Occupancy modelling confirms the first extirpation of a freshwater fish from one of the world's largest river systems

Numerous human-induced changes to freshwater ecosystems are implicated in the loss of biodiversity, including fish extinctions. Dams and weirs have profoundly altered the ecological character of the Murray-Darling Basin (MDB), one of the world's largest river systems, which discharges at the mouth of the River Murray in South Australia. These impacts are evident in the Ramsar-listed final reaches of the river, where regulation, drought, and flow reductions over recent decades have led to the collapse of freshwater fish populations, including Yarra pygmy perch (Nannoperca obscura), which is Endangered on the International Union for Conservation of Nature Red List of Threatened Species. Nannoperca obscura occurs in several major catchments in south-eastern Australia, but the genetically unique population in the MDB only inhabits wetlands near the terminus of the River Murray where wild populations were last recorded in 2008 approaching the end of a prolonged drought. This study involved repeated surveys targeting N. obscura to evaluate its occupancy in the MDB while accounting for imperfect detection. The surveys failed to find N. obscura despite regular detection of the closely related southern pygmy perch (Nannoperca australis), which provides a proxy for the congener. This study presents strong evidence that N. obscura is absent in the survey region, thereby inferring the first freshwater fish extirpation from the MDB. The outcomes of this study should prompt responses required to aid the species' population recovery and present a reliable approach to confirm the status of freshwater fishes for conservation management.