504 



HERSEY AND BACKUS 



[chap. 13 



level due to the activity of bioluminescent organisms (Clarke and Wertheim, 

 1956 ; Clarke and Hubbard, 1959). These layers may vary considerably in their 

 vertical extent but rarely are less than 50 m thick and may be as much as 200 m 

 thick. They are reasonably uniform in their main features over long reaches of 

 the ocean and change slowly with the seasons. (The only seasonal study so far 

 reported is that of Barham (1957) for Monterey Bay, California, where, Barham 

 suggests, a more complex situation is found than is likely for the high seas.) 



, *^ \ ■ ^^ ^~^ - -10 \t 



(19' 8N, ee'isw) wiv-if » \ 4 t ^->.>~, 



26 FEB, 1954 



SUNSET 18J0 



BELOW TRANSOOCen 



i 



4iiK^"gteag i fei?.«tei^tea^i^.iiffiip . ^^ 



y^ 



0' 



10 



:-2o 



30 S 



1649 



+ 4 (OUEEN) TIME 



Fig. 5. A 12 kc/s echo-sounder record made by lowering the echo-sounder transducer to a 

 point midway between the surface and a deep scattering layer and holding it in this 

 position as the layer migrated up past the transducer (see text). The increase and then 

 decrease with time of individual scatterers may be noted as well as their upward 

 progress. The sinuous shape of the echo sequences is due to the rolling of the ship and 

 the consequent rise and fall of the transducer. (After Johnson, Backus, Hersey and 

 Owen, 1956.) 



Generally these layers make a pronounced diurnal vertical migration but some 

 do not. There appears to be no correlation between midday depth and con- 

 spicuous features of the temperature-depth profile although the level to which 

 these layers rise at night may be limited by abrupt increases in temperature. 

 Although the typical deep scattering layer appears on the record of a surface 

 echo-sounder as a stripe of numerous, diffuse, jumbled echoes, the scatterers 

 can be individually resolved when the range to the layer is made short by 



