Todd McLeish: The bizarre (to us) ocean sunfish washes up in New England

Ocean sunfish

A tank at the Monterey Bay Aquarium, in California, provides a size comparison between an ocean sunfish and humans.

From ecoRI News (ecori.org)

The ocean sunfish earned its moment in the spotlight in 2015, when a viral video surfaced of a foul-mouthed recreational fisherman who observed a specimen along the Massachusetts coastline and excitedly tried to guess what it was as the fish calmly rested at the surface.

The largest bony fish, the pie-shaped creature is certainly an oddity to those who are unfamiliar with it — they bask on their side on the water’s surface and can grow to nearly 11 feet and weigh up to 5,000 pounds by eating almost exclusively jellyfish.

Like whales, however, they also sometimes become stranded on beaches or in shallow tidal areas, where they are unable to extricate themselves and die. Almost 350 of them have stranded along the New England coast since 2008, according to Michael Rizzo of the New England Coastal Wildlife Alliance, who studies the species.

Rizzo presented the results of his analysis of ocean sunfish strandings April 13 at the Northeast Natural History Conference in Springfield, Mass.

Also called mola mola — a name derived from the Latin for millstone, a reference to the massive animal’s circular shape — ocean sunfish are found in New England waters each summer and are observed wintering off the coast of the southern United States.

“A lot of them wind up stranding in New England every year, starting in August and continuing through early January, but the busiest months are October to December,” Rizzo said. “When they get into shallow areas, they get stuck and can’t get out. Once the tide goes out and they’re in the mud, you can’t move them.”

A record 81 ocean sunfish were reported stranded in New England in 2017, with an additional 60 stranding in 2018. Staff and volunteers from the alliance attempt to rescue those that are still alive, though few survive. In one case, an ocean sunfish that stranded in a shallow tidal area was towed into open water, only to have it strand again and die a short time later less than a mile away.

The alliance also collects sighting data of live ocean sunfish to better understand their abundance and activities while in New England.

Many ocean sunfish are killed or become stranded as a result of fishing gear entanglements and injuries from boat propellers, but the most common cause is cold stunning.

“That’s a physiological condition an animal can experience due to prolonged exposure to cold water,” Rizzo said. “They become hypothermic and can’t move any more. It’s very similar to what happens to sea turtles.”

Most of ocean sunfish strandings occur along the coast of Cape Cod Bay, though some have stranded as far north as Portsmouth, N.H. Others have stranded on Nantucket, but none were reported to have stranded along the Rhode Island or Connecticut coast in the past decade.

“It seems that most of them are going south and get caught up in the fishhook of Cape Cod and they wander around and can’t get out,” Rizzo said. “Once they get around Cape Cod, it seems as if they take a straight shot south and avoid the southern New England coast.”

Little is known about the population or distribution of ocean sunfish in the area.

“From what we can tell and from what we have read, the mola population is robust but decreasing, which is why they are listed as vulnerable,” said Carol “Krill” Carson, a marine biologist and president of the alliance. “With many threats to the marine environment, including climate change and marine debris, we are afraid that this species will see continued loss in population numbers.”

Since so little is known about them, the alliance conducts a necropsy (animal autopsy) on as many of the dead ocean sunfish as it can, and samples of numerous tissues are collected for scientists to study. Research is being conducted on their diet and toxicity, as well as on the more than 40 species of parasites that have been found infesting various parts of their body. Efforts are also underway to learn how to determine their age and how best to rescue them from beaches.

Scientists hope that additional data on ocean sunfish strandings will help to identify why so many are stranding in certain years. Since cold stunning is the primary cause of most strandings, Rizzo and Carson speculate that warming waters due to climate change may be having an effect on the fish by delaying their southbound migration until it’s too late.

If that were true, Rizzo said, then the number of sea turtles found stranded should correlate with ocean sunfish strandings, and that isn’t always the case.

“It was a big year for sea turtle strandings in 2014, for example, but that was a low year for ocean sunfish,” he said. “We’re going to try to do a water temperature analysis to see if that tells us anything.”

Rhode Island resident and author Todd McLeish is an ecoRI News contributor who also runs a wildlife blog.