Assessing the Impact of the Newly Introduced European Flounder

Image Credit: Tiit Hunt, CC BY-SA 3.0, Image Cropped

Guest Post by Theresa Henke

First record of niche overlap of native European plaice (Pleuronectes platessa) and non-indigenous European flounder (Platichthys flesus) on nursery grounds in Iceland (2020) Henke et al., Aquatic Invasions, In Press

The Crux

Determining whether or not an introduced species is invasive is important, as it determines whether or not management steps need to be taken to slow or eliminate any negative impacts it might have on the local ecosystem. In Iceland, 15 introduced species have been recorded over the past decades but only six of them are currently classified as invasive or potentially invasive. One of these potentially invasive species is the European flounder (Platichthys flesus), a flatfish commonly found in coastal waters of Europe. The flounder is a catadromous fish, meaning it spawns in marine habitats but has the ability to survive in freshwater streams as well.

In 1999, the flounder was firstly identified in Icelandic waters in the southwest of the country. Since then it has rapidly spread clockwise around the country. Currently, it can be found in every part of Iceland, mostly in estuaries but also up in rivers and lakes. Juvenile flounder can be found on nursery grounds in shallow, brackish waters where they overlap with juvenile European plaice (Pleuronectes platessa). Plaice is a commercially important flatfish species native to Iceland. Despite the knowledge of the flounder’s arrival in Iceland in 1999, not much research has been conducted on the impact of this potentially invasive species.

What We Did

In 2017, we collected juveniles of both flounder and plaice from three sites in western Iceland to see what these fish have been eating. To assess their recent diet, we opened the stomachs of 305 fish and identified and counted everything we found. To get a better picture of the long-term diet of both species, we took tissue samples of 113 fish for stable isotope analysis. Stable isotope analysis looks at the presence of alternative forms of certain elements, such as carbon (C) and nitrogen (N). We can then use the composition and molecular weight to figure out what kind of food the fish has been eating. Overall, we looked for overlap in the diet of both species. If an overlap exists, it means there is the potential for competition between the two species. We applied different metrics to calculate the overlap of what flounder and plaice have consumed based on the stomach content analysis as well as the stable isotopes.

Did You Know: Ambicoloration

Blending in with your surroundings is desirable in many situations, and aquatic habitats are no different. Like many other fish and sharks, most flatfish species display countershading, a camouflage strategy where the animals are colored darker on the upper site of the body than on the bottom. In the case of flatfish, the eye side is colored to blend in with the surrounding benthic habitat whereas the eyeless, blind side is of a white color to blend in with the sunlight entering the water. The fish therefore make it more difficult for predators to detect them, irrespective of whether they are swimming above or below the fish.

The term ambicoloration describes a phenomenon observed in some flatfish species where pigmentation of the blind side is disturbed and develops the coloration of the eye side. The coverage of such ambicoloration can range from small dots to large chunks of the blind side and rarely even the whole fish.

What We Found?

Our study is the first to report the co-occurrence of the non-indigenous flounder and the native plaice on nursery grounds in Iceland. We found that both species were present at all sites throughout the sampling season, often with more than one age class of flounder and plaice present. Generally, flounder had a wider variety of prey at all sites, supported by both the stomach content analysis as well as the stable isotope analysis. Furthermore, even though there is a high variation in the size of the fish (often resulting in different prey), there is a high overlap between the two species diets. In areas where both species co-occur naturally, it can usually be observed that each species somewhat specializes in their diet to reduce the overlap, but we can’t yet see whether this is happening for flounder and plaice around Iceland.


As with most studies conducted within the frame of a master´s thesis, the lack of time and data is a problem. This study was not able to take large scale distribution or temporal variation into account. Furthermore, due to the small amount of data, we could not compare between different age groups of both species. A lot more data is needed to confidently assess the potential effects that the occurrence of flounder has on plaice.

So What?

This study is a first step to increase the knowledge about this potentially invasive species in Iceland. Signs of competition don’t necessarily mean that the native plaice will suffer heavily, but it could mean trouble, especially for juveniles. Whilst we can’t yet see if the plaice have begun to specialize as a result of the flounder, we can’t discount it happening in the future.

With invasive species often having detrimental impacts all over the world, it is important to observe the development of the colonization of a new species to be able to respond accordingly if necessary. Studying the colonization of flounder in Icelandic waters is not only important on a local scale but furthermore presents a unique opportunity to investigate the invasion of a fish species in a sub-arctic marine environment. This may allow for some general conclusions to be drawn on invasion biology in these climates.

