It will stop a bullet – but can it really stop fish escaping from their cages?

Published on Monday 5 May 2008 21:57

A SUPER-STRONG material used in bullet-proof armour is the latest weapon brought in to try to stop salmon escaping from fish farms in Scotland.

Two trials will test the new material against the weather and predators such as seals, which can damage conventional cages.

The industry has spent millions of pounds trying to prevent escapes, and wild fish bodies have raised concerns that farmed salmon can carry disease and affect the genes of wild species if they reproduce.

The new tests involve Dyneema, a high-strength, lightweight polyethylene fibre already used in ropes, cables and nets in the fishing, shipping and offshore industries, as well as in bullet- resistant armour and clothing for police and the military.

It offers twice the strength of standard aquaculture nets, which are mainly made from nylon. It is also double the price, with mesh on a single fish farm cage, 15 metres deep and 90 metres in circumference, costing 6,000 to 8,000.

The first tests are being carried out by Loch Duart Ltd at Badcall Bay in Scourie, Sutherland, as part of a research and development project co-ordinated and part-funded by the Scottish Salmon Producers' Organisation (SSPO). Marine Harvest Scotland will start a second trial shortly on Harris.

In 2003 Loch Duart Ltd lost 18,000 young salmon that escaped from pens in Calbha Bay, south of Scourie, costing it 180,000 in lost revenue.

Nick Joy, the managing director, said: "This material could be the answer."

Marine Harvest lost 52,353 fish from a farm in Loch Portain, North Uist, last May.

Between 2002 and 2006, a total of 1.58 million fish escaped from salmon farms. Last September, wild fish bodies complained to the European Commission over the escape of more than 100,000 salmon from sites in the Western Isles over six months.

The SSPO says about 35 per cent of escapes are due to predation, mainly attacks by seals. It says annual escape figures fell from 312,655 to 155,653 between 2002 and 2006, and there were no breaches between January and April this year.

In 2007, the number of escaped salmon represented less than 0.2 per cent of the 70 million smolts (young fish) that were put to sea over the two previous years, the SSPO said.

Dyneema was invented by the Dutch firm DSM and is manufactured by Boris Nets, of Lancashire. It will be tested against standard nets over two years.

Sid Patten, chief executive of the SSPO, said: "This represents a significant investment to investigate the potential effectiveness of the novel net material."

The experiment was also welcomed by wild fish bodies, which have been highly critical of fish farm escapes.

STRONGER THAN STEEL

DYNEEMA is the brand name for an ultra high molecular weight polyethylene (UHMWPE). Described as the world's strongest fibre, it is said to be up to 15 times stronger than steel and up to 40 per cent stronger than rival synthetic fibres on a weight-for-weight basis.

A single strand is capable of carrying a load of 240lb, while a length the width of a little finger can dangle 20 small cars weighing a total of nine tonnes.

Ropes made from the material can last two to four times longer and have a higher breaking strength than steel-wire ropes, but are about a tenth of the weight.