DPI – Blackwater event causing fish kills

by Department of Primary Industries on 24 Dec 2010 What is a blackwater event?

Blackwater is a natural part of the ecology of lowland river systems. Blackwater 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.

It can provide a valuable source of carbon to rivers, but it can also create low levels of dissolved oxygen in water which can be harmful to fish and other aquatic species.

Blackwater usually develops in late spring or summer on 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 blackwater are temperature of the water and the amount of carbon (ie leafy litter and woody debris) present. The heightened risk of blackwater events is influenced by increased amounts of organic material and rising temperatures.

Where is blackwater currently occurring?
Blackwater is currently affecting sections of the River Murray, the lower Goulburn River, Broken Creek and Loddon River. NSW Rivers impacted to date are Edward and Wakool river systems, Lower Darling Anabranch and Murrumbidgee Rivers.

How long will this event last for?
It is unknown as to how long this event will last for. The two most important factors influencing the development of blackwater are temperature of the water and the amount of carbon (ie leafy litter and woody debris) present.

How does blackwater affect aquatic species?
Low levels of dissolved oxygen can cause stress (and even death) to fish, crayfish and other animals which breathe underwater.

Blackwater events can and have resulted in a number of native fish deaths. Crayfish and shrimp have sought refuge out of the water at Swan Hill and along the Goulburn River (anglers need to be reminded to observe bag limits, under the Fisheries Act - it is illegal to remove crayfish during the closed season.)

Some fish deaths have occurred primarily between Loch Garry Shepparton, to below McCoys Bridge in the Goulburn River. Isolated deaths of fish have occurred in the Loddon River system. Field reports indicate that a number of fish have moved into areas that have better oxygen conditions.

What is being done by authorities to manage black water and reduce fish deaths?
The Murray Darling Basin Authority, in conjunction with Victorian and NSW agencies has investigated measures to lessen the impact of black water.

It is not possible to dilute the current areas or blackwater without causing more flooding, as most river systems are currently operating at full channel capacity.

However, when floodplain flows recover, opportunities to mitigate the event through the delivery of environmental or dilution flows, will be investigated and implemented where possible.

Why are fish not being translocated to other waterbodies?
Given the extent of blackwater across Victorian rivers systems, coupled with low dissolved oxygen levels (DO) even where there is no evidence of black water, it is difficult to find suitable waters for fish to survive in. Many fish have moved into areas within the river system that have better oxygen conditions.

Who should I report fish deaths to?
EPA is the lead agency for co-ordinating the response to fish death incidents. Fish deaths can be reported to EPA’s Pollution Watch Line on 1800 444 004.

Am I allowed to catch fish in impacted area?
The taking of crayfish (spiny freshwater crays) during the closed season is prohibited. In both Victorian and NSW water, the closed season is from 1st September to 30 April.

Anglers must observe all fishing regulations during this event and adhere to bag and size limits that apply in both Victorian and NSW waters (Murray River).

As it is fish spawning season, anglers may wish to voluntarily limit their catch to minimise further impacts on native fish populations.

Is it safe to eat fish in systems affected by blackwater?
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.

Will Fisheries Victoria continue stocking in rivers and impoundments affected by blackwater?
Fisheries Victoria is committed to meeting its stocking obligations as identified in the 2010 Vic Fish Stock report. Native fish stockings occur from December through to March.

Fisheries Victoria is working with other agencies in monitoring water quality, particularly dissolved oxygen levels to determine the suitability of identified waters. This may mean the transfer of some fish stocks to other waterbodies that are not impacted by blackwater or low dissolved oxygen levels.

Will Fisheries Victoria increase stocking in waters where fish deaths have occurred as a result of the black water?
Fisheries Victoria will consider the impacts of blackwater events as part of its annual Vic Fish Stock planning program that will be conducted in March 2011.

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