultural mythology as well. As per literature in Japan, the oarfish is called Ryūgū no tsukai, or “messenger from the sea god’s palace.”
Strandings are often interpreted as omens of natural disasters, especially earthquakes and tsunamis. This belief gained traction after several oarfish washed ashore prior to the catastrophic 2011 Tōhoku earthquake. But interestingly, Japanese research that analyzed the oarfish sightings and events of natural disasters such as an earthquake debunk this as simple mythology.
Banner image: A 2.6 meter-long (8.5-ft) oarfish (Regalecus russelii) caught off the western coast of Sri Lanka is the first record of the world’s longest fish in its waters. Image courtesy of Ishara Rathnasuriya.
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Citations:
Rathnasuriya, M. I., & Hapuarachchi, T. H. (2025). First record of oarfish, Regalecus russellii (Actinopterygii, Lampriformes, Regalecidae), from Sri Lankan waters, Indian Ocean. Acta Ichthyologica et Piscatoria, 55, 145-150. doi:10.3897/aiep.55.148496
Fernando, D., Perera, N., & Ebert, D. A. (2015). First record of the megamouth shark, Megachasma pelagios, (Chondrichthyes: Lamniformes: Megachasmidae) from Sri Lanka, northern Indian Ocean. Marine Biodiversity Records, 8. doi:10.1017/s1755267215000512
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