Variation in winter wildfowl numbers on gravel pit lakes at Great Linford, Buckinghamshire, 1974-79 and 1984-91, with particular reference to the effects of fish removal.

Abstract

Counts of wildfowl using the lakes at the Great Linford gravel pit complex (300 ha) in winter, in two 5-year periods in the 1970s and 1980s, were compared. Species emphasis changed from predominately dabbling ducks in the earlier period, when most lakes were newly flooded, to greater numbers of submerged macrophyte feeders in the later period. Diving duck numbers were little changed and competition for food from high populations of stocked coarse fish is suggested as a factor limiting lake use by birds which feed on benthic invertebrates.
There was a large increase in numbers of wildfowl using a 17ha lake after the fish populations were removed in November 1987. Use by herbivorous species, Mute Swan, Coot and Gadwall, increased dramatically in response to new weed growth. Shoveler and Pochard numbers both increased significantly and more Tufted Duck were counted. It is suggested that these changes were related to demonstrated increases in benthic invertebrates post-fish removal.
The implications for manipulating both new and maturing gravel pit lakes are discussed; fish removal is suggested as a useful technique for managing wetland nature reserves.