Abstract
1. Small‐bodied freshwater fishes are commonly overlooked in threatened species management despite being highly imperilled. Before this study, the newly described little pygmy perch (Nannoperca pygmaea ) was known from only 0.06 km2 of habitat in a single catchment in south‐western Australia and a lack of knowledge prevented an understanding of its conservation status and priority actions.
2. The present study determined the distribution, biology, and movement patterns of N. pygmaea and compared these with other small, sympatric percihthyids to assess its conservation status and likely resilience to threatening processes.
3. The current ‘extent of occurrence' of N. pygmaea was determined to be 3,420 km2 and the ‘area of occupancy' was 10 km2. Nannoperca pygmaea inhabited permanent refuge pools in the main stem of an ephemeral, secondarily salinized catchment where salinities remained <6 ppt during summer.
4. During winter N. pygmaea undertook a short upstream migration into a seasonally flowing freshwater tributary to serially spawn. The timing of reproduction was partitioned among three sympatric pygmy perches within the austral winter/spring period.
5. The species qualifies as ‘Endangered' under the IUCN Red List assessment criteria. Its restriction to freshwater refugia within salinized rivers suggests that N. pygmaea may have a limited salinity tolerance similar to the sympatric, threatened Nannatherina balstoni , and that the species is susceptible to prolonged drought, drying of critical baseflow refuges, increasing salinization, and introductions of alien species.
6. The study serves as an example that small‐bodied freshwater fishes need greater research attention to understand the biological and environmental mechanisms underpinning their decline.
