Have you spotted a river incident? Call us: (08) 9278 0900 | 0419 192 845 (after hours)

Fish Kills

Fish killFish kill in upper Swan River in June 2012

The Swan Canning Riverpark regularly experiences fish kill events of varying intensity. These events involve a sudden and significant death of fish and/or other aquatic animals such as crabs and marron, and are characterised by a large number of aquatic animals dying during a short period of time, often in a defined area.

The Swan River Trust conducts investigations to understand and manage fish kills.

Causes of fish kill

There are a number of factors that can affect fish. One of the most common factors behind fish deaths in the Swan Canning Riverpark is the depletion of oxygen.

Fish require oxygen to survive. Evidence suggests that fish will move away from low oxygen areas, however dissolved oxygen concentrations of less than 2mg/L will put fish under stress, and anoxic (no oxygen) waters will result in fish deaths, particularly if the change to anoxic conditions is very sudden.

Fish kills in the upper Swan and Canning rivers often follow high rainfall events in summer, when widespread areas of the waterway become deoxygenated over a short period. It is hypothesized that these events deposit large amounts of organic material to the river where naturally occurring microbes immediately begin to break it down in the warm water conditions. This decomposition process depletes oxygen from the water, particularly where ambient conditions prevent re-oxygenation through diffusion, wind mixing or turbulence.

The lack of oxygen can also have a secondary impact on fish by interfering with nutrient cycling processes. Areas with low oxygen levels can be subject to a build-up of ammonium iwhich can be toxic to fish and other aquatic life.

Algal blooms contribute oxygen to the waterways through photosynthesis during daylight hours. However, at night time, when the blooms are not photosynthesizing they contribute to the depletion of oxygen in water column through respiration. When algal blooms die off, their cells also become part of the organic load on the riverbed and through microbial action will decompose and further contribute to oxygen depletion.

Many types of algae are harmless. However, some species of algae, such as the dinoflagellate, Karlodinium veneficum can release toxins under certain conditions and this can be lethal to fish. Other species of algae affect fish by coating their gills with a mucous-like substance that prevents fish from extracting oxygen from the water.

Other factors that can sometimes cause fish kills in the Swan Canning Riverpark include contaminants that enter waterways through drainage and changes in pH due to acid drainage water arising from earthworks in areas with acid-sulphate soils. In addition, it is common for fishers to leave unwanted fish (such as blowies) on the foreshore and these are often mistakenly reported as fish kill events.

Importantly, fishers should always avoid collecting and eating dead or lethargic fish.

How you can help

The Trust encourages the public to report dead or sluggish fish in the Swan Canning Riverpark on 9278 0900 or after hours on 0419 192 845.

More information

Factors contributing to fish kills and algal blooms [ PDF Document 700 KB]