Fish movement and sensitivity to contaminated mine water

Flowing through the Ranger uranium mine lease and into Kakadu National Park, Magela Creek is home to important populations of native fish species that need to be able to move between the river, floodplain and escarpment country at different times of the year.

Weathering of waste rock from the mine releases contaminants, including magnesium sulfate, a salt. These contaminants are washed out by the rain and are predicted to move through the local groundwater towards Magela Creek. Depending on the concentration, the magnesium sulfate has the potential to affect fish, trees and other freshwater ecosystems in and near Magela Creek downstream from the Ranger mine site.

There were two main parts of the project.

• Acoustic tracking was used to examine the how fish moved up and down the sand channels of Magela Creek past the mine site. With the help of the Djurrubu Rangers, we tagged large-bodied fish (saratoga, sooty grunter and sharp-nose grunter) from Mudginberri and Bowerbird billabongs to track their movements over two wet–dry cycles, and mid-bodied fish (spangled perch, barred grunter and black catfish) from around Mudginberri Billabong to see how they moved upstream at the end of the wet season as the flood flows receded. Ten acoustic receivers were deployed across the study area to track movement of the tagged fish.
• We used high-resolution sonar video surveys to identify and measure individual fish in Mudginberri and Bowerbird billabongs, allowing us to estimate the abundance and sizes of fish species in the dry, build-up and wet seasons. We combined this information with the acoustic tracking data to develop an understanding of the movements, distribution and population structure of fish across the study area.

Location of Kakadu National Park, Magela Creek, Ranger uranium mine and the four billabongs where fish were collected, tagged and detected.

• A significant proportion of tagged fish (~30%) migrated >18 km downstream from Bowerbird Billabong into the mine lease during the wet season. Movement started soon after the first wet-season flows, and most fish migrated back upstream into their previous home range in Bowerbird Billabong during or at the end of the wet season.
• About a quarter of the fish that entered the mine lease used backflow billabongs (Mudginberri, Coonjimba) as habitat during the wet season. Concentrations of mine-derived magnesium are moderately elevated in Georgetown Billabong, while concentrations of both magnesium and manganese may be highly elevated in Coonjimba Billabong. Dissolved oxygen can also reach very low levels at times in both billabongs.
• The sand channels of Magela Creek past the mine are a critical migration pathway for fish due to the connections between lowland reaches of the system and the escarpment refuge billabongs.
• Fish are most likely to encounter mine wastewater during the wet season, when large-bodied fish use sand channels and backflow billabongs as habitat, and at the end of the wet season when the floods recede and fish migrate through the sand channels to reach dry-season refuges.
• The highest abundances of large-bodied fish in the sand channels appear to occur during high flows, which may dilute any mine-related contaminants and reduce the risk to fish.
• Mid- and small-bodied fish continue to migrate at the end of the wet season on recessional, low flows, which pose the highest risk for exposure of these fish to mine wastewater.
• Detailed modelling is required to increase the accuracy of the magnesium concentrations that fish in Magela Creek may be exposed to, so that a more comprehensive risk assessment can be conducted.
• Ongoing monitoring is required to identify legacy impacts and facilitate mitigation of any disruptions to fish assemblages and migration through Magela Creek.

Fish migration during the wet season transports large amounts of nutrients and energy from productive floodplains into riverine food webs. In Kakadu National Park, this connection between floodplains and rivers supports valuable ecological assets including crocodile and waterbird populations, barramundi, prawn and mud crab fisheries, and populations of threatened speartooth shark and sawfish.

Sand channel region of Magela Creek between Bowerbird and Mudginberri billabongs. Photo: David Crook.

Previous work on fish migration in Magela Creek is comprehensive. However, the reliance on visual observations and trapping data limits the inferences that can be made regarding ecological connectivity and the potential future impacts of waste-rock contaminants in Magela Creek.

More detailed information on fish movement was identified by the Australian Government’s Supervising Scientist Branch as critical to improving understanding of the potential legacy impacts of the mine on aquatic plants and animals and for the development of future monitoring programs.

The project used acoustic telemetry to characterise patterns of how different fish from upstream (Bowerbird Billabong) and downstream (Mudginberri Billabong/Magela crossing) areas moved through the Magela Creek sand channels.

This telemetric work was combined with high-resolution sonar videos taken in both billabongs. This sonar technology shows promise for monitoring and assessing fish assemblage condition. Fish can be monitored at night or in very turbid water. The lengths of individual fish can be accurately measured, which can help produce a more robust biomass estimate. The defined sample area also means that fish density and biomass can be calculated – these measures of assemblage condition may be more accurate than those traditionally used to extrapolate from relative abundance measures.

Observations of fish interactions with mine-waters released into Magela Creek in the current study did not find evidence that fish were deterred or avoided water containing magnesium sulfate. In fact, chequered rainbowfish were observed preferentially swimming up discharge channels. Based on the lack of avoidance behaviour of fish exposed to the discharge water, it appears unlikely that concentrations of magnesium up to 11 mg/L will inhibit the migration of fish in Magela Creek.

Access to Bowerbird Billabong  was by helicopter. Photo: Tom Mooney.

Understanding movement patterns of fish was an important objective of this research, and the acoustic tracking revealed important new information about the movements of the larger-bodied species of sooty grunter, saratoga and sharp-nose grunter.

Fish started moving downstream from Bowerbird Billabong towards Ranger uranium mine soon after the first wet-season flows. Large-bodied fish were most likely to be found in channels on the mine site during high-flow periods when flows were greater that 50 ML/d.

Bowerbird Billabong. Photo: David Crook.

One of the most interesting features for all species was the upstream homing behaviour the fish exhibited towards the end of the wet season. Nearly all fish that moved downstream from Bowerbird Billabong onto the Ranger mine lease made return upstream movements toward Bowerbird Billabong at the end of the wet season. Even the fish that weren’t detected in the downstream reaches still had much lower detection rates at the Bowerbird listening stations during the wet season, suggesting a wider range of movement above the mine lease during the wet. Three saratoga and one sooty grunter were also detected in off-channel billabongs during the study.

Plot showing detections of tagged fish at the downstream Bowerbird Billabong acoustic receiver over the study period. Each row of data points shows the detections for an individual fish and the solid line shows creek discharge. This figure demonstrates the reduction in detections in Bowerbird Billabong during periods of high flow, which is attributed to migration into the Ranger uranium mine lease and localised movements upstream of the lease.

The sonar videos revealed that there were generally higher densities of fish in Mudginberri Billabong than in Bowerbird Billabong except during the wet season. Both billabongs had large numbers of small-bodied (<10 cm) fishes, but the density of large-bodied fish (barramundi, bony herring, forktail catfish, tarpon and saratoga) was much higher in Mudginberri Billabong than in the escarpment refuge habitat at Bowerbird Billabong.

Saratoga, sharp-nose grunter and sooty grunter were among the large-bodied fish tagged.