Black water event in the Murray, Murrumbidgee and Lower Darling River catchments: March 2012
What is a black water event?
Black water is a natural part of the ecology of lowland river systems. Black water occurs naturally due to the breakdown of leaf litter, inundated crops and other vegetation on the floor of the floodplain and the release of tannins and lignin causing water discolouration. Past changes to river and land management may have increased the incidence of these events and the way we manage our rivers in future can play a part in reducing the frequency and severity of these events.
Black water usually develops in flooded agricultural lands, wetlands or floodplains which have large accumulations of organic material. It can also occur in rivers if large amounts of leafy or woody material are washed in from storms.
Black water is also often associated with low dissolved oxygen levels which can cause stress and death of fish and other aquatic animals such as crayfish, yabbies and shrimp when the water on the floodplain drains back into the river channel as the flood recedes. Black water associated with low dissolved oxygen is termed 'hypoxic black water'.
Black water is rich in carbon compounds which can enhance fish and aquatic invertebrate production in floodplain rivers in the longer term, acting as the basis for much of the food chain in rivers.
The two most important factors influencing the development of black water are temperature of the water and the amount of carbon (i.e. leaf litter, woody debris and grasses) present. The heightened risk of black water events is influenced by increased amounts of organic material and rising air and water temperatures.
Where is black water currently occurring?
Black water is currently affecting sections of the Murray River (particularly downstream of the Barmah-Millewa and Koondrook-Perricoota Forests), Edward and Wakool river systems, Billabong Creek, Lower Darling and Murrumbidgee Rivers.
How long will this event last for?
The event is expected to continue for several months. The two most important factors influencing the development of black water are temperature of the water and the amount of carbon (i.e. leafy litter, woody debris and grasses) present.
How does black water affect aquatic species?
Low levels of dissolved oxygen can cause stress (and even death) to fish, crayfish and other animals which use gills to extract oxygen from the water to breathe.
Black water events can and have resulted in thousands of native fish deaths over the years. Large fish are usually affected to a greater degree due to their need for large amounts of oxygen. Crayfish, yabbies and shrimp are observed coming to the water’s edge and crayfish will climb out of the water in an attempt to breathe air. Affected fish can be observed gasping for air at the surface.
What is being done by authorities to manage black water?
The Murray Darling Basin Authority (MDBA), government agencies in Victoria and NSW and community groups are constantly monitoring water quality conditions to identify critical areas and determine when dissolved oxygen levels are approaching critical thresholds.
Regulators and irrigation escape channels have been opened to ensure large volumes of dilution water are passing down the main river channels.
The NSW Office of Environment and Heritage is coordinating the delivery of environmental water to help boost oxygen levels in the Edward-Wakool river system. (More info)
Who should I report fish kills to?
NSW DPI is the lead agency for co-ordinating the response to fish kill incidents. Fish kills can be reported to the Department’s 24 hour hot line on 1800 043 536. Information on fish kills and the Department’s fish kill reporting protocol can be accessed on the Department’s website.
The Department has fish kill response kits located at regional Fisheries offices which include water quality testing and fish sampling equipment to allow a rapid response to fish kill events.
Am I allowed to catch fish in impacted area?
The taking and/or possession of Murray crayfish and other spiny freshwater crays during the closed season is prohibited. In both Victorian and NSW waters the closed season is from 1 September to 30 April.
Anglers are required to observe all fishing regulations during this black water event and adhere to bag and size limits that apply in both NSW and Victorian waters. The Murray River is NSW waters and NSW laws apply.
Fishers and others are encouraged not to disturb any Murray crayfish that may have left the water and are currently on the banks of the river. Once the water quality improves, the crayfish will return into the water.
Fisheries officers have been patrolling the black water events to monitor the black water flows and to detect and deter any illegal activity.
Is it safe to eat fish in systems affected by black water?
It is important to use a commonsense approach. Do not eat fish that are dead when you find them or that do not look healthy when caught. Consumption of discoloured or outwardly stressed fish may be a health risk due to their poor condition.
How long term are the effects of black water on fish stocks?
It is hard to estimate the long term effects of these black water events on native fish populations.
In a black water event many fish move into areas within the river system that have better oxygen conditions.
It is also important to note that while there have been fish kills due to black water events, the recent heavy rain events and flooding have boosted native fish populations in other areas.
The increased flows will give fish an opportunity to move widely within the river systems and fish will also have the opportunity to move along rivers, as weirs that usually block their movement are lifted to allow the free passage of flood water.
It is hoped that many of these fish will travel back to areas affected by black water events once the water has cleared.
NSW DPI undertakes fish surveys as part of the MDBA Sustainable Rivers Audit, the NSW Monitoring, Evaluation and Reporting Program and other research projects. Over time these surveys will identify any significant changes in fish populations since these black water events have occurred.
Will NSW DPI continue stocking in rivers and impoundments affected by black water?
NSW DPI is committed to meeting its stocking obligations. Native fish stockings generally occur from December through to March.
The Department is working with other agencies in monitoring water quality, particularly dissolved oxygen levels to determine the suitability of identified waters. This may mean the postponement of upcoming stocking events or reassignment of stocking locations to other areas that are not impacted by black water or low dissolved oxygen levels.