A Cluster of Duck Deaths at the Western Treatment Plant, Werribee, Victoria, During 2005–06

Abstract

Between November 2005 and March 2006 >700 waterfowl were found dead in ponds and on embankments of a sewage-treatment lagoon at Melbourne Water’s Western Treatment Plant, Werribee, Victoria. With few exceptions, these waterfowl were filter-feeders, Pinkeared Duck Malacorhynchus membranaceus and Australasian Shoveler Anas rhynchotis, and were in full moult of primary feathers, making them flightless. Investigations failed to determine a definitive cause of death but three possible causes were considered most likely for the apparently rapid onset of death: avian botulism, poisoning from toxic algae, and drowning after becoming trapped in eight large, 1.5-m-diameter drop-inlets to 22-m-long underwater transfer pipes. None of these possible explanations is supported by all of the known facts, but mesh covers over the drop-inlets have prevented further deaths over two subsequent moult periods. It therefore seems that drowning was responsible for at least some of the deaths, probably combined with an initial die-off caused by toxic algae that drew attention to the site and revealed the ongoing deaths through drowning. This cluster of deaths highlights the importance of identifying and appropriately managing threats where wildlife concentrate in large numbers, including some threats that perhaps have been given little consideration in the past.