Climate change, predators blamed for winter flounder’s demise in RI

Like plaice, sole, halibut and other species of flatfish, the winter flounder doesn’t look quite right compared with other creatures that follow the rules of symmetry. 

It’s like a Looney Tunes version of a fish, one that got squashed by an anvil. Or something imagined by a Cubism-era Picasso.

But what the fish lacks in aesthetics, it makes up for in ingenuity. What sets winter flounder apart from other fish is a unique spawning habit honed by evolution. To avoid predators, it reverses the migratory cycle of most other fish.

While species like scup or black sea bass come inshore to spawn in the summer and spend the cold months offshore, winter flounder do just the opposite, summering in cooler, deeper waters along the Outer Continental Shelf and waiting until November or December to make the trip into Narragansett Bay and other coastal estuaries.  

The fish display fidelity to spawning sites, nearly always returning to the places where they hatched. The ones that come back to the Bay are more or less a distinct group that spends the summer months in Rhode Island Sound, as far east as the waters around Nantucket and Martha’s Vineyard. 

In the dead of winter and usually under cover of night, as water temperatures drop to near freezing, winter flounder start their breeding ritual. Like other fish adapted to extreme cold, the species produces a special protein that acts like antifreeze, preventing the growth of ice crystals in its body.

The female releases batches of hundreds of thousands of eggs to be fertilized by a male partner in January, February or March, waiting for temperatures to dip below about 42 degrees Fahrenheit, when predators should be absent, and creating what’s known as a thermal refuge for their young. 

Larvae hatch from the eggs after about three weeks and grow into tiny creatures that look initially like any other fish that swim upright in the water column. 

But then, after another six weeks or so, they drift down towards the bottom and start to undergo a singular metamorphosis. As a larva transforms into a juvenile, one eye switches sides, moving over the head to sit alongside the other.

The young fish then turns over onto its side and settles in shallow waters, where it spends its first two years in relative shelter, feeding on worms and crustaceans, before growing strong enough to make the trip offshore. 

As a survival strategy, this spawning behavior is only as good as its timing — in scientific terms, its phenology. 

If it’s off by just a little, a place that would otherwise offer sanctuary can suddenly become very dangerous.

The earliest theories about the demise of winter flounder focused on the sand shrimp, a stout-bodied crustacean found in estuaries all along the Atlantic coast.

Described by the late H. Perry Jeffries, a longtime University of Rhode Island oceanographer, as an “inch-long terror” that eats anything it can get its claws on, the shrimp follows the more typical migration pattern, moving back into shallow nearshore areas only when waters warm in the spring. 

But if temperatures rise earlier than normal or if winters are milder, the voracious shrimp overlap with winter flounder eggs and larvae when they’re at their most vulnerable and gobble them up.

More recently, researchers have theorized that summer flounder — a bigger, faster and more aggressive warm-water fish that preys on juvenile winter flounder — is having an impact. It, too, is spending more time in the Bay and in larger numbers, posing a more potent threat to the young of its flatfish relatives.

Scientists have also pointed a finger at the striped sea robin, a mid-Atlantic fish not to be confused with its more benign cousin, the northern sea robin, a native of the Bay. As waters have warmed, striped sea robins have proliferated, the population growing from next to nothing in the 1960s to robust numbers today. 

A fish that URI oceanography professor Jeremy Collie has likened to a vacuum that seems to fly through the water, it’s a skilled predator that winter flounder never had to reckon with before.

And there are those who are looking beyond fish. The harbor seal, for one, is known to eat winter flounder, and while the marine mammal’s numbers appear to have remained stable over the last 25 years, it occupies the Bay during the fish’s entire spawning season.

The double-crested cormorant is another threat. The diving bird returned to Rhode Island as a breeding species in 1981 after being all but exterminated regionally. Cormorant numbers have leveled off, but the bird is a peerless underwater hunter and has been shown to feast on winter flounder.

It’s a formidable lineup of potential culprits, but are there prime suspects among them?

Joe Langan, a researcher at URI, thinks so.

A native of upstate New York, he traces his interest in the underwater world to a childhood spent fly-fishing for brook trout in the headwaters of the Delaware River in the Catskills. 

Langan saw the effects that extreme weather could have on an ecosystem when a succession of floods near his family’s camp silted up the gravel-bottomed rivers and streams that trout prefer. It wasn’t a stretch to start thinking about the nexus between the natural world and climate.

During college in Maine, he studied Atlantic cod, the most iconic of New England fish, before starting his doctorate six years ago at URI, where one of his first jobs was manning the Graduate School of Oceanography’s weekly fish trawl. Similar to the DEM survey conducted every month by the research vessel John H. Chafee, its purpose is to take snapshots of the Bay, collecting data on what species are around and in what numbers at any one time.

Over weeks and weeks of sorting, counting and measuring catches, Langan saw up close the decline in winter flounder. Whereas Candace Oviatt, a URI professor who has been studying the Bay for some 50 years, and other predecessors were scooping up 150 or more per trawl, Langan would find only one or two.

