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Trials of acoustic deterrent devices (ADD) were carried out on the River North Esk and River Conon in Scotland to test the effectiveness of deterring seals from a specific area of the river and as a barrier to upstream movement of the seals. The ADD was switched on and off alternatively for periods of several days and surveys were carried out to estimate the number of seals present within each river. The ADD had no significant effect on the abundance of seals in the survey area, but it did reduce seal movement upstream significantly. The reduction in movement was constant over the four month period of the trial.