Shrimp-like, phototropic mysids were photographed swarming at night. Since 

 they are not present in the water during the day, they are suspected of con- 

 tributing to the observed acoustic nighttime scattering. 



C 60 



TIME (MIN) 



Echogram compared with a recording of an Isotherm movement, shows that the 

 maximum biological acoustic scattering moves vertically with the thermocline. The 

 scatterers undergo 5- to 10-minute periods of oscillation corresponding with the 

 movement of internal waves. 



53 



