Measurement and simulation of the effects of absorptivity due to fish on transmission loss in shallow water

Absorption losses at the resonance frequencies of pelagic fish (sardines) up to 15 dB during night and day, and up to 25 dB at sunrise 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 shallow (83 m) site in the Gulf of Lion. The measurements were made at frequencies between 600 and 5000 Hz with a parametric source. To simulate the most significant features in these data, Saclant Centre´s coupled normal mode model was modified to incorporate absorption layers in the water column, and the directionality of the source. Comparison of computations with measurements permits estimation of the absorption coefficient, and the depth and thickness of absorbing layers