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Closing salmon escape routes

23.03.2007
Icing, operating errors and lack of knowledge of currents and waves led to escapes from fish farms last year. But countermeasures are now being put into effect.

According to official statistics, a million fish escaped from Norwegian fish farms in 2006. The aquaculture industry has asked SINTEF to look at the causes of the escapes in eight cases that together were responsible for half of the escapees.

Most escapes were due to a combination of different factors. SINTEF says that there is good reason to believe that wrong assumptions about the impact of environmental forces such as ice, currents and ocean waves either triggered or contributed to the causes of six of the eight escapes studied. According to the report, operating error was a contributory factor in five cases.

Guidelines for countermeasures

Several of the findings of the study have already led to changes in standards for the design of fish farm facilities, explains SINTEF scientist Østen Jensen, author of the report.

Simultaneously, the Fishery and Aquaculture Industry Research Fund has given NOK 2 million to a project led by SINTEF in which the participants will develop new technology and solutions, based precisely on what this study tells us about the causes of escapes.

All the farms that were studied lie in Northern Norway. The eight escapes all happened in January last year during two periods of bad weather. The first of them produced strong south-westerly winds, while the other arrived with a storm which brought long-lasting strong winds from the southeast, combined with low temperatures.

More ice than expected

The amount of ice that built up on the sea-cages during the storm took many people by surprise. The build-up surpassed the amount of ice that current standards state that sea-cages must be designed to withstand.

The project that has just started will enable better methods to be found for taking icing into account and designing sea-cages to withstand it.

Wrong picture of currents

According to the report, the true current speed at four of the fish farms probably higher than the values used as a basis for approval of the farms. SINTEF believes that this was a contributory factor in at least one of the escapes. According to the report, it would be possible to meet the requirements that the standard sets for current measurements without a sufficient safety margin being designed into the cages.

“In the current project, we will be developing new current measurement methods. The aim is to arrive at a reliable basis for design, based on measurement periods of between one and three months,” says SINTEF’s Østen Jensen.

The study shows that ocean waves were a contributory factor in three escapes. According to the report, the current standard lack adequate descriptions of how we should take such waves into account when fish farms are being planned. Where wave action is regarded as important, companies with special expertise should carry out analyses, concludes the report.

Wrong use should become impossible

The report also points out that some people attach the net wrongly to the floats. In one system, the net was fastened to the ?? uprights, which increased the risk of failure of the attachment. The SINTEF scientists also found an example of weights being attached too high up on the net, which was being worn off by friction.

“One of the aims of the current project is to develop solutions that will make it impossible to use equipment wrongly,” says Jensen.

According to the report, it is likely that the number and seriousness of escapes could have been reduced if the best of already known solutions had been adopted.

“So a system for identifying “best practice” and telling fish farmers about it would be another measure that could prevent escapes,” says Jensen.

Aase Dragland | alfa
Further information:
http://www.sintef.no

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