Although the nature of the sound scatterers was not 

 determined with certainty, their reaction to temperature 

 oscillation was observed. In most cases the scatterers 

 were found to lie near the level of a strong temperature 

 gradient. As the internal temperature waves passed the 

 tower they were continuously followed with the isotherm 

 followers and, at the same time, the sound scatterers were 

 tracked by the echo-sounder (fig. 16). The depth of maxi- 

 mum acoustic scattering underwent vertical oscillation, 

 with periods of from 5 to 10 minutes, in coincidence with 

 the internal waves. It was apparent that internal waves were 

 responsible for short-period changes in the level of maximum 

 sound scatterers. 



i ^P Hi ^il ^ f ^^ ^p^^^iiiiP i i ii il^ i P ^ ^^p Wi ^ i 



I I — 



Figure 16. The NK-7 echogram (A) compared to the 

 depth of an isotherm in the thermocline (B). 



With an upward-directed echo-sounder, it was also 

 found that a stronger scattering layer appeared in the early 

 evening and disappeared again by morning. Figure 17 shows 

 a comparison of an echogram made during the day without 

 noticeable scatterers (A), and one made in the evening with 

 scatterers throughout the water column (B). It was found 

 that the acoustic scatterers (with the exception of fish) also 



21 



