Figure 3. The backscattered intensity averaged over the four acoustic beams at 45 m depth (top), 35 m, 25 m, and 15 m (bottom). The strong diurnal signal (as large as 10 db), rises in the evening (~7 pm) and falls in the morning (~3 AM). This 24-hour cycle is not related to the lunar 12.42-hour tidal cycle, but is sun synchronized and related to biological migration. The lower frequency changes in overall backscattering indicates changes in number and size of scatterers that may be related to changes in biomass.