Rainbow Trout

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 habitats^1-6 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. Scotland^{10, 11}, England^{1, 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 world^{12, 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 wild^{11, 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 populations^{12, 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 available^{1, 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-pens^{21, 22-24}, whilst the biosecurity of land-based RAS facilities means that escape risks are minimal within these systems.