With the renewal of activities in Fishery Research, which took place in the lands bordering the North Sea after the War, fresh lines of investigation were introduced. Amongst others, quantitative surveys of the bottom fauna similar to those carried out by Petersen were undertaken by several countries. The southern part of the North Sea was examined by the Ministry of Agriculture and Fisheries and the results were published by Davis (2, 3). Other work undertaken in the German Bight was published by Hagmeier (6). A small survey was carried out by Petersen (8) in the North Sea off the north-west coast of Denmark. Areas to the north and west of Scotland, as well as the northern half of the North Sea, were surveyed by the Fishery Board for Scotland and a preliminary account published (11). The present paper is, in part, a summary of the results of the survey work carried out by the Fishery Board for Scotland in the northern part of the North Sea during the years 1922-25, supplemented by a subsequent survey of the intertidal areas undertaken privately; in part an analysis of the faunistic divisions of the North Sea based on the distribution of the molluscs, especially lamellibranchs, as recorded in the above papers. So far as the North Sea is concerned, the area covered by the Scottish investigations is bounded on the south approximately by latitude 56° N., and on the east and north by the 200 m. (100 fm.) contour.
In a previous paper (4) the quantitative distribution of the molluscs and polychætes in certain intertidal muds and sands on the Scottish Coast was discussed, but there still remained several other types of shore such as (a) a river mud-flat, (b) grounds at the heads of sea lochs on the West Coast, and (c) the more exposed shores of the Western Islands, where conditions might possibly differ greatly from those already considered, with corresponding changes in the fauna. Since the publication of the first paper quantitative samples have been collected in each of these areas. Taking the survey as a whole, collections have now been made on such widely different parts of the coast that the peculiarities of the distribution of the above two groups can be stated with a fair degree of accuracy. The present paper is divided into two sections, the first dealing with the additional observations, the second with the survey as a whole.
In 1926 a quantitative examination of the fauna of the intertidal sands in the Cumbrae, Firth of Clyde, was begun, and the results proved so interesting that it was decided to extend the survey to other parts of the Scottish coast, the object being to study the variation in density and composition of the fauna, with special reference to the two groups of molluscs and polychætes, from place to place, and at various levels on the beach. During the past two years observations have been made in three widely separated areas, namely, on the Ayrshire coast and Firth of Clyde, on the southern shore of the Moray Firth, and on the east coast of Scotland and in the Firth of Forth. In all, twenty-five bays or stretches of coast have been visited, and over one hundred and fifty quantitative collections worked through.
The first part (5) of a study of the quantitative distribution of the bottom-fauna in Scottish waters, and of the natural faunistic divisions of the North Sea and adjacent areas, dealt with the distribution of the molluscs in the North Sea. In this paper the echinoderms from the same area are treated on similar lines. In addition the natural faunistic divisions, as shown by the distribution of the dominant species of molluscs and echinoderms, and the validity of Petersen's community concept as applied to the North Sea, are discussed.Reference should be made to the earlier paper for information concerning the methods of collecting, the area covered by the Scottish surveys, similar surveys carried out by other workers in the remaining parts of the North Sea, the number of hauls in each square, and other introductory matters.
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