Food pellets laced with a chemical called teflubenzuron have long been given to farmed salmon around the coast in order to control sea lice infestations. The lice eat salmon, killing them or stunting their growth, and can cause multi-million-pound losses for fish farmers.
But salmon excrete the chemical, which then pollutes the seabed around fish farms at levels than can be lethal to marine wildlife. Shellfish are poisoned and prevented from making new shells to protect them as they grow.
Now the Scottish Environment Protection Agency (Sepa) says it has persuaded the company that markets the drug to withdraw it. Campaigners, however, warn that replacement drugs could do just as much harm, and are demanding a much broader legal ban.
The latest research on teflubenzuron was conducted by scientists from the Institute of Marine Research and the National Institute of Nutrition and Seafood Research in Bergen, Norway. They detected high concentrations in sediment around a salmon farm on a fjord north of the city.