Experiments on herring (Clupea harengus L.), sprat (Sprattus sprattus (L.)) and whiting (Merlangius merlangus (L.)) showed that when these fish make rapid swimming movements, such movements are preceded by fast pressure pulses in the surrounding sea water. The first (alpha) phases of these pulses had durations of from 1.5 to 3.5 ms. The pulses could be excited in free-swimming fish by both visual and auditory stimuli and the latencies to the latter ranged from 5 to 8.5 ms. Identical pulses could be elicited by giving electrical stimuli to anaesthetized fish; these pulses had latencies from 3.4 to 7 ms. The pressure fields around the fish were measured on suspended anaesthetized fish stimulated electrically. At any instant the fields of the fast pulses produced by whiting had the same polarity at all positions round the fish and pressure decayed inversely with the 1.5 power of distance. The alpha phase of the fast pulse was usually a decompression. The fields around a stimulated herring were different. The pressures on both sides opposite the centre of the fish were of one polarity while those around the head and the tail were of the opposite polarity, the pattern of pressure being symmetrical about the long axis of the fish. In our experiments the alpha phase opposite the centre of the fish was always a compression. The amplitudes of these pulses declined with distance by the power of 2.5. In all species in our experiments the fast pulses were followed by slower pulses associated with swimming movements; these slower pulses had opposite polarities at corresponding points on the two sides of the fish. The possible importance of the pressure pulses, particularly the fast pulses, in communication between fish is discussed. The sensitivities at various frequencies of receptors of pressure, pressure gradient and net pressure gradient between fish and water are reviewed in relation to the pressures, pressure gradients and net gradients found in the pressure fields of the pulses. It is concluded that the fast pulses could be clearly detected by another fish at several fish lengths from the source fish. Furthermore enough information to determine an approximate position of the source would be received by the receptors of a neighbouring fish though there is no evidence at present that this information is utilized. Different information is transmitted by the slower pulses and also by visual signals and these are discussed.
