Sunfish

Sunfish
  • Scientific name: Mola mola
  • Also know as: Ocean Sunfish, Giant Sunfish, Common Mola, Moonfish
  • Size: Up to 10ft length and 14ft 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 (i.e. excluding sharks). As the unusual appearance of the sunfish 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, something many people put down to global warming and climate change. However, it is worth noting that sunfish have been caught – very occasionally – for decades with the shore caught record of 49lb 4oz (a mere baby by sunfish standards) standing since 1976, and sunfish have been reported 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 the Arctic regions. In European waters they are found throughout the north east Atlantic, Mediterranean and Black Sea, although their range does extend as far as the cooler 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 the 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 further back in the throat of the fish, and crunch up food prior to passing it to the stomach.

Basking Sunfish

A sunfish basking on the surface of the sea.

Sunfish can be found in water down to several hundreds metres deep, and usually feed in the middle of the water column. Sometimes sunfish can be found lying flat on the surface of the sea. A number of theories have been put forward to explain why sunfish do this with some people claiming that they 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 truth is that it is a mystery why sunfish take part in this behaviour. 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.

Breeding and Growth

Sunfish fry

A newly hatched sunfish.

Little is know about the sunfish breeding patterns, although it is thought that they spawn in the Atlantic, Pacific and Indian oceans at points near to the equator. Female sunfish produce around 300 million eggs (more than any other vertebrate) which are fertilised externally. Sunfish fry are just 2.5mm 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. Small sunfish (under a few feet in length) have been observed moving in shoals, although it is thought that this species becomes solitary once it is mature.

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, however, sunfish is classed as a delicacy. However, sunfish numbers have been madly reduced across the whole of the range of the species through due to sunfish 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 Near Threatened and has a decreasing population trend.

Maximum Size

Enormous Sunfish

An enormous 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.

As stated sunfish are the heaviest bony fish in the world. The average size of a mature sunfish is usually several hundred pounds. However, the very largest can be 10ft in length, 14ft vertically and weigh around 4,500 – 5,000lbs, making the sunfish one of the very largest bony fish species in the world.

Other Sunfish Species

There is a second species of sunfish in the Mola genus called the southern sunfish (Mola ramsayi), as well as other closely related species such as the slender sunfish (Ranzania laevis). There are also a number of freshwater species known as sunfish around the world but these are completely unrelated to the ocean sunfish.

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