Tilapia are members of the Cichlid family and are one of many invasive noxious fish species that have spread across many parts of the world causing irreversible damage. Tilapia were introduced into Australia in the 1960’s and now dominate most major Queensland catchments. There are 2 main species within Australian waterways – the Mozambique tilapia (Oreochromis mossambicus) and the spotted tilapia (Tilapia mariae). The Murray-Darling Basin is now the frontline for these fish. Unfortunately these fish are easily spread by people dumping them from their aquarium into waterways (sometimes on purpose), using them as bait and from natural and human water transfers. Once these fish establish themselves they are difficult, if not impossible, to eradicate.
Native fish species are outcompeted and attacked by tilapia species which are highly aggressive and may dominate waterways causing a detrimental shift in riverine and wetland ecosystem function. The biological characteristics that allow them to dominate include their ability to rapidly breed with fast growth rates and low mortality. Further, tilapia can survive in extreme conditions that many native fish cannot. Female Tilapia carry and aggressively defend fertilised eggs and young in their mouth, resulting in high survival rates of young (50-90 %). They can breed year-round in harsh conditions by diverting their energy into reproducing at an earlier age rather than putting that energy into growth.
Turbid, warm and polluted waters low in oxygen are no problem for Tilapia as they can survive temperatures from 8 – 42°C and gulp air from the surface if needed. They can also survive well in seawater. Being omnivorous, they consume anything from algae to other fish species, including their own. As tilapia disrupt the function of native aquatic ecosystems they also indirectly cause a decline in water quality. They can disturb sediment and in-stream plant life by grazing and from extensive construction of nests in substrate. How does this relate to bush or riparian regeneration and conservation? The answer is that Tilapia are more of a symptom of degraded riverine and wetland habitat rather than the cause. They generally prefer well vegetated areas in slow flowing water, particularly in degraded systems containing invasive weed species such as para grass (Urochloa mutica).
Currently the complete eradication of Tilapia from most natural waterways is not a realistic goal. However we can maintain native aquatic ecosystems by increasing the resilience of native species against Tilapia (and other pest fish) incursions. This can be done by taking a holistic approach to river rehabilitation. For example we can increase the resilience of less competitive native fish species by reducing stressors such as pollution and poor water quality through improved land use practices. We can also provide native fish habitat by restoring riparian and in-stream vegetation and by re-snagging creek banks (also by leaving fallen trees in waterways). In addition, removal of fish barriers such as weirs, culverts and crossings can help improve resilience of native fish species by allowing passage for feeding and reproduction.
Article by Daniel Bloom, waterways officer, Sunshine Coast Council
For further information on pest fish in Queensland you can visit the Department of Agriculture and Fisheries website.
Content and images by Department of Agriculture and Fisheries.