Changes in volume reverberation from deep to shallow water in the eastern Gulf of Mexico.

Scattering from fish is a primary cause of volume reverberation and, since fish populations change from deep to shallow water, the character of volume reverberation should also change. However, there are few data available to document expected changes. Therefore, an experiment was conducted in the eastern Gulf of Mexico to investigate possible changes in volume reverberation from deep to slope to shelf waters. Results showed that volume reverberation in outer shelf waters varied more rapidly with respect to both time and space than that in deeper waters. Day-time scattering was similar for deep, slope and shelf waters, total scattering strengths generally increased with frequency. Night-time scattering for the deep ocean and slope also increased with increasing frequency. Scattering modeling suggests that swimbladder-bearing fishes smaller than 10 cm were responsible for the observed volume reverberation. Night-time scattering at the outer shelf location was very different, with strong scattering peaks at low frequencies. Scattering modeling implicates 12-15 cm rough scad and round herring as potential causes of the low frequency peaks. Hence, experiment results confirmed that, as expected, volume reverberation over shelf waters is different and more variable than in deeper waters of the eastern Gulf of Mexico.