Winter Flounder – a Freeze Resistance Fish

A juvenile winter flounder

​There is more to flounder than just a fishy meal, no matter how tasty it might be. Winter flounder or blackbacks as some fishermen call them is a beautiful looking fish with an incredible way to survive the icy-cold winter waters of New York Harbor.
 
As you might guess, the name “winter flounder” comes from the fish’s fondness for cold water. Unlike many local species of fish that migrate out of the harbor for the winter or become largely sluggish and inactive when water temperatures start to dip below 55 degrees F, chilly water does not scare away the winter flounder (Pleuronectes americanus). It not only survives, it thrives in cold water.
 
But how did this small-mouthed flatfish become so well adapted to cold water? No joke, the fish seems to manufacture its own antifreeze. 

​Species of winter flounder have evolved in a remarkable way to endure the frosty and frigid waters of the Atlantic Ocean. Scientists Christopher B. Marshall, Avijit Chakrabartty, and Peter L. Davis in a 2005 article in the Journal of Biological Chemistry found that the fish have adapted to their cold-water environments due to the presence of “antifreeze” proteins (type I AFP) and other hyperactive antifreeze proteins in the plasma and bloodstream that are rich with amino acids, greater than 60 percent. These special proteins are capable of making a freeze resistance fish.
 
The acids bind to ice crystals to prevent the growth of pointed shapes in the bloodstream that can cause damage to internal organs, cells, and blood vessels. With cells and bodies composed mostly of water, the formation of ice crystals in the blood can lead to the irreversible damage to internal tissues. Unique chemical properties within the flat, egg-shaped body of a winter flounder help to make ice crystals grow in a more favored curved or round shape.
 
The combination of these acidic proteins work so well that winter flounder are able to survive in waters at 28.4 degrees F, the freezing point of seawater.
 
This evolutionary innovation is not only found in winter flounder, but in many fish that swim in the north and south poles, including Antarctic toothfish and the Arctic cod. Some over-wintering insects, such as beetles, have also evolved with the antifreeze gene to survive cold and snow.
 
Unfortunately, humans lack this antifreeze gene. So when our skin freezes, we get frostbite. Frostbite occurs when skin and underlying tissues freeze. In areas of the body affected by frostbite, sharp ice crystals form and cells and blood vessels become damaged.
 
Winter Flounder use this remarkable evolutionary adaptation to create offspring and spawn in the bay during the winter.  The specie's ability to tolerate cold water is an efficient strategy, allowing the fish to reproduce when few predators are active that might forage on eggs and young. 

​Winter flounder usually arrive into New York Harbor around December from deep offshore waters where it spends much of summer and fall. Spawning typically takes place in near shore waters from winter to early spring. 
 
Females produce between 500,000 to 1.5 million eggs, which they deposit on sandy or muddy bottoms and algae mats at night. After hatching, larvae have eyes on both sides of their head, just like any other fish. But in five to six weeks, the little flounders will settle to the bottom and begin their metamorphosis. After a few weeks, their left eye will miraculously migrate to the right side of their body, completing their transformation into a juvenile flounder. A single fish can live up to 18 years, growing as big as 25 inches and weighing up to 8 pounds, though 1 to 3 pounds are more typical.
 
After a winter spawn, adult flounder will start to feed. When estuarine waters begin to warm up sometime around May, adult fish will start to migrate and move offshore to cooler waters.
 
Many juvenile winter flounder, however, will remain in the seasonal waters of New York Harbor until they are around two years of age. The little fish will use the shallow waters of coves and waterfronts as a nursery to feed on smaller fish, shrimps, amphipods, and sandworms. One of their favorite foods is the siphon tip of the hard clam.
 
Yet, the winter flounder population we experience today is just a small fraction of what it used to be. Winter flounder once supported a massive recreational and commercial fishery along the northeast. Flounder fishing today is nowhere near what it was, especially back in the 1970s. The species started to decline around 1984 largely due to overfishing.
 
Winter flounder is now a highly regulated aquatic species. The good news is that the population of the winter flounder is well enough for both commercial and recreational fishermen to harvest, albeit at much lower levels. The bad news is that winter flounder populations still seem sluggish or stagnant.
 
The reasons for the current plight of the winter flounder are many and unclear, but global warming and water pollution almost certainly have a part to play. Jason McNamee, chief of marine resource management for the Rhode Island Department of Environmental Management, suggests that many young fish are not surviving to adulthood. It could be lower amounts of dissolved oxygen in the water due to dirty water from too many nutrients, made worse by warmer waters. McNamee goes on to say that another reason might be increasing predators due to the lack of bay ice and really cold winters from an increase in air temperatures due to global warming.
 
Is there a future for a beloved fish in New York Harbor that loves winter in an ever-warming world?