Hatfield Marine Sci. Ctr., Oregon State Univ., Newport, OR 97365
To control the distribution of seals around salmon hatcheries, pond tests were conducted using swept frequencies between 2 and 20 kHz. This did not affect salmonid movements or reproduction. Aperiodic 12- or 17-kHz pulses of varying duration were effective at levels of 187 dB re: 1 (mu)Pa in reducing seal abundance near several hatcheries and pen aquaculture facilities. A few larger (possibly older) seals habituated or were less sensitive, and foraged with modified techniques. For sea lions, the same system produced a dramatic initial startle response but was otherwise totally ineffective. Many marine mammals react to moving sound sources even if loud stationary sources are tolerated. Early in this experimentation, swept frequencies were eliminated for simplicity. However, the illusion of motion as simulated by Doppler-like sweeps may have been lost in the process. Operant conditioning research suggests that an aversive stimulus is best maintained when used aperiodically. If several stimuli have a deterrent effect, this effect can be sustained longer by intermixing the stimuli to avoid or delay habituation. These principles may also be applicable to intentional harassment and industrial effects.