When marine biologists Rob Harcourt and Matthew Kertesz saw the giant dorsal fin poking out of the water immediately ahead of their tiny boat, they thought the worst. It was not unusual to spot a big shark in Jervis Bay, south of Sydney, an area famed for its rich ocean life and stunning clear water. But on closer inspection, Harcourt felt his heart beat faster with excitement as he realised that a very different kind of creature loomed below him in the water.
As it wheeled over onto its side, the animal revealed itself to be a giant ocean sunfish, normally found hundreds of kilometres out to sea, which had somehow meandered into the shallow waters off the coast.
The two marine specialists realised what a rare treat stumbling across the fish was. They lost no time donning flippers and masks, and leapt into the 40 metre deep water with a camera.
Up close, the animal was breathtaking. This was just a little one: about three metres across, but Harcourt knew these ocean giants could reach over four metres in length and weigh up to 2000 kilograms.
The huge eye, the size of a fist and not at all afraid, loomed close as Harcourt took in the body shape, possibly one of the strangest in the already strange world of fishes.
The head was enormous: in fact, the animal appeared to be all head, with a tail fin running down the back of the 'neck'. Emerging from this head/body were the fins. Lacking most of the fins a normal fish has, sunfish have just two: giant couch-sized appendages protruding from top and bottom of the body. In front of the huge doleful eyes were the lips, as large and luscious as Mick Jagger's.
The amazing sunfish
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The sunfish is the largest and most fertile bony fish in the world. Some sharks (such as the whale shark and great white) can grow larger, but these are cartilaginous fish, rather than bony fish. Sunfish can produce massive numbers of eggs: one female caught off Florida was carrying 300 million eggs. This makes the cane toad look quite modest, producing a mere 60,000 eggs per clutch.
Sunfish tend to lie on their side close to the surface of the ocean, appearing to bask in the warmth of the sun, say researchers at the Large Pelagic Research Lab at the University of New Hampshire. They may be 'thermally recharging' after diving to depths where their bodies have been significantly cooled by the deep water.
Although a rare sight near the coast of Australia, researchers say sunfish have been seen off Perth and at Ningaloo Reef, and also in the harbours of Darwin and Sydney. They inhabit all the main oceans of the world, both tropical and temperate.
There are also anecdotal sightings of sunfish made by the general public. Responding to a Sydney Morning Herald story about the Jervis Bay sunfish sighting, readers variously reported seeing sunfish at Botany Bay and Wollongong, and in New Zealand at the Three Kings (northern New Zealand) and Palm Beach (west of Auckland).
The group Molidae includes four species of sunfish: Ranzania laevis (slender mola), Masterus lanceolutus (sharptailed mola), Mola mola (common mola), and Mola ramsayi (Southern Ocean Sunfish). Sunfish are frequently called 'mola' even though only two of the four species are in this genus. ('Mola' in Latin means "millstone" and describes the ocean sunfish's somewhat circular shape.)
Up close and personal
Anyone who spots a sunfish at sea is pretty lucky. Normally, the only way to get a close-up look at a real sunfish is to wait until an injured or dead individual washes up on a beach. But there is at least one place in the world where you are almost guaranteed to see a sunfish and even dive with one of these marvels of nature.
Near several tiny coral-fringed islands off the east coast of Bali, sunfish congregate each year from August through to October. Mysteriously, the normally ocean-loving fish move towards the coast at this time of the year to feed and live in waters as shallow as 25 metres, making it one of the best places in the world to see them.
Since arriving in Bali from Perth eight years ago Jonathan Cross, who runs dive company Blue Season Bali, has led hundreds of sunfish dives, but still finds it fascinating.
He'll typically see up to 20 sunfish each day across several dive sites: "They are amazing things ... very difficult to describe. You can go right up to them and they've got this deep, black eye set right back in their frame. They look so prehistoric, they seem to have been around for such a long time. "
"They swim around and may scoot away if you scare them or blow bubbles at them but you can get as close as 1 metre from them. They can be very curious...When they are coming towards you, they look quite thin, but then when they turn side out, they are just massive."
There are a number of theories as to why sunfish flock to Bali each year. One reason may be to get parasites removed by the cleaner fish which abound on the reef. The huge surface area of the sunfish is a magnet for parasites, and up to 40 different species of isopods, worms and molluscs have been found picnicking off one fish.
The area also experiences a peculiar upwelling of cold, deep, water from August through to October. This nutrient-rich water is a rich food supply for tiny marine animals which may attract sunfish.
Travelling to Bali may be the best way to see sunfish up close, but where they spend the rest of their time remains something of a mystery.
Mola mystery
In America, researchers have been devising ingenious ways of following the normally elusive sunfish to try and get a handle on even the most basic information about their biology, development and movements.
Inga Potter, a researcher with the Large Pelagic Research Laboratory at the University of New Hampshire, is one of only a handful of researchers who focus on sunfish. Her group has attached satellite tags to sunfish off the north-east coast of the United States, and tracked their movements over summer. But even attaching tags is no easy matter. Smaller species such as turtles can be caught and hauled aboard to be tagged, but not a 2000 kilogram sunfish. The only way is to jump into the water with the fish and physically attach the tag, all the while hoping you don't encounter any unpleasant visitors!
Potter's tagging studies are beginning to reveal that sunfish travel vast distances: one fish tagged off the coast of New England travelled 3000 kilometres in 130 days, eventually ending up in the Gulf of Mexico before returning north. So why the big trip?
Potter suggests that sunfish leave the north east to begin their trek southwards in response to cooling sea surface temperatures. As the sea temperature declines, plankton numbers go down, and so in turn do the jellyfish which feed on plankton. Jellyfish are the main food source for sunfish, so once jellyfish numbers drop, sunfish beat a hasty retreat south in search of more tucker.
Sunfish may also be moving south in order to spawn in the warm southerly waters, as both tuna and swordfish do. "I would love to know more about mola spawning and reproduction," says Potter, "but I am not sure how that question will ever get answered, certainly not in our present study!"
The satellite tagging has provided information not just about the distances sunfish travel, but also about the depths they go to, and how they move about vertically within the water column. It also answers a question which Potter had been puzzling about: how do the southward-travelling sunfish tackle the rapidly northward moving waters of the Gulf Stream which lies directly in their path? The satellite tracking data revealed that when sunfish encounter the current, they actively swim under it, diving to depths of up to 750 metres.
A life in danger
As researchers struggle to learn more about sunfish, they face a vulnerable future. Although not intentionally caught for human consumption (except in Japan), thousands of sunfish are killed each year as part of the bycatch of other fisheries. According to Potter's research group the common sunfish M. mola makes up a large portion of the bycatch of the Pacific and Mediterranean bluefin long line fishery. A massive 70 to 90 per cent of the total swordfish driftnet catch was made up of the common sunfish between 1992 and 1994, and in the Pacific, ocean sunfish make up a quarter of the total haul of swordfish driftnet fisheries.
In Australia, sunfish are also killed as bycatch in the tuna and billfish fishery, according to records from the Commonwealth Pelagic Longline Fishery Data. Yet virtually no research has been done on the sunfish which live around the coasts of Australia. Nothing is known of how far and where they travel, or when and where they breed. Until a more focused research program appears, we'll have to rely on the occasional lucky encounter to learn more about this spectacular animal.
Credits
Thanks to Jonathan Cross, Director of Blue Season Bali for the use of photo and video images of sunfish and for sharing his experience of diving with them. Thanks also to Rob Harcourt from Macquarie University for photo images of sunfish and to Ingrid Potter from the University of New Hampshire for providing information about her research.
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