Sea Shorts: Wonders of the Winter Flounder

This curious little fish, which was in Newfoundland’s Petty Harbour Mini Aquarium, is a winter flounder.

A wonderful winter flounder. Warning! Video is quite noisy (you may want to turn the sound off). Credit Samantha Andrews/Ocean Oculus

Winter flounders live off the coast of North America in the Atlantic Ocean, from Labrador in Canada all the way down to Georgia in the USA. Even though this particular fish is quite small, winter flounders can reach 70 cm in length and weigh in at over 3 kg!

Winter flounder belong to a larger group of fish known as flatfish – I don’t think I need to explain why! Winter flounder have both their eyes and their mouth on the right side of their body. It may look pretty odd, but it is very useful. With this flat shape, the fish can stay on or just under the surface of the sea floor with only its eyes peeping out. Here, well-hidden and camouflaged with its mottled skin, it can lie in wait for some unsuspecting animal that will become dinner. Of course being so well camouflaged means they are also protected from becoming someone else’s dinner.

Look carefully and you will see another species of flounder - known as a peacock flounder - nicely camouflaged on the sea floor. Credit  Dr Dwayne Meadows, NOAA/NMFS/OPR  (Public Domain)

Look carefully and you will see another species of flounder - known as a peacock flounder - nicely camouflaged on the sea floor. Credit Dr Dwayne Meadows, NOAA/NMFS/OPR (Public Domain)

Flatfish aren’t born looking like flatfish. For the first few weeks of life when they live in the water column, the baby flat fish (the larvae) look like any other baby flatfish. As it gets older, its skull starts to bend, and one eye ‘migrates’ towards the other. Eventually, the transformation is complete, and the flatfish move to live on the sea floor, where they will live out the rest of their days.

Flatfish developmental sequence. Images by Dr. Alexander M. Schreiber

Not all flatfish have their eyes and mouth on their right-hand side. Some like the ‘summer flounder’ have them on the left, and some like spiny turbots can have their eyes and mouth either on the right or the left!

Sadly for winter flounders, they are rather tasty for us humans and has suffered massively from overfishing, which means that their numbers are much lower than they used to be. In the USA, NOAA has been working hard to try to ensure fishing for these flatfish is more sustainable. For example, they have reduced the amount of winter flounder (the quota) fisheries are allowed to take and introduced time-area closures. Time-area closures essentially block fishing during the time and in the places where the fish spawn. With overfishing no longer occurring in NOAA’s stocks*, we would hope to see some signs of recovery… right?

Well, the situation is a bit complex. In some stocks, we have seen some increases, in others not so much. Just to make it more complicated, increases and decreases can vary from one year to the next! This is why NOAA and other fishery managers look at many years’ worth of data and information to get the big picture and decide how well each of their stocks is doing.

There are many reasons why recovery may not be as good as we would like. For example, some of their habitats may have been lost of heavily degraded by fishing (notably bottom trawling) and human activities in estuarine and coastal areas. Sea temperature increases caused by the climate crisis may be making some of the areas winter flounder used to favour increasingly too warm. Finally, years of overfishing had impacts on genetics. Essentially, the more individuals you remove from a population, the less there are to have babies and pass on their genes. Because every individual has a slightly different genetic makeup, the overall population eventually loses genetic variability. Why is this a problem? The ability of an individual to survive and reproduce under different conditions is partly the result of its genetic makeup. Some individuals may be better able to cope with less food than others, some may be more resistant to a disease, and some may be able to cope with slightly warmer waters than others. When we have a population with low genetic variability, the population is less able to cope with changes. As a result, the population is at greater risk of being wiped out.

In the coastal waters off Long Island in the USA, one study from scientists at Stony Brook found that the numbers of fish gathering to spawn was so low, inbreeding was common. Inbreeding has all sorts of problems associated with it for the individual fish, like a higher risk of death, lower reproductive output (fewer babies), and a higher risk of mutations.

* NOAA group all the winter flounder in USA waters into groups known as stocks for management purposes – this is very common all over the world

Header image: Winter flounder at Petty Harbour Mini Aquarium, Newfoundland. Credit Samantha Andrews/Ocean Oculus