Abstract
Scattering layers beyond the Continental Shelf, in deep water, were often observed during cruises with the charter vessel of the Regional Fisheries Survey in West Africa. The layers were located at depths of 100 to 350 m during daytime; at night, they generally ascended to the surface. They consisted mainly of myctophids and small squids. In this report, results of first observations on deep-scattering layers of Sardinella aurita (Cuv. et Val.) are given. The layers occurred off Mauritania during the months of hydrological transition, characterized by low water-temperatures. In these months, S. aurita were rarely caught over the shelf. Apparently, they migrate north and south in deep water, and were not detected by commercial vessels. The environmental conditions and the difficulties for the industrial fishery during this transitional period are briefly described. the temporal and spatial occurrence of deep-scattering S. aurita is given, and diurnal vertical migrations are shown, together with examples of echo records. Results of fishing operations made in the layer, and some biological observations on S. aurita are presented.
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