In order investigate how a decline in river water quality might affect the foraging behaviour of bats in riparian habitats, bat activity and feeding rates were measured at sites upstream and downstream from 19 sewage outputs. Paired sampling was used to control for variation in bat activity due to environmental parameters. But activity (bat passes) and attempted prey captures (terminal buzzes) recorded from broad band bat detectors were counted at each site, and bat species groups could be distinguished. Overall but activity and foraging activity were reduced downstream from sewage outputs, by 11% (total reduction in passes) and 28% (total reduction in buzzes). Both phonic types of Pipistrellus pipistrellus were less active at downstream sites than at upstream sites (total reduction in activity was 55% for the 45 kHz phonic type and 51% for the 55 kHz phonic type). The 45 kHz phonic type of P. pipistrellus concentrated its foraging effort at upstream sites (total reduction in counts of buzzes was 87%), while Myotis spp. foraged at higher rates downstream from outputs than upstream (total increase in foraging rate was 112%). For the conservation of P. pipistrellus, the maintenance of high standards of water quality may be important. Myotis daubentonii may be able to benefit from autrophication.
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