Live-trapping surveys recorded populations of the platypus, Ornithorhynchus anatinus, in 73% of 45 reaches in the Dandenong Creek and Werribee, Yarra, Maribyrnong, Bunyip, and Lang Lang River catchments near Melbourne, Victoria; however, many populations occurred at low density. Our study investigated the relationship between population status and water and sediment quality along 28 stream reaches, including 17 reaches supporting a population of O. anatinus and 11 reaches lacking a resident population. Stream attributes included surface water-quality variables (summer concentrations of dissolved O2, total P [TP], NOX, total Kjeldahl N [TKN], dissolved organic N, NH4-N, and 50th, 75th, and 90th percentiles of suspended solids [SS]), concentrations of sediment toxicants (Zn, Pb, Cd, As, Cr, Cu, Hg, Ni), extent of catchment urbanization (as indicated by % imperviousness), and daily discharge. Reaches supporting a medium-density population (mean number of ≥0.5 adults or subadults captured per pair of nets set overnight) were characterized by significantly lower concentrations of streamwater TP, TKN, and SS (90th percentile), significantly lower Cd, Pb, and Zn in sediments, and significantly lower catchment imperviousness than reaches lacking resident animals. The maximum imperviousness associated with a population of O. anatinus was 11%, suggesting that this species is sensitive to urban-related change. Capture rate was not significantly correlated with median summer discharge, but was inversely correlated with streamwater TP and TKN. Further studies are needed to determine if pollutants may limit urban O. anatinus populations through direct toxicity or indirectly by pollutants reducing their benthic macroinvertebrate food resource.
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