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Fish like this have been caught recently (this Autumn) in many freshwater bodies in Sweden (like Dalbergsån) and no one knows what they are supposed to be (as written in this article). Apparently, the fish was caught on worms.

enter image description here

It looks like a common roach, but the eyes are different colour and the red around the gills is uncharacteristic. Some pictures I have seen show a yellowish hue on the fins. I'd guess it is an invasive new species and probably someone out there knows what it is.

Here is a gallery with other images.

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    I guess it's more likely to be some kind of sickness (from poisoning): web-search results or see this thread for example
    – OddDeer
    Commented Nov 8, 2016 at 10:10
  • You may really be on to something.
    – Kenji
    Commented Nov 8, 2016 at 16:42
  • Environmental conditions or even some foods could play a part in these conditions to exist in fish, if it is a common roach.
    – Ken Graham
    Commented Nov 10, 2016 at 17:54
  • You guys were almost right. I just read an article where a scientist explains this case as a pigment deficiency.
    – Kenji
    Commented Nov 24, 2016 at 16:54

2 Answers 2

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Well, it seems that the mystery was solved: it is a common roach with a guanin deficiency that causes them to have a different colour, as explained by this ecologist (in Swedish).

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Two fish in the palm of a hand

After netting one of our stock ponds, for a stock check. We discovered these two beauties in the net, the pond had a mixture of species, Carp, Tench, Roach and Rudd. I’d never seen anything like them before, I placed them in a holding tank while we continued with the netting. I took some pictures and emailed them to some friends within the industry to see what they thought.

A few days and several referrals later. I received an email confirming that they are Roach with a pigmentation disorder, he said we are lucky to have them as they are very rare. He has worked on a fish farm, supplying freshwater species for restocking rivers and lakes for over twenty years and hasn’t seen one in the flesh before.

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