Escapes and Introductions — Seafish

Atlantic Halibut

Hippoglossus hippoglossus

Escapes and Introductions

Еscapees 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, 2. In addition, escapes represent a financial loss to farmers, so it is in their interest to prevent them as much as possible.

The potential of fish escaping the farming enclosures is usually higher in ‘open’ culture systems, such as net-pen farms.

Escapes of Atlantic halibut within its native range (where they are presently farmed) have not historically been considered an issue, however there is now concern that escaped reared fish may interbreed with wild fish and produce offspring with compromised genetic integrity/reduced fitness1. Escapes from halibut farms do occur, e.g. nearly 7,000 halibut escaped from a Scottish fish farm in 20133. However, the limited evidence available indicates that farmed halibut survive poorly in the wild, restricting potential impacts. In 2006, Atlantic halibut stocks released by vandals from a Scottish farm were subsequently found dead along the sea shore in large numbers4, 5. This incident illustrates the importance of farm security systems to deter and prevent sabotage.

The use of on-shore systems (RAS and PAS) significantly limits the potential for escapees. However even though unlikely, escapement is still possible through outlets in flow-through systems e.g. due to accidental release of fertilised eggs, larvae or market-size fish during handling and sorting in proximity to outlets2.

To reduce escape risks farms should have trapping devices such as screens and grills on all water inlets, outlets and drainage channels that are suitably sized to match the size of the stock. These screens should be regularly inspected, maintained and such actions recorded. Rearing facilities should be of adequate size and standard, able to retain stocks during periods of flood and regularly inspected and maintained. There should be no intentional release of stock from the farm.

When Atlantic halibut are farmed in the marine environment, containment systems should comply with the latest technical engineering standards which reduce the risk of escapees. Authorities can ensure that new Atlantic halibut facilities apply for the appropriate licence or permit and should provide evidence that containment systems will prevent escape and escapees will not establish.

Many accreditation procedures for other marine species (e.g. Atlantic salmon) require the presence of farming 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. Such an approach is also recommended for Atlantic halibut farming, even if agreed technical standards are not in place for such a niche species.


  1. Cook, E.J et al, 2007. Non-Native Aquaculture Species Releases: Implications for Aquatic Ecosystems. Chapter 5 in Aquaculture in the Ecosystem, pp155-184
  2. ICES
  3. Scottish Government
  4. The Times
  5. BBC News