Sunfish

Sunfish
  • Scientific name: Mola mola
  • Also know as: Ocean Sunfish, Giant Sunfish, Common Mola, Moonfish
  • Size: Up to 10ft length and 13ft vertically (including fins). Up to 5,000lb.
  • UK minimum size: N/a
  • UK shore caught record: 49lb 4oz
  • ICUN Status: VU (Vulnerable)
  • Distribution: Found in tropical seas around the equator, although an increasing number are appearing around the south and west of the British Isles.
  • Feeds on: Mostly feeds on jellyfish, but can also hunt small fish and squid.
  • Description: Striking looking huge, heavy, laterally-compressed fish. Usually grey but can have orange, yellow and green colouration. Single large triangular dorsal and anal fins set far back on the body. Unusual beak-like mouth which the sunfish is incapable of fully clothing. Frill like fin exists at the back rather than a tail.

The sunfish is an incredibly strange-looking fish that can grow to gigantic sizes. It is the heaviest bony fish in the world. As the name suggests this species originates in tropical seas around the equator, but it appears to be getting more common around the south of England in the summer months due to warming sea temperatures. However, it is worth noting that sunfish have been caught in UK waters (on a rare basis) for decades with the shore and boat caught records (49lb and 108lb respectively) both being set in  1976. Sunfish have also been observed on an irregular basis all around the UK, not just in the south.

Distribution

Sunfish have an extremely wide distribution and are found on a worldwide basis. They are generally a warmer water species found around the equator but their range does extend into temperate and cooler seas, only being absent from the cold waters of the Arctic regions. In European waters they are found throughout the north east Atlantic, the Mediterranean and the Black Sea, although their range does extend as far as the waters of the Nordic countries and into the Baltic Sea.

Diet and Behaviour

Sunfish are slow-moving creatures with their small fins offering only a limited ability to move their great bulk around, as the video below (© Petra Schriever) shows. They feed by sucking easy-to-catch jellyfish into their strange beak-like mouth, which they are not capable of fully closing

It is thought that sunfish have to eat vast amounts of nutritionally-poor jellyfish in order to maintain their size. Sunfish also have a limited ability to hunt and will use whatever speed they can muster to catch small fish and squid. They may also feed on starfish and brittle stars if this source of food is present. Sunfish do have teeth but they are located in the throat of the fish and crunch up food prior to it passing to the stomach.

Basking Sunfish
A sunfish basking on the surface of the sea.

Sunfish usually feed in the middle of the water column. Sometimes sunfish can be found lying flat on the surface of the sea. The reason for this is unknown but theories range from claims that sunfish are resting after swimming energetically, floating on the currents to save energy, warming up in preparation for a deep dive into cooler water or allowing seabirds to land on them and pick off parasites. The size of sunfish means that they have few natural predators themselves, although seals and sharks can attack sunfish – these pictures show a seal biting chunks out of a slow-moving sunfish.

Reproduction and Growth

Sunfish fry
A newly hatched sunfish.

Little is know about the sunfish spawning patterns, although it is thought that they spawn in the Atlantic, Pacific and Indian oceans at points near to the equator. Female sunfish produce hundreds of millions of eggs which are fertilised externally. Sunfish fry are just a few millimeters long when newly hatched (as the picture, right, shows), but will go on to grow extremely quickly, with evidence suggesting that sunfish can increase their weight by hundreds of pounds in just a few years. Indeed, the largest sunfish are believed to weigh between fifty and sixty million times their birth weight. Small juvenile sunfish (under a few feet in length) have been observed moving in shoals, although it is thought that this species becomes solitary once they are fully grown.

Commercial Value and Population Trends

 Sunfish Dish
A dish made with sunfish meat.

Sunfish are not caught commercially in Europe (indeed, the trade of sunfish meat is banned by the EU and it is an offence to sell or buy it) but in Asian countries such as Japan, China and Taiwan sunfish is classed as a delicacy. The main threat to sunfish has been through the species being inadvertently caught as bycatch. Sunfish are caught on long-line, drift nets and other forms of fishing gear, with the IUCN (International Union for the Conservation of Nature) stating that in some fisheries (usually those targeting species such as swordfish in the southern Atlantic and Pacific) the bycatch of sunfish is greater than the total of the target catch. A further issue is caused by plastic pollution with sunfish choking on plastic carrier bags and other forms of waste which they mistake for jellyfish. On a global basis the IUCN classes sunfish as a species which is Vulnerable and has a decreasing population trend.

Maximum Size and Other Sunfish Species

Enormous Sunfish
An enormous ocean sunfish caught by W.N. McMillan off the coast of Santa Catalina Island, California on April 1st, 1910. Its weight was estimated at 3,500 pounds.

The average size of a mature ocean sunfish is usually several hundred pounds, but at their very largest can be 10ft in length, 13ft vertically (if the fins are included) and potentially weigh up to 5,000lbs. Another very closely related species of sunfish, Mola ramsayi, also known as Ramsay’s sunfish or the bump-headed sunfish, has been confirmed as the largest bony fish in the world, when, in 2021, a specimen weighing 6,049lbs was found dead in the waters of Faial Island in the Azores. Until 2017 it was believed that the Mola mola and Mola ramsayi were the only two species of sunfish. However, in summer 2017, Danish PhD researcher established Marianne Nyegaard that there was a third species of sunfish in Australian and New Zealand waters. This was named the hoodwinker sunfish (Mola tecta) due to its ability to remain undiscovered as a distinct species for so long.