Aquatic life

Bitterling, Rhodeus sericeus or Rhodeus amarus

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Special interest

The Bitterling is unique amongst fish in that it lays its eggs in the body cavity of freshwater mussels. It can therefore be considered parasitic on the mussel. However mussel larvae are parasitic on fish - so it was always assumed that the bitterling/mussel relationship was best considered as symbiotic! But new research from the University of Leicester disproves this!

The bitterling is a particularly interesting aquarium fish. It is also of considerable scientific interest.

British Isles - distribution

The Bitterling, Rhodeus sericeus or Rhodeus amarus or Rhodeus sericeus amarus (taxonomists list several sub-species) is not an indigenous British fish: its native range includes most of central and eastern Europe and Asia Minor. It is officially classed as a European endangered species and is therefore protected in Europe! However in recent years, presumably as pollution has reduced, its range has increased quite dramatically and its protection is therefore somewhat ironic!

Shropshire, Lancashire and Cheshire

According to Jonathan Newdick, there are ponds in England, particularly in Shropshire, Lancashire and Cheshire where the fish has been introduced. Probably as a result of releases from aquaria.

A stock survey by the EA in 2000 found Bitterling in Smithy Brook, a tributary of the River Douglas in Wigan, Lancashire.

Bitterling were first recorded in the Blackbrook canal, near St. Helens, in 1920.

Cambridge Botanical gardens

A pond in Cambridge Botanic Gardens has had Bitterling present for decades - certainly there were bitterling there in the 1970s. It was once mooted that those in the River cam had escaped from here, but the pond's overflow is quite small and shallow and does not seem suitable for bitterling to escape. The Botanic garden say that such an escape is not possible.

Cam and Ouse river system

The Bitterling is present in this river system, probably from an aquarium dump sometime in the late 1960s - which of course at that time was not considered harmful. They have been present in Burwell Lode where I first netted some in the 1980s. Burwell Lode joins the river Cam at Upware. Bitterling are also present in the rivers Cam and the Great Ouse, as well as the Lark and, upstream, I am told, to the River Ivel in Bedfordshire. I personally have caught Bitterling some 12km downstream of Burwell Lode, in Roswell lakes at Ely. Furthermore, a Google search for "fish records" bitterling will show the record Bitterling was caught in Barway lake (off Soham lode, fed from the river Ouse) in 1998. 21 gm! So one must assume the whole River Cam and Ouse and any connecting waters are now populated with bitterling. Denis Flack (who holds this record) says they are common throughout Cambridgeshire, including the river Lark (8th July 2007).

Although the Bitterling is a foreign native that is under legal restriction, there is no evidence that its presence in large numbers in this river system has done any harm. In fact they may have been beneficial - being a small fish they are a natural prey species for pike, zander and other predatory fish so they may well have reduced predation on indigenous species. Lancaster Canal The ebook The Ultimate Angling Bucket List mentions bitterling in the Lancaster canal just north of Preston. He has also used (with permission) the photo below.

This canal is effectively isolated from the rest of the canal system as a tidal estuary must be crossed to enter the Leeds and Liverpool canal.

Llandrindod Wells, Powys

Monday the 2nd of August, 2010. Bailey Goodman informs me that there are large shoals in the lake at Llandrindod Wells, Powys. He says that as far as he is aware they have not spread to any of the rivers there.

Llangollen Canal

The Canal and River Trust states The bitterling has been present in the Llangollen Canal for decades. It appears to have little impact on native fish stocks.

Manchester ship canal

I also have it on good authority that it is present in the Manchester ship canal.

Thurne, Norfolk

9th May 2007 An informant says they are present in large dykes at Thurne, Norfolk.

River Trent

A pdf from the Reichard Lab - Fish Evolutionary Ecology states Evidence of self-sustaining populations was found for three non-native fish species ,.. bitterling Rhodeus amarus (Bloch) (in the River Trent). The paper does not locate the fish.

This same paper also says The species (Bitterling) is thought to have become established in the River Cam, a tributary of the Great Ouse, in the 1970s,

Other locations

If you know of other places where Bitterling are to be found, let me know please as a distribution list would be interesting. Reports I already have are listed above. These are as supplied and have not been substantiated.


The Bitterling is a small (maximum around 70mm), deep-bodied (though the young are considerably less deep-bodied) fish that lives in ponds, dykes and other lowland waters. This, combined with its peaceful, yet lively character, hardiness and extremely interesting breeding behaviour and coloration, makes it an ideal aquarium fish.

The Bitterling is an extremely hardy fish. Being european, it survives being cooled down to 4°C without problem, and is happy in tanks up to around 20°C, or even higher, provided the tank is well aerated, as, being a cold water fish, it is used to water with a good oxygen content. Oxygen solubility (as does that of any gas) decreases with increasing temperature, so tropical fish live with a much lower oxygen content in the water than do cold water fish.

What impressed me about the Bitterling's hardiness is that they show no sign of distressed when moved from an outside tank (temperature around 10°C) straight into a tropical tank (around 25°C) - a move which would possibly kill most other cold water fish!

This robustness combined with the Bitterling's peaceful yet lively character makes them an ideal fish for a community tank. I have a shoal of maybe 10 Bitterling in a tropical community tank: this is a Juwel tank, with integral filter, and I have fitted the Juwel Venturi which suffuses half of the tank with very fine air bubbles - about the best aeration you can get, but entirely necessary to keep cold water fishes healthy at the elevated temperatures of a tropical tank! The Bitterling seem very happy at 25 or even 27°C in this tank.

