The 1988 RWB Stephens Lecture: Sonar Ichthyology

Sonar ichthyology refers to the interaction of underwater acoustics and fish.described here for audio frequencies and for clupeoid fish in the Bristol Channel.A quantitative review is given covering three sets of phenomena.First we have the long distance attenuation due to dispersed fish, possible because of the high absorption and scattering losses associated with their swim bladders.The main evidence comes from the differences between daytime with aggregated fish and night with dispersed fish.but there are several other manifestations.Second there is an acoustic propagation fluctuation with period about one minute.due to individual fish.Third as our main subject we have sonar records showing fish.sometimes shoaled and sometimes dispersed.out to ranges of many tens of km.Besides the diurnal variations our observations include tidal, Doppler, modal interference and storm effects.There are major differences between pilchard and sprat as regards behaviour and the consequent pictorial records.