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 rapid breakdown of leaf litter on the floor of the floodplain causing water discolouration and often low dissolved oxygen levels. Weather conditions where there is prolonged drought followed by heavy flooding contribute to the severity of the these events.

River regulation has also reduced the more regular flooding regime within floodplain wetlands and, combined with a prolonged drought, can result in the accumulation of high loads of leaf litter and woody debris being inundated. This also exacerbates the severity of black water events.

Black water events can provide a valuable source of carbon to rivers which can stimulate fish breeding and food production, but they can also create low levels of dissolved oxygen in water which can be harmful to fish and other aquatic species.

Black water usually occurs in flooded 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.

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 and woody debris) present. The heightened risk of black water events is influenced by increased amounts of organic material and rising air and water temperatures.

In 2010-11, black water affected sections of the Murray River (particularly large wetland areas such as the Barmah-Millewa and Koondrook-Pericoota Forests), Edward and Wakool river systems, Lower Darling and Murrumbidgee Rivers.

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 rely on oxygen in the water to breathe.

Black water events can and have resulted in a number of native fish deaths. Crayfish are observed leaving the water and shrimp are often seen near the water’s edge. Affected fish can be observed gasping for air at the surface.

In the 2010-11 event, some fish deaths occurred in a number of locations on the Murray River downstream from the junction with the Goulburn River. Deaths of fish have also occurred in the Edward River system near Moulamein and in the Murrumbidgee River system near Balranald. Field reports indicate that a number of fish have moved away from the black water and into areas that have better oxygen conditions.

What can authorities do to manage a black water event and reduce fish deaths?

The Murray Darling Basin Authority (MDBA), in conjunction with government agencies in Victoria and NSW, is constantly investigating measures to lessen the impact of black water.

In the 2010-11 event, it was not possible to dilute the black water without causing more flooding, as most river systems were operating at full channel capacity.

Freshwater scientists from NSW DPI have joined a consortium with experts from the University of NSW, the Arthur Rylah Research Institute (Victoria) and CSIRO’s Murray Darling Freshwater Research Centre to evaluate scientific information about black water events, synthesise existing knowledge about black water generally, identify knowledge gaps and formulate plans for future data collection.

Why are fish not being translocated to other waterbodies?

In large events such as occurred in 2010-2011, the extent of black water across NSW river systems, coupled with low dissolved oxygen levels (DO) even where there is no evidence of black water, meant that it was difficult to find suitable waters for fish to survive in. Many fish moved into areas within the river system that have better oxygen conditions.

During the Edward River fish kill in early December 2010, Murray Cod were translocated from the river to Moulamein Lake, but unfortunately a number of these fish subsequently died. Where options for translocation are identified, the Department will endeavour to save fish via translocation where feasible and safe to do so.

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.

Where events occur during fish spawning season, anglers may wish to voluntarily limit their catch to minimise further impacts on native fish populations.

Fishers and others are encouraged not to disturb the Murray crayfish that have left the water and are on the banks of the river. Once the water quality improves, the crayfish will return into the water.

Fisheries officers patrol the black water events to monitor the black water flows and to deter and detect 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 unhealthy looking 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, follow-up heavy rain events and flooding boost native fish populations in other areas. The floods promote spawning and juvenile survival in many species as the main breeding season for many native fish including the Murray Cod and threatened species such as Silver Perch and Freshwater Catfish is during the Spring and early Summer when water temperatures increase. The increased flows give fish an opportunity to move widely within the river systems. Fish 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 travel back to areas affected by black water events once water quality improves.

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 works with other agencies in monitoring water quality, particularly dissolved oxygen levels to determine the suitability of identified waters. At any given time, 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.

Will NSW DPI increase stocking in waters where fish deaths have occurred as a result of the black water?

The Department has considered the impacts of the 2010-11 black water events as part of its annual planning program for stocking in 2011.