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Winter Flounder


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Winter Flounder Adults

Winter flounder (Pseudopleuronectes americanus) are right-eye flatfishes (Family Pleuronectidae). They may look flattened on the belly, but are actually flat sideways and lying on their left side, with both eyes together on the right side of the head. The left (blind) side, pressed against the sea floor, usually stays white. Winter flounder normally grow to about  18 inches and two pounds. They can be distinguished from other flounders in New Jersey nearshore waters by their small pointed mouth with small teeth and eyes up high and close together. By contrast, summer flounder (Paralichthys dentatus), another common species, has eyes on the left side of the body, set wider apart, and have a square jaw with big teeth. Winter flounder range from Labrador, Canada to Georgia, USA, but in near shore waters tend to stay north of the Delaware Bay. They like it cold and even have an antifreeze protein in their blood to help them survive freezing temperatures.

Importance to Humans

Winter flounder are good food. They are fleshier than most other Atlantic Coast flatfishes and have a delicate white meat. Commercial fishers catch them in bottom-dragging trawl nets offshore, and sport anglers target them with small baits such as worms and shrimps in winter and spring when they come inshore. Commercial catches tripled between the early 1960's to 1982 in the Gulf of Maine to a high of nearly 3,000 metric tons, matched by recreational fishing harvest, then catches collapsed dramatically and stayed low despite a high level of fishing effort. The sale of winter flounder to consumers and the sale of tackle and boats to winter flounder sport anglers contributes to the economy  of Mid Atlantic and New England states.    

Winter Flounder Reproduction

Winter flounder are unique among fishes of the Mid Atlantic and New England states. Unlike most coastal marine fishes in this region, winter flounder enter estuaries (where rivers meet sea) to spawn in fall and winter, when most fishes are migrating offshore or south. In the estuaries, winter flounder lay clutches of eggs that stick to the bottom, unlike the floating or "semi-buoyant" eggs that are typical of most other coastal fish. While this increases the chance that young (larval) winter flounder are in food-rich estuaries in the following spring, it also makes the eggs vulnerable to harsh winter conditions as well as to disturbance by dredging, which is needed to keep estuarine harbors navigable.

In laboratory tanks, adult female winter flounder deposit eggs at night while swimming in tight circles just above the bottom as following males shed sperm. Winter flounder spawn repeatedly, with an average of 40 spawning events reported per female and 147 per male over a 60-day period in a large aquarium These behaviors should result in highly localized egg distribution.  

Winter flounder spawning appears to be sensitive to temperature. Winter flounder spawn in estuaries from New Jersey to Canada and tagging shows a high degree of repeat visits to specific estuaries. However, little is known of specific spawning habitats within an estuary. We do know that winter flounder are adapted to and spawn in cold water and are protected by an anti-freeze protein to temperatures as low as 1.4 oC. They can recover from cold shock but not heat shock. Winter flounder spawn from February to May, with an April peak in the Gulf of Maine and an earlier onset and peak in the Mid-Atlantic Bight. Tagged winter flounder south of Cape Cod appeared to seek cold temperatures in deep water during summer, whereas winter flounder in the colder Gulf of Maine do not. However, winter flounder from  both the Middle Atlantic Bight (Cape Cod, Massachusetts to Cape Hatteras, North Carolina) and the Gulf of Maine move into shallow and estuarine waters to spawn.  Eggs hatch after 15-18 days depending on temperature. Researchers have found winter flounder eggs in other estuaries on sandy bottom and algal mats, yet some estuaries such as the Navesink River have mud bottoms and  eelgrass, but winter flounder still spawn somewhere in them.