One of the most severe anthropogenic impacts on river systems throughout the world has been drastic alterations to the natural flow regime. In Australia, many native freshwater fish have declined in range and abundance in response to anthropogenic changes to the flow regime. The Murray-Darling Basin has experienced one of the worst hydrological droughts on record, this combined with extensive river regulation resulted in unfavourable conditions for spawning and larval survival for flow-induced spawners. The drought was ended by a sustained period of high rainfall, which resulted in widespread flooding and a flow that was more representative of natural flows than the managed regime. This research investigated the annual change in larval assemblages of native and alien species in the Lower River Murray during varying hydrological conditions. Larval fish were sampled during spring/summer during three distinct hydrological periods:
Data were analysed for annual, spatial and seasonal variations using multivariate techniques and correlations were identified between changes in larval assemblages and environmental variables (discharge, water level, conductivity and temperature). Three types of spawning characteristics were evident in the changes in the larval assemblage:
The majority of small to medium-bodied native species spawned in low flow years, while for some species changes in abundance was more strongly correlated to seasonal variation. Larvae of Macquaria ambigua ambigua and Bidyanus bidyanus were not recorded during the low flow years, low abundances of larvae were recorded during the within-channel flow pulse year and significant increases in eggs and larval abundances were recorded during the high flow and over bank flood year.
Restoring a more natural flow regime throughout the MDB is integral to maintain and potentially restore native fish populations. The challenges in heavily regulated river systems are vast, but a combination of environmental water allocation and river management has the potential to provide a flow regime more reminiscent of natural conditions, and return ecologically important within channel flow events. This project found that extended drought and low flow periods can have a significant negative effect on populations of large-bodied native species and that mitigation strategies, including water allocations aimed at maintaining or restoring necessary conditions for fish spawning and recruitment, should be planned prior to action being required.
This project highlighted that the abundance and distribution of larval M. peelii and T. tandanus was greater during higher flow conditions, and importantly larval M. a. ambigua and B. bidyanus were present only during the within channel flow pulse and high flow, overbank flood years. While a number of small to medium-bodied species will spawn and develop during low flow conditions in the heavily regulated weir pool environment, for M. a. ambigua and B. bidyanus continued low flow conditions likely pose a significant threat. This project has demonstrated that during years of low water availability the minimum Entitlement Flows into SA are not sufficient to induce spawning in flow dependent spawners.