From Dutch news agency ANP (translated):
SITTARD – The numbers of bullheads, prehistoric looking fish, in and near the Meuse and Rhine rivers has gone up strongly in a few years time. …
Research of Nijmegen University compared the present bullheads to old specimens in museums in order to be able to say something on species.
Everyone was surprised that they turned out to be not the protected species Cottus gobio.
It turned out it was not anywhere in The Netherlands.
On the other hand, there is explosive growth by an hybrid of two other species (Cottus rhenanus and Cottus perifretum).
This new hybrid bullhead feels at home both in big rivers and open waters, like the big lake Ijsselmeer, and in fast flowing brooks.
More about bullheads in the Netherlands: here.