River invaders

Sir, The problem of swans on some rivers (report and letter, August 23) is related to two factors.

First, the degradation of many rivers in the south of England by overabstraction. The Stour below Sturminster Newton is reduced to a trickle of water flowing through dense reed beds. A once-rich habitat is now useless for water weed, fish and swans, which are forced to concentrate on rivers where the flow is better, such as the Wiltshire Avon.

Secondly, the competition from alien species, in particular black swans. These ugly interlopers need to be removed from our rivers before it is too late. The damage done to rivers by introduced mink and coypu should be remembered and effective action taken now.

By rectifying the environmental damage which is putting such pressure on their habitats our most beautiful birds will once again have room to flourish.