The weed is growing well in the faster current and, if you look carefully, you can see a shoal of small fish. Derek thinks that there were more than a hundred small fish, and that theys are Minnows. (By reference to an ancient Observers Book of Freshwater Fishes!)
The behaviour and the appearance including white tubercles and the very occasional flash of red as well as their liking for gravel beds at this time of year all seem to lead to that identification. We have in the past caught minnows here during sampling sessions and have also seen shoals elsewhere. We have also seen sticklebacks, trout and other fish.