Clatworthy Reservoir, situated in the Brendon Hills in west Somerset, was created in the late 1950s by damming the headwaters of the River Tone. With the primary function being to provide a water supply on the western side of the county of Somerset, it also has a recreational function in providing angling, specifically fly fishing for trout. A highly biodiverse aquatic ecosystem developed, including a breeding population of rainbow trout, derived from fish introduced to enhance the sport for anglers. By the end of the 20th century, American Signal Crayfish had become established in the reservoir, having been introduced to the UK under government licence some decades previously. The local source of this alien species is uncertain. Within twenty years, the Signal Crayfish appeared to have more or less wiped out most other aquatic fauna apart from Sticklebacks, and the artificially-reared Rainbow Trout with which the lake is stocked. With the added stresses resulting from more frequent severe reductions in water level, partly attributable to climate change, it is doubtful if the lake could now be described as containing a functioning ecosystem.