I. G. Pola collaborated in laboratory work. We are grateful to many individuals and institutes who kindly provided samples for this work. Samples in the U.K. and Ireland were collected by the Environment Agency, Rivers and Fisheries Associations of Scotland and Central Fisheries Board of Ireland. J. de la Hoz (Spain) provided Spanish samples. G.M.S. was supported by a grant from the Agencia Española de Cooperación Internacional (AECI, Spain). This study was financed by the European INTERREG projects ASAP II (Project No: 203) and A.A.R.C., and the Spanish National Project CGL2009-08279.