SALMON FARMERS are again using a toxic pesticide years after it was thought to have been phased out. The chemical, teflubenzuron, known commercially as Calicide, is given to salmon to kill sea lice parasites. Concerns about the polluting effects of the substance, 90% of which is excreted by the salmon into the sea, have been raised by shellfish farmers.

A 1999 report by the Scottish Environment Protection Agency (Sepa) found teflubenzuron to be "potentially highly toxic to any species which undergo moulting within their life cycle. This will therefore include some commercially important marine animals such as lobster, crab, shrimp and some zooplankton species." Safety reports commissioned by the manufacturer, Nutreco, revealed Calicide can still be found in sediment on the sea bed nearly two years after use.

Sepa grants licences for Calicide, but sets strict limits on concentration levels, rendering adoption by salmon farms largely impractical. The chemical disappeared from use in 2005-2006, yet Sepa has now revealed its re-emergence at three different sites this year, as salmon farmers try to tackle resistance of sea lice to two other treatments.

At a meeting of fishing industry heads and government officials, a regional development officer announced the reintroduction of Calicide in the Western Isles due to the diminishing effectiveness of SLICE and Exis treatments.

The recently released minutes of the Tripartite Working Group state: "Certain producers had started to source and use Calicide as an alternative. Sepa were asked to review their discharge licences to allow the use of greater quantities."

David Oakes, a Western Isles scallop farmer, said: "I was told by Sepa years ago that Calicide wasn't being used. It's a chitin inhibitor, and most shellfish have chitin as part of their make-up, so it cannot be good for us. It's been shown that it's detrimental to the environment so it's a big concern."

A leading Scottish marine biologist, who wishes to remain anonymous, believes Sepa's safety limits regarding the toxicity of Calicide are not sufficiently severe.

He said: "It affects all animals that produce chitin, which is a huge number of species in the sea and on land. If it's toxic to sea lice, it's toxic to any animal that produces chitin. I think there should be a moratorium on its use until further scientific study is done."

To date, Sepa has given 268 fish farm sites licences to use Calicide. Douglas Sinclair, Sepa's aquaculture expert, said there was no evidence of the chemical being used in dangerous quantities.

He said: "Use of Calicide is rising a little bit, but there should be no problem as long as it's within our limits. It's inappropriate use could cause a risk to sea life but the use of Calicide within the limits we set poses no greater risk than any other sea lice medicine."

The salmon industry is keen to stress that no regulations have been broken. Dr John Webster, technical director of the Scottish Salmon Producers' Organisation, said: "For anyone to imply what we're doing is in any way improper or environmentally damaging or sinister is absurd."

And he added: "Some of it Calicide goes through the fish, some of it lands in the sea bed, but it soon becomes completely innocuous Sepa control the amount so the effects make it very localised and very temporary."

Concerned that competitors in Norway are gaining advantage through less strict controls, salmon producers are urging Sepa to allow Calicide to be used more widely and in greater quantities.

Webster said: "The margin of safety is a bit too high. Because of that, the way the industry can develop is being hampered. It's about getting toward a reasonable balance."

Sepa's Douglas Sinclair said: "There's no grounds to review our licensing of Calicide."