Earlier researchers had long since decided that overfishing was not the driving force behind the demise of the species. As far back as 1974, after a dip in numbers in the Bay, Jeffries was sounding the alarm in the pages of The Journal about warmer-than-normal winters.

More recently, Richard Bell, a fisheries scientist with The Nature Conservancy in Rhode Island, looked at the trajectories of winter flounder and summer flounder and compared their differences. While both were overfished in the 1980s and ’90s, only the summer flounder stock rebounded after fishing was restricted. He concluded that temperature increases were standing in the way of a winter flounder recovery. 

Langan set out to learn exactly why by weighing all the threats facing winter flounder against each other. 

For his dissertation, he reconstructed the life cycle of the fish using information from the two long-term Bay surveys, as well as sampling data from Brayton Point and the results of a DEM seine-net survey for juveniles. He also worked with Conor McManus, the deputy chief of the DEM’s Division of Marine Fisheries, to sample for larvae and added the results to a computer model he created.

After estimating numbers of the fish at each life stage, from egg to adult, he was able to pinpoint where the population was losing numbers, finding that the steepest drop-off is coming within the first year of life. 

Not surprisingly, he found that fishing isn’t much of an issue anymore. And he determined that the predator that is having the greatest impact is the striped sea robin, a fish whose presence in the Bay is a direct result of overall warming.

But, according to Langan’s model, it’s not the only climate-related factor at play. A more significant element appears to be an increase specifically in summer temperatures, the effects of which, unlike predation, are indiscriminate. They are being felt in two ways. 

For one, warmer waters hold less dissolved oxygen than colder waters, and when temperatures in June, July or August spike, it can further deplete oxygen levels in the Bay, forcing young fish to use more energy for survival and retarding their growth rates. As a result, juvenile fish are smaller than they would normally be. And smaller fish are susceptible to more predators.

More importantly in the long run, winter flounder can only tolerate so much heat. When average summer water temperatures in the Bay climb above about 72 degrees, the chances of survival for the fish drop, especially in the shallower coves and rivers preferred by juveniles that get even hotter. 

Records going back to 1959 show that temperatures never got so high until this past decade. But they’ve crossed that threshold four times since 2012.

“Temperature seems to be the problem and it leads to all these downstream effects,” Langan says.

Langan emphasizes that his findings are far from the final word on the cause of the winter flounder crash.

His research doesn’t mention sand shrimp, but only because of a lack of the right data. The shrimp primarily eat winter flounder eggs, which, unlike other fish eggs that float in the winter column, are deposited on estuary bottoms. Because they’re so difficult to find and collect, it’s impossible to know how many are being eaten. 

But Langan doesn’t doubt the shrimp’s impact, citing past studies by David Taylor, at Roger Williams University, and others that found the stomach contents of shrimp were full of eggs and larval flounder.

He also doesn’t discount that cormorants are having an effect. His model found that the predatory bird is playing a role, just not as big as other factors. And he readily acknowledges that there could be something else going on that his model hasn’t captured.

Others who know the Bay well don’t contradict Langan’s findings, though some have their own theories for what happened to winter flounder. 

Lobsterman Lanny Dellinger still points to cormorants, saying their rise in numbers came at the same time as the drop in the flounder population. He’s watched the birds “line up like soldiers” in Wickford Harbor, taking turns to dive for fish.

“They’re incredibly efficient,” he says. “Every time they’d come up, they’d have one in their mouth.”

Jerry Carvalho, a retired commercial fisherman who trawled the Bay for winter flounder in the fish’s heyday, believes that sand shrimp and seals have played a part. And he gives special attention to summer flounder, which spawn earlier than winter flounder and grow much faster, allowing its juveniles to prey on the young of the other species.

As past president of the Rhode Island Fishermen’s Alliance, he filed a lawsuit in the 1990s challenging the ban on fishing for winter flounder in the Bay, and he has never believed that overfishing was responsible for what happened to the fish.

“We never ran out of winter flounder,” he says. “Not until we saw the effects of temperature and predation.”

In his paper comparing summer flounder and winter flounder, Bell described how the specific conditions needed for winter flounder or similar specialist species to survive are not only governed by physical boundaries but also temporal ones.

“… habitat is not the simple sum of the area of each estuary, but a moving envelope, which varies at multiple scales in both space and time,” he wrote.

It’s this aspect of time that has become a problem for winter flounder. In more ways than one, the fish appears to be running out of it.

There are still plenty of sheltered areas with muddy bottoms in the Bay that the species prefers. But even with all the physical habitat in the world, if the right temperatures don’t occur at the right times or for the right durations, winter flounder will struggle.

As a fish with discreet spawning needs, it may be at a disadvantage when compared with other, more generalist species.

Oviatt, who describes Langan’s work as built on “rock-solid statistical methods,” says his model shows how climate change can lead to a cascading series of impacts on a fish like winter flounder. 

“It just has all the strikes against it,” she says.

But even though winter flounder may be suffering more, it’s not the only inhabitant of Narragansett Bay that’s experiencing change in a warming world.