Rainbow Trout

Oncorhynchus mykiss

Escapes and Introductions

Escapees from aquaculture facilities can potentially impact on habitats and species in the receiving water bodies. Problems could occur due to competition, potential disease transfer, establishment of non-native species, interbreeding with wild populations, and impacts on sensitive habitats1-6 and potentially attracts mammalian and bird predators (e.g. otters, mink, heron, osprey, etc.) and thereby increase the level of predation on wild fish7.

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. However, minimising the escapes of non-native aquaculture species must be a high priority for resource managers, conservationists and the aquaculture industry8. Potential problems associated with the introduction of rainbow trout as a non–native species needs to be assessed by regulators prior to introduction.

A with any fish farming system there is the potential for stock to escape and prevention is the key to mitigating rainbow trout escapes. Escapes from farms can occur both through repeated low-number incidents and through large-scale events9. In land-based farms that such escapes could happen, for example due to floods, blockages, accidents while handling and sorting, and vandalism. Where fish are reared in marine cages, technical and operational failures can lead to escapes: cages can be breached in storms; holes can appear through wear and tear; predator interaction and operational accidents can occur, and so on.

Escaped rainbow trout have been noted in various areas, e.g. Scotland10, 11, England1, 2, Serbia3, Chile12. However, it has to be noted that rainbow trout, of essentially the same genetic make-up as farmed fish, are also intentionally stocked for sport in many places around the world12, 13.

Losses due to escapes represent a considerable financial loss to a farm, so it is in its interest to prevent them. Mitigation should ideally include the following five elements14:

  • Mandatory reporting of all escape incidents
  • Establishment of mechanisms to analyse and learn from escape reporting
  • Conduct mandatory, rapid, technical assessments to determine the causes of escape incidents involving large numbers of fish
  • Mandatory training of fish farm staff in escape-critical operations and techniques
  • Introduction of technical standards for sea-cage aquaculture equipment coupled with an independent mechanism to enforce the standard, where fish are farmed in the sea

The extent of a potential interaction between escapee fish and wild fish will depend upon the body size, number (relative to the wild populations), survival and behaviour. There is evidence that escapees from fish farms survive poorly in the wild11, 15, attributed to genetic selection for the farm environment and a lack of experience of the wild environment8. Escapees tend to lose condition and feed poorly on wild prey. Nevertheless, a small proportion of individuals can survive15 and establish populations12, 16. Rainbow trout are considered an invasive species having established populations outside their natural range. Few self-sustaining populations are known in the UK5.

Chromosome manipulation, widely used in rainbow trout farming (‘all-female’ and ‘triploid’), will prevent reproduction and establishment of self-sustaining populations from escapes17.

Authorities can ensure that new trout facilities apply for the appropriate licence or permit and should provide evidence that containment systems will prevent escape, and escapees will not establish. To reduce the risk of escapes, farms should have well built and maintained ponds, screens/grills (suitably sized to match the size of the stock) on all water inlets and outlets. These screens should be regularly inspected and maintained, and such actions recorded. Retaining embankments (bunds, levees) should be of adequate height and standard to retain stocks during periods of flood and be regularly inspected and maintained. Trapping devices to capture any escapees should be placed in drainage channels and outlets. There should be no intentional release of stock from the farm. When trout are to be farmed in net-pens, systems should comply with the latest technical engineering containment standards which reduce the risk of escapees.

Many accreditation procedures require trout farmers to have standard operating procedures (SOP) that incorporate an escape risk assessment. In addition, there should be evidence of farm staff capacity and capability, including training of staff prior to starting work and regular subsequent training to understand and address risks from escapes and follow the defined SOP. Furthermore, any estimated unexplained loss of farmed trout in net-pens is required to be made publicly available1, 18, 19. Most of the above issues are addressed by certification standards.

When reared in marine waters, recapturing rainbow trout after escape (at or close to the point of escape) is 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. These two factors mean that few attempts to recapture farmed fish after large-scale escape incidents have been successful20. Improved netting materials, enhanced engineering standards and equipment build quality, better staff training around vessel handling and use, and efforts to deter predators, have done much to reduce escape incidents from net-pens21, 22-24, whilst the biosecurity of land-based RAS facilities means that escape risks are minimal within these systems.


  1. Walker, AF. 2004. An investigation of escaped rainbow trout in the upper River Earn and Loch Earn during 2002/03. Fisheries Research Services, Scottish Fisheries Information Pamphlet No 24, 21p.
  2. Turnpenny AWH, O’Keeffe N. 2005. Screening for intake and outfalls: a best practice guide. Environment Agency, 153p.
  3. Lenhardt M, Markovic G, Hegedis A, Maletin S, Cirkovic M, Markovic Z. 2007. Non-native and translocated fish species in Serbia and their impact on the native ichthyofauna. Reviews in Fish Biology and Fisheries. 21, 407-421.
  4. Albarino RJ, Buria LM. 2011. Altered mayfly distribution due to strong interactions with alien rainbow trout in Andean streams of Patagonia. Limnologica. 41, 220-227.
  5. Fausch KD. 2007. Introduction, establishment and effects of non-native salmonids: considering the risk of rainbow trout invasion in the United Kingdom. J Fish Biol. 71 (Supp. D), pp1-32.
  6. Zwol JAV, Neff BD, Wilson CC. 2012. The influence of non-native salmonids on circulating hormone concentrations in juvenile Atlantic salmon. Animal Behaviour 83, pp119-129.
  7. Walker, AF. 2005. Rainbow trout on the loose in Scotland. Trout News 39, pp17-19.
  8. Cook, E.J et al, 2007. Non-Native Aquaculture Species Releases: Implications for Aquatic. (Chapter 5 in Aquaculture in the Ecosystem, pp155-184)
  9. Hansen LP, Windsor ML. 2006. Interactions between aquaculture and wild stocks of Atlantic salmon and other diadromous fish species: science and management, challenges and solutions. ICES J Mar Sci 63, 1159-1161.
  10. Marine Scotland Science 2018.
  11. Walker, AF. 2003. Status of rainbow trout in Scotland: the results form a questionnaire survey. Fisheries Research Services, Scottish Fisheries Information Pamphlet No 23, 28pp.
  12. MBA
  13. MBA
  14. Jensen, O et al, 2010. Escapes of fishes from Norwegian sea-cage aquaculture: causes, consequences and prevention. Aquaculture Environment Interactions. 1, pp71–83
  15. Blanchfield PJ, Tate LS, Podemski CL 2009. Survival and behaviour of rainbow trout (Oncorhynchus mykiss) released from an experimental aquaculture operation. Can J Fish Aquat Sci 66, pp1976-1988.
  16. Consuegra S, Phillips N, Gajardo G, de Leaniz CG. 2011. Winning the invasion roulette: escapes from fish farms increase admixture and facilitate establishment of non-native rainbow trout. Evolutionary Applications 4, pp660-671.
  17. FAO
  18. ASC
  19. GAA BAP
  20. Dempster, T. et al, 2016. Recapturing escaped fish from marine aquaculture is largely unsuccessful: Alternatives to reduce the number of escapees in the wild. Reviews in Aquaculture, April 2016
  21. Bridger, C.J.et al, 2015. Physical containment approaches to mitigate potential escape of European-origin Atlantic salmon in south coast Newfoundland aquaculture operations. DFO Canadian Science Advisory Secretariat Research Document
  22. MBA
  23. MBA
  24. MBA