However there are limitations to acoustics. One is that you don't know which animals are actually in the water column reflecting the sound back to you. Also as important, is the fact that different animals scatter sound with extremely different efficiencies. Sound may be scattered by other processes in the ocean though, sediments, air bubbles and even changes in the temperature, salinity and density of the water itself will reflect sound energy. This talk presents one way of analyzing multiple frequency acoustic data to infer whether or not biology is the source of scattering.
Internal waves are frequent oceanographic features seen throughout the worlds oceans.
Their formation and importance are best left for someone else to discuss who is more knowledgeable. However we can say that they can be seen quite easily by acoustics. Luckily Dr. Tim Stanton and others have developed theoretical models for not only biological scatterers but also for acoustic scattering from turbulence. The figure to the right is taken from a paper in 1994 by Stanton, et al. which discusses how scattering from these two sources differs. I then measured the scattering level along the upper and lower layers of the internal wave and discovered that the top layer was consistent with a negative slope, while the bottom layer had a positive slope.