Natural events big fish killers

Fish kills are common occurrences in the tropics and can occur throughout the year but are most common during the build-up and beginning of the Wet.

Fish kills can be due to a single factor, or two or more factors interacting.

Fish kills are common occurrences in the tropics and can occur throughout the year but are most common during the build-up and beginning of the Wet.

This is generally due to higher water temperatures (and consequently lower dissolved oxygen levels), sudden storm/flood events, and lower water levels in freshwater river systems and billabongs.

Three main factors play a major role in fish kills:

■ Environmental factors (e.g. dissolved oxygen levels, salinity, temperature and acidity levels),

■ Toxicants/pollutants, and

■ Infection with disease pathogens.

Fish kills can be due to a single factor acting alone, or two or more of these factors interacting together.

During this time of the year heavy, early rainfall often results in extensive surface run-off, which carries large loads of plant material and organically rich soil into a waterway.

As this material decomposes it creates a high oxygen demand, which then results in very low levels of dissolved oxygen in the water column. Fish kills from this cause most often occur in enclosed bodies of water such as billabongs.

Another common cause of fish kills are algal blooms or red tides which occur when colonies of algae grow out of control due to favourable environmental conditions, producing toxic or harmful effects on aquatic life.

Tides, prevailing winds and currents often concentrate the algal blooms in parts of estuaries and waterways where fish kills then invariably occur.

The species most frequently observed and reported as being affected by kills include mullet, catfish, bony bream, rainbow fish, barramundi, freshwater crustaceans and molluscs.

Hardy species such as barramundi, mullet and catfish appear to be susceptible to fish kill events because they often occupy habitats such as seasonally landlocked billabongs that are subject to environmental influences as previously mentioned.

If you witness a fish kill please report it to the Department of Primary Industry and Fisheries on the Fish Watch Hotline: [phone] as soon as possible.

This will enable a quick response and investigation into the cause of the kill.

https://www.facebook.com/FisheriesNT