Fish kills in NSW 2020-2021

What are fish kills?

Fish kills are defined as a sudden mass mortality of wild fish.

Fish kills can occur at any time although data indicates fish kills are more likely to occur in summer or following sudden changes in temperature.

Recent reports of fish kills are included in the table below and we will update the table as we receive reports.

Some of the key causes of fish kills are outlined in the information below.


Blackwater occurs naturally over time when leaf litter and woody debris build up on the floodplain and then get washed into the river system during a flood. It’s an important part of the aquatic food web as it provides food for insects which feed fish, frogs and other aquatic wildlife.

When there is a prolonged dry period, the leaf litter and organic matter on the floodplain builds up until the next flood event. This can result in a significant amount of organic matter returning to the river with any big flushes of water after prolonged drought, making blackwater events more likely.

Hypoxic blackwater

When there is a lot of organic material washed into the river, its rapid decay can consume dissolved oxygen from the water. If dissolved oxygen drops to very low levels, this can cause ‘hypoxic’ water which can be stressful for native fish and can lead to fish kills.

More information on blackwater can be found on the MDBA website and in this factsheet by DPIE Environment, Energy and Science (NSW DPIE EES).

To notify the department of potential blackwater events email or to report a fish kill, contact the Fishers Watch Phoneline on 1800 043 536.

Bushfire impacts on native fish

Find information on the impacts of bushfires on native fish.


Stratification is when the surface of the water heats up more than the deeper water. That warmer layer tends to be warm and well-oxygenated compared to the deeper water that is colder and oxygen-depleted.

Thermal stratification


  1. Blue Green Algal Bloom
    Algal flourish in warm and still conditions, particularly in the absence of high flow
  2. Surface Water Layer
    Warm, high nutrient load from run off.
    Dissolved oxygen by day, depletion overnight
    Limited fish habitat
  3. Deep Water Layer
    Cooler, low light penetration, low dissolved oxygen (hypoxia), Poor fish habitat
  4. Decomposition of organic matter
    Depletes dissolved oxygen
For an accessible explanation of this graphic contact the author

De-stratification ("Turn over")


  1. Sudden weather events or small increases in flow
    Can mix the warmer surface water and cooler deep water – essentially breaking down the temperature stratification
  2. These changes can mix the warmer surface water with cooler deep water
    Algal blooms may also be disrupted, potentially increasing decomposition (and further depleting oxygen). This means even at the surface dissolved oxygen levels can become critical, killing fish.
For an accessible explanation of this graphic contact the author

Simple things you can do

  • Be on the lookout for any changes in water quality - i.e. changes to the colour or odour of your local waterway and report them to us via the Fishers Watch Phoneline on 1800 043 536.
  • Contact us on the phoneline if you observe any dead fish or fish starting to gasp at the water surfaces.

Investigating and reporting

Recent fish death events

Updated: 15 December 2020 (3pm)

1 July 2020 - 30 June 2021

Location and date kill observed

Cause and extent

Murray Darling catchments in NSW

Peel River, Tamworth (7 December)

Report of 10 to 20 dead fish including European Carp and Murray Cod.

Cause was episodic rainfall event the preceding weekend that caused short and sharp flow in the river. This can cause a rapid reduction in dissolved oxygen levels due to large volumes of organic material entering the river system.