Use of underwater playback to reduce the impact of elders on mussel farms

One of the most commonly employed methods of reducing damage by diving duck s to mussel stocks on mussel farms in Atlantic Canada and in Scotland is ch asing birds by boat. While effective in the short term. the frequency of ch ases is often restricted by high costs. both in time and furl. Tests in Sco tland used underwater recordings of chase-boat engines replayed at regular intervals on continuous loop tapes through on underwater loudspeaker in an attempt to reduce predation pressure by elders on mussel farms. Trials of t he underwater playback system (UPS) gave significant reductions in eider nu mbers of 50-80% while a control trial with the playback of an unassociated noise gave no reduction in numbers. The mean return time of birds to the fa rm after chasing by boat also increased significantly. As the presence of w orkers on mussel farms reduces the number of eiders feeding there, the UPS is a useful deterrent when workers are not present. The long-term habituati on of ducks to the system was negligible when workers are absent, providing there is occasionally reinforcement of the deterrent by boat chasing. Pote ntial factors effecting the efficacy or the UPS are discussed. (C) 2001 Int ernational Council for the Exploration of the Sea.