Microhabitat characteristics of feeding sites used by diving duck Aythya wintering on the grossly polluted Manchester Ship Canal, UK

Nationally important wintering populations of up to 2700 Pochard Aythya fer ina and 2000 Tufted Duck A. fuligula feed around Salford Docks in the upper reaches of the Manchester Ship Canal. The system is grossly organically po lluted and devoid of aquatic macrophytes, but holds high densities of oligo chaetes and other pollution-tolerant invertebrates. The species fed togethe r and differences in feeding distributions were more apparent between day a nd night than they were between species or winter period. We related the sp atial patterns of feeding by ducks to water depth, channel width, total org anic benthic carbon levels, and invertebrate densities. Separate analyses w ere performed for each species, during nighttime and daytime, and for diffe rent periods of the winter. In only half these analyses could feeding patte rns be explained by differences in the microhabitat variables. However, fee ding by both species was concentrated in the wider portions of the docks, i n areas with high densities of oligochaetes, but particularly in areas of h igh total benthic organic carbon. These were areas where large amounts of s ewage matter are deposited in the benthos. There are plans to improve water quality in the docks by modernizing sewage treatment systems upstream and by oxygenating the water within the docks. These may generally improve the biology of the system, but they may have a serious impact on the numbers of diving ducks by reducing the densities of their invertebrate prey.