A lucky glimpse of an American invader

Just upstream from this point on the river Tees at Barnard Castle they've installed a weir that creates a broad area of calm shallow water at its top. This morning I was leaning over the wall, watching the fish rise and idly peering down into the water, and spotted ..........

..... this - a large American signal crayfish, Pacifastacus leniusculus, identifiable by the pale spot at the base of its claws. This species was introduced into Britain sometime after the 1960s and has been spreading rapidly ever since. It has been largely responsible for the decline of the native white-clawed crayfish, through competition for habitat niches but most of all through the transmission of the fungal disease Aphanomyces astaci, known as crayfish plague, to which the American species is immune.