During our recent holiday in the Dales we took a walk along the Swale towards Muker and when checking out the rocks in the side of the river we found several of this fish:
This was a first for me, and quite interesting. It's clearly well adapted for a fast flowing river, such as the Swale. It looks very much like a loach but is actually a member of the Scorpionfish family. It's a species that is indicative of good water quality - which you'd hope would be the case for the Swale, high up near it's source. According to the NBN website:
It's quite widely distributed, favouring northern rivers, but also found down south - even near to me in Kent! If you zoom into the exact area around Muker:
you can see there are 2 close-by records, but interestingly none from the stretch of the Swale itself, which runs north from Muker. We saw ours somewhere just above the "u" of Muker.
There's lot's more info at the Encyclopedia of Life: entry.