Theresa Henke is a PhD student at the University of Iceland Research Center of the Westfjords, Iceland. She is interested in the ecology of invasive species and their impacts on native environments. Currently, she is researching the establishment of the European flounder in Iceland. You can contact her at for further questions or find her on ResearchGate.


  • I don’t want to sound ironical here but isn’t the European flounder a pretty frequent visitor to restaurant menus and fish markets? Seems to me a good part of the answer is more recipes, IE to catch and eat more of them. I will happily take a dozen or so whole ones, freshly frozen, per month and I assure you they won’t get far from our icebox. Here in south Florida, for example, ex-fishtank tilapia were thought to endanger our native sunnies and bluegill. Once it was discovered how tasty they were (kosher restaurants, especially, used to offer farmed or imported tilapia as “St. Peter’s Fish”), the wildlife folks turned their attention to our pythons and tegus. Now, any fisherman having a good day on the canal bank is likely to throw his sunnies back and keep his tilapia.


    • I think one of the problems with popularising invasive species is that it can often lead to illegal stocking. There are plenty of invasive species around the world that have a negative effect on local ecosystems, but since fishers love them, there’s no effort made to remove them. It might not necessarily happen, but if Iceland sees a rise in popularity of flounder as a food fish it might (counter-intuitively) end up leading to a massive population boost.

      Regardless, there’s not much that can be done against an invader like this, so those concerned will just have to sit back and hope that the flounder don’t have too much of a negative effect on the plaice.


  • I understand the overstocking problem (even after returning from a fishing trip on Lake Taupo, where big rainbows are supposed to jump unbidden into your boat without ever managing to snag a boot). However, in contrast to that I recall the Grand Banks fishery collapse of some decades ago and wonder how intensively a market needs to exploit a particular food fish to offset the colonization of one ecosystem by an interloper like our flounder here. Here in Miami – among our many other ecological peccadillos – we’ve got a slowly evolving market in lionfish fillets and dumplings (which would be more robust if not for all the small bones in the fillets) but obviously not quite enough to make a dent in them; the moray eels seem to be the only ones really making a party of them. We don’t seem yet to have a viable formula for striking a balance when such things happen.


    • 1. Lionfish dumplings! I am so used to Australia being the victim of invasive species that I get shocked when I read about a species native to Australia being invasive elsewhere (apologies to any Kiwis reading this).
      2. Yeah it’s an interesting balance to have to strike. We’ve tried to increase demand for carp in Australia to get them out of our rivers (ironically most of the time it’s done to protect brown trout fisheries), but they just end up being used as dog food or fertiliser.
      3. Peccadillos is a MARVELLOUS word.


      • It is a marvelous word. The they used to be pretty common on the campus at Texas State University in Laredo…aw, never mind. Anyway, I can’t think of any Australian species which have ever been an issue here in the states. But then, who could distinguish a redback from our native black widows? On the other hand, the first time I saw a bandy-bandy making its way across a yard in Brisbane I thought I was looking at a small California kingsnake. But since it only had one head, I wasn’t very interested. The good news is, no one over here is particularly worried about long-necked turtles establishing themselves in our canals the way folks down under are apoplectic if they find a common snapping turtle in the Murray. If I’m not mistaken, Sean Penn was supposed to be hung for bringing one in disguised as a dog…..


  • With keeping the issue of overstocking in mind, we are indeed exploring ideas on harvesting flounder. Before putting effort into developing any methods though on how to catch flounder without damaging any (highly valuable) salmonid stocks, we are currently trying to evaluate whether there is actual interest among Icelanders to buy and use flounder.
    If there will ever be a flounder fishery it will likely not reduce the overall problem in Iceland. Within just 20 years the flounder established (often highly abundant) along ~5000 km coastline in countless fjords and rivers that are often not easily accessible. So the population will probably bounce back easily if you don´t take all individuals out of the system. We are more hoping to use harvesting flounder as an additional tool to decrease the pressure on native species locally, especially keeping highly valuable salmon rivers in mind where it is feared that the flounder feeds on the salmonid eggs


    • If true I have to admire the flouders’ exquisite taste. I’m fond of a number of preparations of salmon roe (ikura, the Japanese word for it, is the preferred term amongst the elevated pinky set). But seriously, why would you focus on the market for the fish in Iceland alone? I should think there’d be a much bigger export market for them – priced right, anyway, since here in the states the fishmarket prices for flatfish have climbed to the roof and are now jealously eyeing the steel-framed skyscrapers (it might amuse you to know that, lionfish notwithstanding, the newest fish fad, at least locally, is barramundi, would you believe?).


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