However, at a temperature somewhere below 10°C the fishes physiology undergoes a change, as it goes into winter mode. In this mode, they need a significant time to adapt to warmer water and even when allowed to warm to 15°C over 6 hours, putting them into a tank at 25°C kills many of them.

The Bitterling is mentioned in at least four books I possess and without exception, all of the writers recommend it as an excellent aquarium fish.

The picture shows a group of 7 Bitterling in their ordinary coloration, out of the breeding season. Click on the picture for a full-size view.

The Bitterling is distinctly a shoaling fish, and I would suggest a shoal of 12-20 as being a sensible number to have in a tank to observe them at their best.


Since I have a plentiful source of Bitterling, in September 2004, I obtained about 50 to fill a 4ft tank I had spare, and also as food for a 45cm long tyre-track spiny eel I have, who has a significant appetite for small fish.

To my surprise, the spiny eel would not initially touch even quite small bitterling. The reason, for this, I surmise, may be as stated by Rev Gregory C Bateman in his The Book of Aquaria. The Bitterling is bitter to the taste. Since the eel seemed to know that they were inedible, they must also exude a distasteful 'smell' into the water to warn off would-be predators. This would make sense, as Burwell lode is known to possess plentiful pike and Zander - yet the tiny bitterlings multiply readily and are present in their thousands. Bateman also says that they do not make good bait for fishing,

Of course, the spiny eel's initial refusal to eat Bitterling may simply be because they are silver, and most of the fish he had been eating were red (platties), black (mollies) or blue (gouramis). Spiny eels are notoriously finicky eaters and it is not easy to change their diets! However, the 15 bitterling I put in the tank did eventually start to vanish; some 2 weeks later, only one was left. Seems Eel has developed a taste for them, as a change to his normal diet of frozen prawns!

The latin name, Rhodeus amarus, is also relevant, for amarus is the latin for 'bitter'! However reports I have had from various people say this is a misnomer: see below: Bitterling as live bait.

As an addendum to the above, one good authority has confirmed that the flesh of the bitterling is not actually bitter at all! However, as they are very small fish, they really could only be eaten whole (as whitebait) and the gut contents of fish can be very bitter - depending on their recent diet.

Feeding and keeping

The bitterling appears to be not a particularly fussy eater, but it's a small fish so likes small foods. Various books say different things but most agree the bitterling is mainly vegetarian.

From my own observations, they certainly eat Daphnia and other planktonic crustaceans. They can also be seen pecking at the leaves of various water plants and in the mulm at the bottom, presumably picking off microscopic animals (rotifers, small worms etc). I have also fed them on chopped spinach and chopped meat, though they have rather small mouths, so food has to be fairly finely chopped. They are also partial to spirogyra - the comon threadntype alga.

The larger glass larvae, bloodworms and mosquito larvae appear to be too large for their rather small mouths, at least in the smaller specimens.

The legal situation

The legal situation with Bitterling (Rhodeus Sericeus) changed in 2003. A license is now required not only to sell these Bitterling, but you also need a license simply to keep them. Apparently no regular dealers have applied for a license to sell them! However there are many different species of Bitterling and only the European Bitterling, Rhodeus Sericeus, is listed. Other species do not need a licence (but appear to be unobtainable!)

So I in May 2007 I investigated the possibility of getting a license to sell these fish. Getting this would tell me how easy it is likely to be to get a license to keep them. It seems that nobody else in the country has applied for such a license.

The outcome of my request was a provisional yes; provisional on my finding a suitable source of fish to extract. Defra have responsibility for public waters, here the fish abound, but will not let me extract! The license application has now lapsed, and I am not pursuing suitable sources.

By 2016, the position had changed. A contact has some bitterling, caught locally and he applied to DEFRA for a license to keep them. This was refused "Your application to keep Bitterling has been declined. This is because Defra policy is not to permit their keeping in the ornamental trade or by hobbyists." What he can do with them is a mystery - for it is illegal to release bitterling!

Keeping Bittering

There is no fee for Defra licenses and anyone who can obtain bitterling and wishes to keep them should be able to get permission. See the licensing advice page.

There are catches: an application for a license will ask where you sourced these. As far as I know there are no licensed suppliers. However it is illegal to release listed non-native species - including Bitterling. So if you do catch them in the wild - it is illegal to release them!

It is common practise to catch small fish as live bait. Including Bitterling! I have found no legislation regarding this practise - if you know of any please let me know.

So you will see form the above that it is virtually impossible for all of the law to be obeyed - let alone enforced!

Bitterling as live bait

Bitterling are taken with relish by other fish: I used to keep a fire-eel and he would eat bitterling, after taking a while to get used to them. I am also informed by a bait shop (Sovereign Superbaits) that a customer uses bitterling as live bait and they are much preferred to minnows, especially by perch. I have also heard it from one person who has eaten bitterling himself that they are not in fact bitter and are good to eat. However, like minnows and gudgeon (also good eating, treated like whitebait!) they can be bitter if they have been eating the wrong foods! I guess the same must apply to bitterling.

Be warned however that the environment agency is unlikely to take kindly to your using bitterling as live bait unless in waters where they abound - using then in other waters could count as introducing an alien species which is, of course illegal.

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Page first published 28th September 2004
© 2004-2024 Richard John Torrens.
Last modified: 2024
Page written and © by Richard John Torrens