Oreochromis / Sarotherodon / Tilapia spp.
Escapes and Introductions
Escapees from aquaculture facilities can potentially impact on habitats and species in the receiving water bodies. Problems can occur due to competition with wild species, impacts on sensitive habitats, potential disease transfer, establishment of non-native species; and interbreeding with wild populations.
There are some concerns over the use of hybrids, genetically improved strains (such as GIFT) and non-native tilapia species, in relation to their potential interbreeding with native strains of tilapia. As a mitigation strategy the authorities in Lake Akosombo in Ghana for instance restrict the farming of any tilapia strain other than the native Akosombo strain. There is also a potential for farmed Nile tilapia and hybrids escaping in regions where they are non-native (e.g. much of Asia) and establishing self-reproducing feral populations. It is estimated that tilapia for farming purposes have been introduced into more than 140 countries1. At present, 55% of all countries in the world have reported populations of non-native tilapia species established outside of aquaculture. At the country-scale at least 26% of all known tilapia introductions were associated with ecological impacts1.
While the contribution of non-native species to the growth of the global aquaculture industry and the economic benefits that it has brought to many countries cannot be underestimated, minimising the escapes of non-native aquaculture species must be a high priority for resource managers, conservationists and the aquaculture industry2.
Losses due to escapes represent a considerable financial loss to a farm, so it is in the farmer’s interest to prevent them as much as possible. Thus, authorities and farms need to ensure that technical standards are in place, and are adhered to, that minimise the risks of tilapia escaping and hybridising with local wild tilapia populations or establishing reproducing feral populations. The use of sterile stock can help significantly in this regard. Experimental trials on triploidy have shown good results, however this is not currently achievable in commercial tilapia culture.
Escapes are a concern particularly from net-pens sited in large water bodies3 but escapes can also be an issue from pond systems that are connected to river systems4. It is important that cages can withstand heavy storms and also resist being breached by collisions with vessels or by predators.
To reduce escape risks, pond-based farms should have trapping devices such as screens and grills on all water inlets, outlets and drainage channels; these should be suitably sized to match the size of the stock. These screens should be regularly inspected, maintained and such actions recorded. Pond embankments, bunds, and levees should be of adequate height and build standard to retain stocks during periods of flood and regularly inspected and maintained. There should be no intentional release of fish stock from the farm.
Recapturing fish after escape, at or close to the point of escape, may seem a logical management option. However, evidence suggests that fish tend to disperse rapidly from the point of release and recapture efforts are often delayed after large-scale escape events which typically occur during storms and flooding events.
Generally, there has been increasing regulation, uptake of Best Management Practices (BMPs), codes of conduct and certification schemes in tilapia aquaculture which set standards for escape prevention measures5 and help in tackling escape issues.
- Deines, A.M., Wittmann, M.E., Deines, J.M. & Lodge D.M. 2016. Tradeoffs among Ecosystem Services Associated with Global Tilapia Introductions, Reviews in Fisheries Science & Aquaculture, 24:2, pp178-191
- Cook, E.J. et al, 2007. Non-Native Aquaculture Species Releases: Implications for Aquatic. Chapter 5 in Aquaculture in the Ecosystem, p155-184