Many urban watercourses, polluted by heavy industry since the 19th century, have now shed so much of their pollution that they can support populations of the aquatic insects on which trout and grayling feed, such as mayflies, and fish populations are thriving.
The phenomenon is being seen across Britain, from south London to Sheffield, and from Merthyr Tydfil to Glasgow, according to conservationist and fisherman Theo Pike, in his book Trout in Dirty Places.
Read on...