Measurements of the absorption coefficient were made at 12 km between 0.6 and 5.0 kHz at a shallow water (83-m) site in the Gulf of Lion in September, 1995. At night absorption lines due to dispersed sardines at 1.3 kHz at 25 m and sardine schools at 1.5 kHz at 60 m were evident. One hour before sunrise the resonance frequency, f, of dispersed sardines increased with time, becoming 2.7 kHz at sunrise, as dispersed sardines descended to 65 m; and in the following 15 minutes f decreased to 1.7 kHz, as dispersed sardines formed schools, as evidenced by echo sounder records. The signal loss at 2.7 kHz at sunrise, attributed to dispersed sardines at 65 m, was 35 dB; the signal loss at 1.7 kHz during daytime, attributed to sardine schools at 65 m, was 15 dB. The shift in f and the decrease in absorptivity that occurred during the transition are in accord with theory: Close proximity between fish in school at 65 m causes both f and the scattering cross section to diminish (Feuillade, 1996). The latter is proportional to the extinction cross section which, together with n (number per unit volume), control the absorptivity. Estimation of n from absorption spectroscopy measurements, and the hypothesis that marine mammals might exploit this phenomenon, will be discussed.