SECT. 4] SOUND SCATTERING BY MARINE ORGANISMS 537 



F. Sound-Scatterers as Markers of Water Boundaries 



Aggregations of scatterers often accumulate at the boundaries between 

 waters of different properties. These aggregations may occur at such places for 

 a number of reasons. Since the i^hysical properties of the water are changing 

 rapidly with depth at such levels, the probability that an organism may find 

 an optimum set of conditions here is somewhat increased. Furthermore, an 

 animal may be '^trapped" at such a level, forced downward by its negative 

 response to light, let us say, but limited in its downward movement by a 

 sudden decrease in temperature. Finally, animals may simply come to rest in 

 such places at the very level at which they find themselves neutrally buoyant. 

 In any event aggregations occur at such points and they may be used to map 

 the boundaries which they mark. Some success has been had, for instance, in 

 relating the distribution of sound-scatterers to the distribution of Atlantic and 

 Mediterranean water-masses in the Strait of Gibraltar (Frassetto, Backus 

 and Hays, in press), and Weston (1958) has described certain properties of the 

 summer thermocline in the North Sea from echo-sounder records of scatterers 

 accumulating there. Such a technique should be useful in studying the properties 

 of internal waves. From Fig. 4a there is a suggestion that a wave is travelling 

 in the region of the temperature inversion shown in Fig. 4b. 



References 



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