Fish Kills: What are they and how do we stop them?

It’s that time of year again!
No, not the holiday season – it’s fish kill season.


With summer rapidly approaching, dead fish have already been found in the Burdekin’s local rivers, creeks and lakes, causing concern for local fishers and residents.

Fish kills can be devastating and have many causes, however, they are most often triggered by low dissolved oxygen levels in water.

NQ Dry Tropics is anticipating the problem will get worse once the rains arrive, and if enough fish die off, it could permanently change the number and proportion of species in creek ecosystems.

NQ Dry Tropics Project Officer, Scott Fry, said that major rain events can reduce oxygen levels by washing soil and plants into the water.

“Following a long drought, the extremely dry soil struggles to absorb much initial rainfall, and it runs off into the water”, he said.

“As bacteria breaks down this organic material, it sucks up the dissolved oxygen in the water, leaving none for the fish.

“Fish need dissolved oxygen in the water to breathe through their gills, and in low-oxygen water this can lead to massive fish kills”, Scott said.

He added that algae blooms and warm temperatures can also reduce aquatic oxygen levels.

“Algae blooms occur because of excess nutrients in the water, and this decaying plant matter uses up oxygen, which kills the fish”, he said.

“Hot weather can also be a factor because warmer water is less able to hold onto dissolved oxygen.

“Fish kills don’t always happen immediately; water slowly draining into creeks from paddocks is usually very low in oxygen due to a combination of decaying matter and warm water temperatures, and can result in fish kills several days after a rain event”, he explained.

Some fish species can tolerate low-oxygen conditions more than others, with some introduced species such as mosquitofish, tilapia, and carp able to survive while many native species die.

In order to prevent damage to local ecosystems and reduce the spread of introduced species, NQ Dry Tropics, in partnership with Lower Burdekin Water and the Burdekin Fish Restocking Association, has developed strategies to reduce the frequency and severity of fish kill events.

They are taking preventative measures, such as monitoring and creek flushing, to stop fish kills before they happen.

Lower Burdekin Water pumps oxygenated water into low dissolved oxygen areas, preventing fish kills before they happen, however, because of this year’s severe drought there may not be enough available water to pump all the creeks.

NQ Dry Tropics and TropWater are also training the Burdekin Fish Restocking Association to monitor dissolved oxygen levels in local creeks to better understand the oxygen threshold for fish kills.

Alan Griggs, of the Burdekin Fish Restocking Association, said: “We’ve got water meters to measure dissolved oxygen levels, and if they are too low we contact Lower Burdekin Water and get them to turn the pumps on at night to flush that water out.

“It helps that we’ve got a strong working relationship with Lower Burdekin Water.It’s just being prepared for it.  Sometimes there’s nothing you can do but we try and minimise the effects,” he said.  “We have the capacity, equipment and appropriate permits to capture belly-up fish, a common sign in the lead up to a kill, and relocate them to areas with better water quality”.

Scott Fry said that monitoring and preventative strategies would help reduce the number and severity of fish kills in the Burdekin this year, but landholders could also play their part to prevent their occurrence in water holes on their own property.

“Landholders can use fire-fighter pumps to increase oxygen levels by spraying water onto the surface for a short time, causing the water below to circulate,” he said.

“If we all do our bit and work together, we really have a chance of reducing the number of fish kills this year,” he added.

To make sure that fish kills aren’t caused by something more serious, such as pollution or disease, all fish kills should be reported to the Department of Environment and Heritage Protection (EHP).

In order to report a fish kill in your area, please contact the EHP on its pollution hotline 1300 130 372 or at