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The propagation and fluctuation of sound have been studied in shallow coastal waters off the British Isles. The environment and the special environmental measurements are described. Acoustic measurements were made for various ranges between about 2 and 137 km, with bottom-laid transducers. Frequencies used were mainly 1, 2 and 3 kHz, most often transmitted continuously but sometimes pulsed. The investigations have extended over several years, and amplitude and phase fluctuations have been found with periods ranging from a year to less than a second. The nine fluctuation mechanisms which have been identified may be summarized as: ( a ) seasonal in amplitude, (seasonal in phase, ( c ) attenuation due to fish which sometimes causes a greatly reduced amplitude at night when the shoals break up, ( d ) storm effects, (e) tidal changes in depth which sweep an interference pattern past the receiver, ( f ) tidal changes in the shear flow or the water structure which also affect the interference pattern, ( g ) phase effects due to tidal changes in the mean streaming velocity, ( h ) fluctuations of a minutes period, some due to fish, ( i ) surface wave effects, which depend critically on the position in the tidal interference cycle. The above nine effects are really all subjects in their own right, and here large advances are described for seven of them. Most of the effects are both new and important, to be measured in many tens of decibels and in hundreds of phase cycles, but perhaps special attention should be drawn to the significance of the work on fish.