Early studies of these diel vertical movements of plankton and other creatures were mainly confined to populations of shallow water, but during World War II it became apparent that this phenomenon is of very wide occurrence throughout the deep oceans. Physicists investigating the use of underwater echoes for the location of submarines obtained records during daylight hours of a sound-reflecting layer in the deep water beyond the continental shelf (Dietz, 1962; Farquhar, 1977). On echogram tracings this layer gave the appearance of a false bottom at about 300 m where the sea floor was known to be far below this. Towards the end of the day this sound-reflecting layer was observed to rise until close to the surface. During darkness it was less distinct, but at the dawn twilight the layer formed again near the surface, and descended gradually to its usual daylight level as the sun rose. Further detailed investigations revealed that there are sometimes several of these mid-water sound-reflecting layers at depths between 250 and 1000 m and they are now usually termed sonic scattering layers (SSLs), or deep-scattering layers (DSLs).
It was first thought that the SSLs were due to discontinuities between layers of water differing in some physical property, for example temperature, but this would not account for such marked diel changes of position. It is now generally accepted that the tracings are caused mainly by echoes returned from marine creatures which change their depth between daylight and darkness. Where the SSLs comprise several sound-reflecting zones, these are caused by concentrations of different groups of animals at each depth, e.g. euphasiids and fish.
These layers have subsequently been detected in all oceans and vary in distinctness from place to place, being especially faint in the Arctic. They are generally indefinite beneath areas of low fertility, presumably because of the smaller population of these waters. They are usually strongest below warm, productive surface waters. Their detection and distinctness depend to some extent on the frequency of the sonic equipment.
The nature and number of animals existing in the layers are difficult to ascertain due to difficulties of sampling at these depths. Major groups known to contribute to the DSL include the lantern fish (Myctophidae) and other similar mesopelagic fish, various crustaceans such as euphausiids, sergestid and pasaphaeid shrimp, squid and siphonophores. Thus the traces mainly result from larger animals following their plankton prey up and down as well as from the concentrations of plankton themselves. The positions of SSLs on echogram tracings change with alterations of the sonic frequency (Greenlaw, 1979). Evidently the strength of sound reflection is influenced by structural features of organisms, different species giving strongest echoes at different frequencies. Those that contain vacuoles or gas-filled cavities are likely to give specially strong echoes by resonance at particular frequencies determined by the dimensions of the resonating spaces. Where a species includes a wide range of sizes it is likely to reflect sound over a correspondingly broad frequency band.
It is now thought that, at frequencies up to about 35 kHz, echoes come mainly from small fish possessing gas-filled swimbladders. Commonly captured within the SSLs detected at these frequencies are Myctophids (lantern fish), Sterno-ptychids (hatchet fish) and Gonostomatids, all small fish mostly measuring about 4-10 cm in length. Various Crustacea, mainly euphausids and sergestids, are often caught with these fish but probably do not give much echo at these frequencies. Strong echoes at frequencies of 10-12 kHz may also come from gas-filled nectophores of siphonophores such as Nanomia bijuga. Larger fish e.g. herring, cod or redfish, give echoes at 4-6 kHz, and these may account for some of the indistinct SSLs found in the Arctic where mesopelagic fish are not abundant. Sound scattering is also effected by organisms which do not contain resonant cavities. Above 40 kHz reflections may be received from euphausids or pteropods, and above 300 kHz even small copepods may give echoes.
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