Some observations on the relative abundance of biological sound scatters in the North-eastern Atlantic Ocean, with particular reference to apparent fish shoals

Abstract

In various months of the years between 1960 and 1972, the R.R.S. “Discovery II” and R.R.S. “Discovery” carried out a number of echo-surveys in the North-eastern Atlantic Ocean between Latitudes 12° and 62°N, and from the European and African coasts to a longitude of approximately 29°W. The prime objective was to acquire data on sea-floor topography. In addition, numerous echo-traces of biological scattering present in midwater between depths of approximately 35 and 460 m were obtained. The traces were of variable quality, but were considered suitable for a preliminary evaluation of the scattering mainly as a basis for further investigation. The scattering recorded in daytime was broadly divisible into three types:

• (1) Diffuse layers, which were probably due to dispersed single fish. These were not analysed further.

• (2) Very small discrete echoes, mostly derived from single fish and very small shoals. These were mapped to show geographical regions of greater abundance.

• (3) Larger discrete echoes, most of which were almost certainly given by fish shoals. These were analysed in detail and information obtained on depth distribution, shoal density, geographical distribution and relative abundance. Small and moderate-sized shoals were found to be very numerous in spring and summer in oceanic water to the West and North-west of the British Isles. Large numbers of shoals of various sizes were also observed at various time of the year in the Bay of Biscay and near certain parts of the European and African Continental Shelf, near certain seamounts, and near islands such as the Azores, Madeira, and the Cape Verde Islands. In general, shoals appeared to be relatively sparse in the more southerly temperate and subtropical regions of the open ocean. The depth distribution of shoals in open water varied greatly but, on average, the maximum daytime depth of those in the more southerly area of the survey was slightly greater than that of those in the more northerly area. Shoal size also varied, shoals to the north of Latitude 50°N were on average slightly smaller than those detected in southerly regins. Possible identification of the fish is discussed.