THE RELATION BETWEEN THE SCATTERING LAYER AND THE EUPHAUSIACEA

Abstract

A layer which reflects sound, and which lies at a depth of 200-300 fathoms in the daytime, is of worldwide distr. It migrates at night to near the surface. This diurnal migration indicates that the layer is composed of animals. Of the various possibilities, Euphausid shrimps appear the most probable. In certain areas a correlation has been shown between the strength of echo and the abundance of Euphausids, and also between the depth of the layer and the dominance of deep- or shallow-living shrimps. A deeper and a shallower layer, the latter with a reversed diurnal migration, are briefly described. A theory has been developed relating geogr. distr. of euphausids to two environmental factors[long dash]surface temp. and a combination of deep temp. and illumination. The difference in the environments of shallow-living and these deep-living, diurnally-migrating forms is discussed. In an appendix consideration is given to the estimation of illumination at these depths, and to the effects of such factors as cloud, sea-state, etc., on the estimates.