Flow intermittency subjects fish populations to frequent drought disturbance. Their population viability depends on availability of waterhole refuges for individuals to survive drought (resistance) and subsequent recolonisation by recruitment and dispersal once flow returns (resilience). We combined remote-sensed mapping of waterholes that lasted through an extreme drought in the northern Murray Darling Basin, with an assessment of the impacts of in-stream barriers on limiting opportunities for fish to move and repopulate after drought.
At the peak of this 2017-2020 drought, waterholes were few and generally small – representing only 11% of the channel network. All the fish surviving the drought were concentrated into this limited refuge habitat.
Even small instream structures, such as minor weirs, were fish movement barriers, causing large long-term reductions in opportunities for fish to move between river segments during flow events. Almost all the 104 known barriers reduced fish movement opportunities by more than 50% and many by over 70% and up to 100%, when compared to opportunities for movement if the barriers were not present. This large connectivity impact is a consequence of flow intermittency and is likely to reduce the capacity of regional fish populations to recover after drought.
Combining information on the risks from limited refuge habitat availability during drought and from reduced movement opportunity following drought allowed us to identify river segments where drought poses the greatest risk to viability of local fish populations. The spatial arrangements of these risks indicated the barriers that pose the greatest threat, and which are thus the highest priority for mitigation measures, such as by weir removal or the addition of fishways to improve fish movement opportunities.
This approach, considering resistance and resilience processes, is transferable to assessing and prioritising the management of drought and barrier risks to fish population viability in any non-perennial river setting.