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 in the sea, 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 up to 5,000
pounds by eating almost exclusively jellyfish.
Like whales, however, they also
sometimes become stranded on beaches or in shallow
Rizzo presented the results of his analysis
of ocean sunfish strandings at the Northeast Natural History Conference in
Springfield, Mass., on April 13.
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 U.S.
“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.”
The overwhelming majority 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 last 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.”
Because so little is known about them, the
alliance conducts a necropsy (animal autopsy) on as many of the dead ocean
sunfish as they 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.”
This article first appeared on EcoRI.org on April 21, 2019.
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