The ability of cod to detect intense 38 kHz pulses was studied in an anechoic tank, using a classical cardiac conditioning method. All 15 cod studied reacted to 3 ms pulses. The average threshold was 194.4 dB re 1 muPa (s.d. 5.4 dB, s.e.m. 0.3 dB). The data do not reveal the sensory modality involved. A number of additional experiments indicate that the ultrasonic energy per se is the effective stimulus. The threshold is too high for most sources of marine ultrasound to be detected, but is low enough to allow the cod to detect echolocating odontocetes at a range of 10–30 m. The results also indicate that cod can detect echo-sounders of the fishing fleet at similar ranges.

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