Commonwealth Environmental Water Office Long-Term Intervention Monitoring project: Edward-Wakool River system selected area evaluation report 2016-17

This report documents the monitoring and evaluation of ecosystem responses to Commonwealth environmental watering in the Edward-Wakool River System Selected Area in 2016-17. It is the third annual report of the Long Term Intervention Monitoring (LTIM) Project (2014-2019) funded by the Commonwealth Environmental Watering Office. This project was undertaken as a collaboration among Charles Sturt University, NSW DPI (Fisheries), Monash University, NSW Office of Environment and Heritage, La Trobe University and Murray Local Land Services. Field sampling for the project was undertaken by staff from Charles Sturt University, NSW DPI (Fisheries), and NSW Office of Environment and Heritage.

This report provides details of the Commonwealth environmental watering actions, indicators and an evaluation of the ecosystem responses to flows in the Edward-Wakool Selected Area during the 2016-17 watering year. This report evaluates ecosystem responses to:

The large unregulated flow events from August to November 2016 that occurred following record-breaking rainfall in parts of the catchment
Environmental watering actions from irrigation canal escapes that commenced in late October 2016 and continued until December 2016 to provide refuges from hypoxic water for fish and other aquatic biota
Environmental watering actions in Yallakool creek and the Wakool River from January 2017 to provide flow recession at the end of the unregulated flow event. In Yallakool Creek this action continued into late autumn/winter 2017 to maintain base flows during winter, when under normal river operations there would be a period of cease to flow in some of the smaller streams when the regulators are shut down in winter.

Indicators monitored in 2016-17 for the Edward-Wakool selected area evaluation were: river hydrology, water quality and carbon, stream metabolism, riverbank and aquatic vegetation, fish movement, fish reproduction, and fish recruitment (Murray cod, golden perch and silver perch). The fish community was monitored in only zone three for the basin-scale evaluation. No selected area evaluation for the fish community was undertaken in 2015-16 as this is scheduled to be monitored in only years 1 and 5 of the project.

The following responses to the unregulated flood events from August to November 2016 were observed in the Edward-Wakool system:

Extended period of overbank flows that flooded forest, cropping land, grazing and pasture lands and increased connectivity through backwaters, flood runners and anabranches
Increased dissolved organic carbon and nutrients (especially nitrogen and phosphorous) and a rapid decrease in dissolved oxygen in October 2016 resulting in hypoxia and widespread fish deaths
Large reduction in the cover and richness of submerged and amphibious plants during the flood and an increase in the cover and richness of terrestrial riverbank plants following the recession of the flood
Increased movement of golden perch and silver perch over 100’s of kilometres, which was an order of magnitude greater than movements during the preceding months
Very low numbers of Murray cod larvae present compared to previous years, as the hypoxic event occurred at the time of year in which Murray cod spawn in this system
Absence of Murray cod, silver perch and golden perch recruits following the flood
Very high biomass of tadpoles and invertebrates observed in nets, providing a source of food for native fish and other animals.

The responses to the environmental watering actions from irrigation escapes to create local refuges from hypoxic blackwater differed between sites. The response was influenced by the extent to which the environmental water contributed to the river flow. The watering actions from the Edward escape and Wakool escape had positive outcomes, improving DO and DOC downstream of these escapes. No targeted fish monitoring was undertaken near these escapes, however landholders and Fisheries Officers observed fish congregating in the flows from these escapes, suggesting the water actions were successful in creating localised fish refuges during the hypoxic blackwater conditions. The watering action from the Niemur escape did not result in any change in indicators downstream of the escape because the environmental water contributed less than 7 % of the total flow in the Niemur River. The watering action at the Thule escape was influenced by other localised effects. The watering actions from the Niemur and Thule escapes may have provided local refuge habitat for fish and other organisms.

During the flood recession and winter environmental watering action the water quality was maintained within an acceptable range. Compared to fish spawning recorded at the same time in previous years, there was increased spawning of carp, carp gudgeon, Murray River rainbowfish, gambusia and bony herring on the recession of the flood. However, there were no fish recruits of Murray cod, silver perch or golden perch recorded in autumn 2017. There was no detectable response of aquatic vegetation on the recession of the flood or into autumn and winter 2017, because almost no aquatic vegetation survived the flood event.