Mozambique tilapia (Oreochromis mossambicus)* is a major pest fish species in Australia. A successful invader, it has managed to dominate the natural waterways into which it has been introduced. It is not currently found in the Murray–Darling Basin (MDB); however, it has established thriving populations in catchments neighbouring the Basin’s northern rivers.
Despite the high risk of introduction, little work has been done to estimate the potential range Tilapia might occupy in the Basin, or to predict its possible impacts on natural, economic or social assets. Research conducted through this project has boosted our knowledge about this species.
Mozambique tilapia has a wide and varied diet and can occupy a diverse range of habitats. The one factor that appears to affect it is a vulnerability to cold temperatures. Based upon minimum temperature tolerated by Tilapia and the minimum water temperature data available, Mozambique tilapia have the potential to infest the northern Basin in Queensland and parts of New South Wales, through the western inland catchments of NSW and down to the Lower Lakes and lower Murray in South Australia. This equates to a potential distribution occupying approximately half of the MDB. The species could possibly benefit from future climate scenarios. For example, an increase in minimum winter temperatures of 2oC would allow Mozambique tilapia to overwinter in most lowland areas of the Basin
Tilapia is capable of sustaining reproducing populations under the conditions found in much of the MDB, as breeding and feeding can occur for significant portions of the year. In the northern parts of the Basin, and many southern parts, median water temperatures could see a breeding season of at least 3–6 months in duration, with around 4–6 broods for each female in each breeding season.
Tilapia impacts have been recorded in a number of locations both in Australia and overseas. The key impacts recorded include major declines in commercial and traditional fisheries, fish extinctions, destruction of beds of aquatic plants and declines in water quality. Some of the predicted direct impacts of Tilapia on MDB native fish and other biota include:
- direct predation by Tilapia;
- competition for resources (food, habitat);
- destruction of macrophytes and other aquatic plants used as breeding or nursery habitat by native species;
- habitat disturbance;
- transmission of diseases and parasites;
- competitive exclusion of native fish from favourable habitat by Tilapia’s aggressive behaviour;
- increase of blue-green algal blooms (through resuspension of nutrients);
- winter die-offs of Tilapia (polluting waterways); and,
- undermining river banks due to destruction of river plants and nesting behaviour.
Recent experiments showed that Tilapia consume juvenile native fish, including members of genera that occur in the MDB such as rainbowfishes (Melanotaenia spp.), carp gudgeons (Hypseleotris spp.), hardyheads (Craterocephalus spp.), Bony herring ( Nematalosa erebi) and Agassiz’s glassfish (Ambassia agassizii).
Implications for native fish
Invasion of the MDB by Tilapia could be disastrous for many (up to 18) native fish species found in the Basin’s waterways. This project has highlighted the areas and species most at risk from Tilapia and the likely impacts if invasion occurred. The study recommends a ‘prevention is better than cure’ approach, and highlights education and awareness as a key factor. This education campaign is most pertinent in areas close to current distribution of wild Tilapia populations (i.e. north-eastern MDB).
* Recent genetic studies have indicated that there have been several introductions of Oreochromis into Queensland possibly comprising four different species. Most “Mozambique” tilapia individuals are probably inter-species hybrids.
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