Ocean Sunfish—Mola Mola
It was fall of 2014, and the scene was like a 1950s-era sci-fi movie in miniature, set along our coast. Boaters offshore encountered broad expanses of ghostly-white disks of various sizes, floating at the surface. The spectacle looked like a shipment of mutant Frisbees© that had fallen off a freighter. Sadly, beachgoers found many onshore that had died and washed ashore—for these were fish-festooning the surf line, fins missing: likely the result of fishing gear entanglement. No alien invasion, it was a host of ocean sunfish. This species of fish is known colloquially and scientifically as mola mola, and that autumn they had appeared in unusually high numbers in our coastal waters.
The ocean sunfish has been variously described as a: floating garbage can lid; seagoing flying saucer; or a garden stepping-stone. Examined closely, it resembles an oversized fish head with a small tail; a seeming afterthought. Its body is flattened laterally, giving it a full-moon shape. When dorsal and ventral fins are both extended, they are nearly circular. It is the heaviest bony fish known to science, with adults weighing 2,200 lbs. on average.
Ocean sunfish are cosmopolitan: found in tropical and temperate waters around the globe. Locally, they seem to like the warm temperatures that the Davidson Current brings to our north-central California waters in the fall, which coincides with peak jellyfish abundance in the Greater Farallones sanctuary. Jellies, especially sea nettles, are their dietary staples. Since jellies offer little nutrition, molas consume them in huge quantities. But they also eat crustaceans, sea stars, sponges, mollusks, squid, algae, plankton, and small fish.
Sunfish have a noticeable presence in north-central California waters: singly, or rafts of them, riding the swell. From their seemingly idle behavior, one would not know they are in fact waveborne wonders. This open-ocean fish can dive to depths of over 2,000 ft. to feed on sponges and corals. To recover from these cold depths, this otherworldly creature basks at the ocean’s surface to soak up the sun’s rays, and readjusts its internal chemistry to sea level norms. While resting, it may offer a buffet for hungry gulls who have learned to pick parasites from its sandpapery skin. But once it’s time to go hunting again, it uses a powerful sculling body motion, its tiny tail functioning as a rudder.
Females can claim the fecundity crown among vertebrates (animals with backbones): sunfish hold the egg-laying record, each capable of laying 300,000,000 eggs at a time.*
Adult sunfish are vulnerable to few natural predators, with the exception of sharks, killer whales, and sea lions. In fact, for some sea lions, it’s not just a sunfish – it’s a funfish. They've been seen grasping small molas and flinging them around, a’la Ultimate Frisbee©.
Biologists have raised the alarm that populations in the western Pacific, and some parts of Europe, are declining. They are not a marketable food fish in U.S. waters, but are considered a delicacy in some parts of Asia. While not fished commercially here, molas nonetheless are the most common victims of bycatch of the drift net fishery, comprising over 25 percent of the California fisheries bycatch. It is the undiscriminating drift gill net that entangles all that comes its way. NOAA Fisheries is currently working with the fishing industry to learn more about their habits, and to help minimize this bycatch, much of which occurs in the swordfish fishery. Drift gill net fishing is illegal in most states, but still legal in California.