First noticed during the latter stages of the Second World War when hydro-graphic survey ships equipped with echo sounders showed a ‘phantom seabed’ appearing on the graphs, the Deep Scattering Layer (DSL) often appeared many hundreds of fathoms above that of the true seafloor that could clearly be seen on the graph.
It was noticed that this layer moved vertically according to the time of day. At night it would rise to the surface while, during the hours of daylight, it would descend to a depth that rarely exceeded 2,000 fathoms.
Although it had long been known that that certain fishes migrated vertically in order to feed on the plankton and other life that was more abundant in the surface waters, this Deep Scattering Layer (DSL) – appearing on the echo-sounder graphs as a level diffuse band – lacked the sharp, broken outlines generally associated with shoals of fish.
In the hope of discovering what it was that reflected, scattered, or absorbed sound rays, plankton nets were lowered down but failed to detect anything. Underwater cameras lowered in a similar manner took many thousand of photographs which, when analysed, showed nothing other than small specks of light, the occasional squid, and little else.
In later years, with the use of more sophisticated sonar equipment, it was discovered that instead of one single, thick layer, the Deep Scattering Layer actually consisted of two or three separate bands, one beneath the other.
With the advent of scientific submersibles built to withstand enormous ocean depths, visual observations of the DSL became more frequent, these sightings confirming that the DSL was biological in content, the top layer consisting of a species of shrimp called Sergestid, which lives on the planktonic plants that exist at the very bottom of the ocean food chain; the second layer made up of another, larger, shrimp, the Euphasid – more popularly known as ‘krill’ – that forms the staple diet of whales, beneath which was a third layer, layer of Lantern Fish, that not only feeds on plankton but also its two upstairs neighbours in the DSL.
Although not regarded as being part of the DSL, the rich feeding opportunities that it provides also attract large numbers of squid.
While these creatures spend the daylight hours in deep water, as night approaches they begin their ascent towards the surface to feed on the plankton that constitutes their diet. At the approach of morning they begin their descent, the lantern fish leaving first, followed by the euphasids, with the sergestids leaving last of all.
(Although the impression given of the DSL is that of a densely populated band, it’s worth remembering that in an area as vast as the ocean even the presence of ten or so of these small creatures in a cubic metre of water is considered a crowd!)
Exactly why these creatures perform this daily vertical migration remains a mystery. One suggestion is that they may be sensitive to light. A theory supported by the fact that while the DSL is common to all of the oceans of the world, in polar waters, where there is continual daylight during the summer months, the biological layer becomes more erratic until it finally disappears. (It’s sometimes argued that shining a light from a vessel’s side will draw lantern fish to it, but this is offset by the fact that when light reaches a certain threshold it repels rather than attracts the fish.)
Another theory is that they move to the deeper, darker waters during the hours of daylight to avoid the more numerous predators that inhabit the upper layers. But since all three of these creatures are noted for their luminescence and brilliant surface displays it would seem that they are not afraid of attracting attention to themselves.
This article first appeared in Sport Diving Magazine in 2008 as part of ‘Things that flash in the night’, a story on bioluminescence taken from a paper that I wrote in 1969.