Detection and reaction of fish to infrasound

Hearing in fish has until a few years ago been considered to cover a sound frequency range from 20-30 Hz up to below 1000 Hz in most species. It is now known that the frequency spectrum extends down in the infrasound range (below 20 Hz). For cod, plaice, perch, roach, and salmon, hearing thresholds have been established by a conditioning technique for sound frequencies down to below 1 Hz. Tests on the behavioural reaction to sound in juvenile salmon have been performed in a large tank. Infrasound (10 Hz) produced spontaneous avoidance responses, while no such responses could be seen at 150 Hz. Tests on down-river migrating salmon smolts have also been performed. During a stimulation period of 170 min, only six fish passed the operating 10 Hz sound source, whereas 338 fish passed during a silent period of the same duration. The 150 Hz stimulation had no evident effect on the migration.