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The ocean sunfish is the heaviest bony fish in the world; it can grow more than 10 feet long and pack on a whopping 5,000 pounds, and yet its flat body has no real tail to speak of. (Mike Johnson)

Unraveling the Mysteries of the Ocean Sunfish

Marine biologist Tierney Thys and researchers at the Monterey Bay Aquarium are learning more about one of the largest jellyfish eaters in the sea

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Thys has since tagged and tracked molas in Taiwan, South Africa, Bali and the Galapagos Islands, and in doing so, she has become one of the world’s leading experts on the fish. She runs a website, Oceansunfish.org, which serves as an information hub on the species, and she asks citizen scientists to report any sightings. “Nearly every day I have people reporting,” says Thys. Molas have been seen north of the Arctic Circle and as far south as Chile and Australia. “I just got a report from Mozambique,” she says. “I would love to go to Mozambique.”

Since ocean sunfish are neither known to be endangered nor are they commercially important (outside of Asia, particularly Japan, Taiwan and the Philippines, where they are eaten), research funding can be hard to come by. Thys scrapes together small grants and squeezes tagging expeditions into her busy schedule as a mother of two, National Geographic Explorer and science media filmmaker and consultant on several marine education projects. “I moonlight on the sunfish,” she says.

The methods for tagging vary by location. In California, Thys and her Monterey Bay Aquarium colleagues often use a spotter plane. From the air, the pilot spies the white outlines of molas and radios their location to a team in a boat below. The ocean sunfish owes its name to its tendency to bask in the sunlight near the surface. In some cases, gulls on the water’s surface also indicate the presence of molas, since Western gulls and California gulls clean the fish of the dozens of species of parasites that live on them. In Bali, where molas don’t spend much time on the surface, Thys and her team tag the fish underwater with modified spear guns. But in other places, it is just a matter of scanning the surface from the bow of a Zodiac boat. “They are just goofy,” says Thys. “They stick their fin out of the water and wave, ‘Hello, I am over here.’ ”

Once a mola is spotted, the group speeds up to it and traps it in a hand net. Snorkelers wearing wetsuits and gloves to protect against the fish’s spiny skin (Thys compares it to “36 grit sandpaper”) jump into the water and corral the fish alongside the boat, while someone inserts the tag at the base of the fish’s dorsal fin.

This past September, Thys had what she considers one of the most amazing sunfish encounters of her career. At a place called Punta Vicente Roca, on Isabela Island in the Galapagos, she and her team came upon a group of about 25 molas, each about five feet long, while diving at depths of up to 90 feet. “I didn’t even know where to look,” says Thys, showing me video footage she took with a small, waterproof camera mounted like a headlamp on a strap around her head. Adult sunfish are loners and do not school, so it is rare to see more than a couple at time. But this spot was a cleaning station. The molas were suspended in a trance-like state, their heads pointed upward while juvenile hogfish pulled off their parasites. “It was awesome,” she adds.

Thys likens molas to “big, slobbery Labradors.” (In addition to parasites, the fish are covered in mucus.) O’Sullivan calls the slow-moving, awkward fish “the Eeyore of the fish world.” Needless to say, molas are harmless and generally unperturbed by humans. Wild encounters, like this one, make Thys wish she could follow the fish to see where they go and what they are up to. That is where satellite tags come into play.

Most of the time, Thys uses pop-up archival transmitting (PAT) tags that release from the fish at a pre-programmed time, drift to the surface and transmit data about the fish’s movements—its locations and depths, as well as water temperatures—by satellite. In the Galapagos, however, she tagged five sunfish with acoustic tags; on two of them, she also placed Fastloc GPS tags. An array of underwater listening stations detects the unique signal of each acoustic tag, while GPS tags reveal sunfish locations in real time. One of the GPS tags, programmed for nine months, released after less than two, but it revealed some interesting details. The fish had traveled nearly 1,700 miles from the archipelago, for reasons unknown, and had made a record dive down to 3,600 feet. Another Fastloc tag is due to pop off this month; its real-time reporting capabilities failed but it could still relay some data.

“We are starting to unravel a bunch of the mysteries,” says Thys. Pockets of mola researchers around the world have found that molas are powerful swimmers that buck ocean currents—dispelling a myth that they are lethargic drifters. Scientists are looking into what factors drive molas’ migrations, though one seems to be temperature. The fish prefer water ranging from 55 to 62 degrees Fahrenheit. Molas also dive up to 40 times a day. They descend to depths, on average, of 310 to 560 feet, most likely to forage in a food-rich zone called the deep scattering layer. Presumably to recover from temperatures as low as 35 degrees Fahrenheit at that level, they then sunbathe at the surface.

But every discovery, in turn, leads to more questions. Molas are found in temperate and tropic waters worldwide, but how big is the total population? The fish make up a large percentage of the unintended catch in fisheries in California, South Africa and the Mediterranean. How is that bycatch impacting overall numbers? Female molas can carry an estimated 300 million eggs, making them the most fecund fish in the sea. Where do they spawn, and at what age?

Molas eat gelatinous zooplankton, such as moon jellies, as well as squid, crustaceans and small fish, including hake, and their eating habits may change as they grow. But how much do they have to eat to keep their portly figure?

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