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Understand Escapes To Improve Prevention Strategies


Researchers from the Centre for Research-based Innovation for Aquaculture Technology (CREATE) at SINTEF Fisheries and Aquaculture have published an extensive review regarding the causes of escapes from Norwegian sea-cage aquaculture over the past decade. Written by O.Jensen, T. Dempster, E.B Thostad, I.Uglem and A.Fredheim.

3.93 million salmon, 0.98 million rainbow trout and 1.05 million cod escaped from 2001 to 2009. Salmon and trout mostly escaped after structural failures of sea-cages, while a far greater proportion of cod than salmon escaped through holes in nets.

The officially reported statistics suggest that after the Norwegian technical standard (NS 9415) for sea-cage farms was phased in, the total number of escaped salmon declined from over 600,000 fish per year (2001 to 2006) to less than 200,000 fish per year (2007 to 2009). This was despite the total number of salmon held in sea-cages in Norway increasing by 44 per cent during this period. The marked decrease in the reported number of escapes is primarily due to a decrease in the number of large escape events involving more than 10,000 fish.

Dr Østen Jensen said: “Large-scale escape events of more than 10,000 individuals represented only 19 per cent of the escape incidents reported, but accounted for 91 per cent of the number of escaped fish. Therefore, a focus on preventing this small proportion of large-scale incidents will have a great effect in diminishing the consequences of escapes. This is where the Norwegian technical standard (which introduced requirements for site survey, risk analyses, design, dimensioning, production, installation and operation regulations;) has had greatest effect and been a useful prevention tool at an industry-wide scale.”

Causes of escapes

Escapes are caused by a variety of incidents related to farming equipment and its operation.

Fisheries following escape events during the period from September 2006 to December 2009 indicate that escapes of Atlantic salmon (by total number of fish escaped) are dominated by structural failures of equipment (68 per cent), with operational related-failure (eight per cent), escapes due to external factors (eight per cent) and escapes from land-based facilities (11 per cent) making up lesser percentages.

For cod, evidence suggests that additional reasons for escape exist beyond those present for salmonids.

This stems from behavioural differences in the way cod interact with cages, through biting of the netting, which may increase wear and tear and contribute to the creation of holes, and a far greater level of exploratory behaviour near the net wall, which may increase the chances of cod swimming through a hole.

Of the 56 escape events reported from September 2006 to December 2009, external (38 per cent) and biological (25 per cent) causes accounted for the greatest numbers of escaped cod. In addition, the reason for escape was undetermined for a significant percentage of the total number of fish that escaped (27 per cent). In contrast to salmon, there was no specific seasonality in the occurrence of large (>10 000 individuals) or medium (1000 to 9999 individuals) escape incidents.

Economic consequences

Relatively little information exists on the direct costs of escapes, although the European Union’s 7th Research Framework project Prevent Escape (www. is currently assessing the cost of escape to the fish farming industry across Europe, and the true cost of escapes is thus likely to be known by 2011.

In Norway, as reported escapes of salmon on average account for losses of <0.2 per cent of the fish held in sea-cages each year, the relative direct economic cost to the industry is small, even when the cost of replacing damaged equipment or paying for recapture efforts is accounted for. Insurance claims are likely to offset these costs.

This may mean that little direct economic incentive exists for salmon farmers to invest further time and resources to prevent escape events. As the percentage of cod that escape is considerably greater than that of salmon, direct economic losses are more significant and may stimulate investments in containment technologies.

The greatest cost of escape to the industry, however, is indirect, as escapes damage the industry’s reputation. The popular press often report escape events widely, which sheds negative light on the industry’s environmental credentials and fuels criticism from environmental groups.

The extent to which this restricts the industry from expanding the number of sites it uses and the amount of fish it produces is immeasurable, but is likely to be significant, as the threats that escapes pose to wild populations are strong counterpoints in debates regarding industry expansion.

Environmental consequences

Escapees can have detrimental genetic and ecological effects on populations of wild conspecifics, and the present level of escapees is regarded as a problem for the future sustainability of sea-cage aquaculture.

Over 325 million Atlantic salmon are held in sea-cages in Norway at any given time, which outnumbers the ~500 000 to 1 million salmon that return to Norwegian rivers from the ocean each year to spawn. A single sea-cage may hold 100s of thousands of cultured fish, meaning that sites with multiple cages may contain more than 1 million fish.

Due to the large numerical imbalances of caged compared to wild populations, escapement raises important concerns about ecological and genetic impacts. Evidence of environmental effects on wild populations is largely limited to Atlantic salmon, as these interactions have been intensively studied, with more limited information for Atlantic cod.

Advice to prevent escapes

To better prevent escapes of juvenile and adult fish as sea-cage aquaculture industries develop around the world, SINTEF recommended that policy-makers implement a 5 component strategy:

1. Mandatory reporting of all escape events, including:

  1. the number of fish that escaped and their size;
  2. a description of the sea-cage technology involved;
  3. categorisation of the operational circumstances or environmental conditions at the time of escape; and
  4. an estimated cause of escape.

2. A defined mechanism to collect, analyse and learn from the mandatory reporting. This information must then be effectively disseminated to equipment suppliers and fish farmers so improvements can be made. Within Norway, the AEC has this role, although the formation of a full commission to achieve this may not be necessary in other countries.

3. As causes of escapes estimated by farmers are often inaccurate, we recommend mandatory, technical assessments to determine the causes of ‘large-scale’ escape incidents. Based on escape statistics in Norway, ‘large-scale’ escape events can be considered to be those that cause the loss of more than 10,000 fish. The technical assessment must occur rapidly (within 48 h) after the escape event. At present, no mechanism for this is in place in Norway.

When technical assessments are made, they are often done weeks to months after the incident. This can lead to a loss of evidence, often making the root cause difficult to ascertain. Learning from each large-scale escape event would assist recommendations for the design and properties of seacage systems and help improve technical standards.

4. Introduction of a technical standard for sea-cage aquaculture equipment coupled with an independent mechanism to enforce the standard. Within Norway, the highly detailed NS 9415 technical standard has perhaps been the most useful tool at an industry-wide scale to prevent escapes. Voluntary standards are unlikely to be effective; therefore, we recommend enforcement through legislation. Within the next few years, NS 9415 is likely to be developed into an international standard (ISO), which will facilitate implementation in sea-cage industries worldwide.

5. Certain operations within fish farming (e.g. correct anchoring and mooring, connecting net-cages to floaters and correct weighting of net-cages in currents) are likely to pose a higher risk of an escape event occurring if they are done incorrectly. Therefore, these key processes should be identified, and mandatory training of staff who undertake these processes would likely reduce human errors that lead to escapes. Many other industries have similar mandatory training requirements for operators to perform particular tasks, and thus legislative precedents would likely exist in most countries that could be drawn upon.

September 2010