Norway’s Laerdal river has just been deliberately poisoned with rotonone in order to control a deadly parasite, Gyrodactylus salaris. The tiny worm was infecting scores of Atlantic salmon, but while rotonone destroys the parasite, it also kills most fish and many other acquatic species. The treatment has been controversial since its introduction more than 20 years ago and its use in the river has brought opposition to a head. Vera Frankel has our report.