BIOLOGICAL OCEANOGRAPHY 



At first light scattering layers begin descendiyig to various depths at which they remain througkoui the day. 



At night layers ascend and merge near the surface. 



THE DEEP SCATTERING LAYERS 



Another biological phenomenon which has gained increasing 

 importance to both mariner and oceanographer during the last two 

 decades is the sea's "deep scattering layers (D.S.L.s)". Briefly, these 

 are the horizontal sound reflecting bands that exist at various depths 

 over broad reaches of the world's oceans. Mariners today are 

 probably quite familiar with the physical aspects of the D.S.L.s as 

 they often produce "false bottoms" on the recording traces of echo- 

 sounding devices. Indeed, it has been widely conjectured that 

 misinterpreted "echograms" may have led to the charting of non- 

 existent shoals. Today, determination of the exact composition, 

 behavior, and distribution of the layers is a continuing oceanographic 

 project, for much more information is needed regarding their possible 

 influence on sonar systems. 



Discovered by accident during World War II, the "layers" have 

 a relatively brief but interesting history. A group of physicists experi- 

 menting with sonic submarine detection gear consistently, and annoy- 

 ingly, recorded echoes from a uniform layer some distance above the 

 sea's floor. During daylight hours an exceptionally well-defined 

 layer was frequently observed at 150 to 175 fathoms. At night it 

 disappeared. The feature could only be attributed to a suspended 

 stratum of some sound reflecting, or scattering, agent, and thus 

 derived the name "deep scattering layer". The name was pluralized 

 after subsequent experiments revealed the existence of multiple 

 layers. 



Soon after initial discovery, a causative solution was sought in 

 some physical property of the sea capable of producing the sonic 

 reflection. To this end, attempts were made to correlate the phenom- 

 enon with density discontinuities, or abrupt temperature differences, 

 in the sea. However, workers were unable to suggest any physical 

 effect that would account for the layers' characteristic of ascending 

 to the surface at sunset and descending to depth at sunrise. 



Observing the close parallel between this characteristic and the 

 diurnal vertical migrations of certain marine animals, biologists 

 postulated that the scattering layers were of biological origin. This 

 theory has endured the test of time and is universally accepted today. 



On the basis of this correlation and later field experiments it was 

 concluded that huge aggregations of tiny planktonic animals were 

 reflecting some of the sonic impulses, or "pings", from sound appa- 

 ratus. The animals, it was suggested, rose to the surface at nightfall 

 to feed upon the abundant phytoplankton. At daybreak they would 

 again seek the darkness of depth, either through the fear of predatation 

 or their natural sensitivity to light. Further investigations have shown 

 that during the day the layers remain at depths roughly between 



700 and 2,400 feet. At night they rise almost to the surface and 

 diffuse, or they may merge into a broad band as much as 500 feet 

 thick. Most places in the deep ocean usually have three layers, the 

 deepest at an average of 1,900 feet. Sometimes, sounding traces show 

 very diffuse layers that stay at the same depth day and night. 



Tiny fish and crustaceans found in scattering layer. Skoion at approximately 

 actual size are two forms of lantern fish, a euphausiid, and a sergestid. 



From physical evidence as a result of sampling and bathyscaph 

 observations, many researchers are of the opinion that shrimp-like 

 crustaceans called euphausiids and sergestids are the reflecting agents 

 in the layers. Others, however, argue that crustaceans rarely occur 

 in sufficient densities to produce reflection layers. In rebuttal, the 

 the latter workers suggest that fish and similar animals with swim 

 bladders, or gas filled bubbles, are the causative agents. Quantities 

 of mydophid or lantern fish, 2- to 3-inch predators which feed upon 



20 



