The bottom fauna of the English Channel. Part II.
Holme, N.A. (1966) The bottom fauna of the English Channel. Part II. Journal of the Marine Biological Association of the United Kingdom, 46 (2). pp. 401-493.
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An earlier paper (Holme, 1961) described the bottom fauna off the south coast of England, as sampled by a modified anchor-dredge. Sampling was later extended to cover the whole English Channel, a further 144 stations being worked, making 311 in all. This paper describes the results of the later survey which, as previously, was concerned mainly with molluscs and echinoderms. Distribution charts for the majority of species identified are given, in which are incorporated the results of both surveys. Environmental conditions in the Channel are briefly summarized. Much of the bottom in central and southern parts of the Channel is exposed to strong tidal streams and wave disturbance so that the bottom is covered by coarse sediments of gravel, stones or boulders. Consequently species inhabiting sands or finer sediments are virtually absent from the samples. ' Distribution patterns have been classified into categories which differ only in detail from those already described. Some species previously thought to be limited to the western half of the Channel were found to penetrate far into the eastern Channel off the French coast. Conversely the limits of true West Channelspecies were found to lie to the west of the Channel Island region, because around the islands vertical mixing of water probably occurs throughout the year, as in the eastern half of the Channel. The depth distribution of species with a limited vertical range is discussed. The occurrence of suitable sediments at different depths is only one factor causing apparent depth zonation, which in certain species seems to be related to other factors directly connected with depth. The animal associations which occur are classified following the system described by N. S. Jones, and a chart is given (Fig. 35) showing the distribution of different associations in the Channel. This emphasizes the widespread occurrence of gravel associations over much of the area. The nature of the different faunistic boundaries is discussed, with particular reference to temperature and vertical mixing. Although the anchor-dredge samples are not strictly quantitative, comparisons have been made of the numbers of species and individuals per haul. Although the western parts of the Channel have more species, the number of individuals per species in the eastern Channel is higher, so that the overall population density may be equally high. Changes in the density of the benthos which have occurred since the 1920'S are discussed in the light of changes in plankton, fisheries and hydrography. The standing crop is very much less than it was in the 1920'S, and the rate of production of fish food is probably even more reduced because of the scarcity of young individuals suitable as food for fish. In the last section the main trends in distribution are summarized and the changes which may have occurred during the last forty years are shown diagrammatically (Fig. 39).
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