Absorption losses at the resonance frequencies of sardines up to about 18 dB at 1.3 kHz at night, 15 dB at 1.7 kHz during the day and 35 dB at 2.7 kHz at dawn were observed at a range of 12 km during Modal Lion, a multidisciplinary experiment designed to isolate absorptivity due to fish from other effects on long‐range propagation at a relatively shallow (83 m) site in the Gulf of Lion. Comparison of transmission loss measurements with a numerical sound propagation model that incorporates absorption layers in the water column permitted estimation of the average absorption coefficient, depth, and thickness of absorption layers. The depths and thickness of layers estimated from sound propagation measurements were in good agreement with echo sounder data. The measured resonance frequencies of dispersed fish at night were within 13% of theoretical computations, based on swim bladder dimensions which were derived from near coincident samples of adult (∼16 cm long) sardines. A smaller absorption line at 3.9 kHz at night is consistent with the resonance frequency of dispersed juvenile sardines, which are known to be ∼6 cm long, based on historical data. Measured resonance frequencies of sardines in schools were about 0.59 times the resonance frequency of dispersed sardines. The observed frequency shift is analogous to previously observed frequency shifts associated with ‘‘clouds’’ of bubbles. The results presented here suggest the possibility of long‐term tomographic mapping of fish parameters over large areas using transmission loss measurements. [This work was supported by The Office of Naval Research.]