Mark–recapture modelling can index population trajectory in the platypus

Semi-aquatic freshwater mammals are a heterogeneous global group that are subject to a range of anthropogenic disturbances, with many species now being recognised as threatened and some viewed as indicators of freshwater ecosystem health. The platypus (Ornithorhynchus anatinus) is one such species where concern has been raised about declining populations. Population monitoring is required, but the most effective way to do this is yet to be determined. We used data from 20 years of mark–recapture in two pools in the Upper Shoalhaven River, New South Wales, to demonstrate an approach to describe population trajectory over time. Modelling of capture data for adult females, juvenile females and males showed that only the adult female data produced reliable estimates of annual abundance, which ranged from 5.9 (95%CI, 5.1–11) to 17.7 (15.7–26.0) individuals. Population growth estimated from these values suggests that the population increased over the 20-year period, which encompassed a severe drought. In adult females the probability of initial capture was 0.61 (95%CI 0.45–0.75), but that of recapture was 0.35 (95%CI 0.23–0.48), suggesting a negative response to initial capture. Males showed no difference, with the probability being estimated at 0.19 (95%CI 0.08–0.41) per occasion. Our study employed two capture sessions per year, which could be increased, here and elsewhere, to improve the precision of parameter estimates. Given that platypuses are long-lived, annual capture surveys are not required to index a population. This study provides an example of how a platypus population can be monitored to guide future conservation efforts.