Range‐wide population genetics study informs on conservation translocations and reintroductions for the endangered Murray hardyhead (Craterocephalus fluviatilis)

Freshwater ecosystems worldwide harbour disproportionately high numbers of endemic species under threat from human activity, particularly accelerated habitat fragmentation. The Murray-Darling Basin in south-eastern Australia, one of the country's largest and arguably its most vulnerable freshwater ecosystem, is inhabited by a number of small-bodied fishes that are threatened with imminent extinction. Here an extensive microsatellite dataset was used, supplemented by additional allozyme and mitochondrial DNA analyses, to assess the genetic diversity, population structure and contemporary migration patterns in the Murray hardyheadCraterocephalus fluviatilis, one of Australia's most threatened fishes. Genetic diversity estimates, primarily based on 413 fish collected during the latter period of intense drought (1997-2010) from 23 sites and genotyped at 14 microsatellite loci, were higher than those previously detected for other regionally co-occurring small-bodied freshwater fishes. Population structure analyses identified a subtle primary split between 'lower Murray' (lower river reaches) versus 'upstream Murray' (upper river reaches) and a total of nine genetically similar sub-populations. This includes unexpected sub-population differentiation in the Lower Lakes, a region at the terminus of the Murray-Darling Basin that most often has inter-connected habitat. Very low levels of contemporary migration were detected between most inferred populations (<2%) during the drought, with all exceptions involving moderate levels of migration from an upstream sub-population into an adjacent downstream sub-population. This article describes how these genetic data have guided translocation and reintroduction efforts in recent years. We advocate the use of assisted gene flow as a central component of continuing efforts to rescue this species from imminent extinction. 1.2.3.4.5.6.