Unprecedented decline of fishes in the northern Murray-Darling Basin, Australia: Darling Hardyhead experience major losses

Authors: Peter Unmack, Karl Moy, Alejandro Trujillo-Gonzalez

Year: 2021

Abstract

Southeastern Australia experienced extreme drought during 2019 which caused the sudden decline of the Darling Hardyhead, Craterocephalus amniculus (DHH). DHH went from being a relatively common species at known sites to being extirpated across a significant portion of its range. We surveyed most known populations in Dec 2019 just prior to widespread rainfall which ended the drought, but failed to find DHH at most sites. In some cases sites were completely dry, in others there was water that had persisted, but DHH could not be found. The big problem is how do we know if they are still there or not? How long should it take to expect them to bounce back if they have survived in a refuge somewhere. We combined traditional sampling using seine nets which are usually good for sampling DHH with a more sensitive eDNA based technique. In November 2020 we sampled most localities where DHH had previously been recorded. We sampled 21 sites across the Namoi catchment, one DHH individual was captured by seine and five additional sites were positive based on eDNA. All but the site the one individual was captured at had low detection rates. In the Gwydir catchment five sites were sampled with zero detection. DHH populations recovered well in the Macintyre catchment with several previous populations (but not all) supporting DHH detected via seine net and eDNA. DHH was IUCN assessed as being Least Concern in early 2019, by the end of 2019 it was probably deserving of at least Vulnerable or Endangered, highlighting the broad and extreme impact the 2019 drought had on fish populations across the entire northern Murray-Darling Basin. In theory these fish could easily be reproduced in captivity to re-establish populations. However, the long term prospects for this species are bleak in the face of climate change which is predicted to make droughts more extreme. Funding from the W.L. Minckley Memorial Grant helped to make this project possible.