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This investigation of the bottom fauna of St Austell and Mevagissey Bays was prompted by the feeling among local fishermen that the increasing amount of china-clay waste being deposited in the bays was adversely affecting the local fishery. Physical pollution on a large scale can harm a fishery in a number of ways. The smothering of shellfish grounds is often to be expected, and where the pollutant contains a high proportion of very fine particles there is the danger that unstable clay banks will build up and constitute a purely mechanical hazard to trawling and potting. In this paper, however, we are concerned only with the effects of clay-waste pollution upon the bottom fauna, since it had been suggested that clay deposition might be impairing St Austell and Mevagissey Bays as feeding grounds for demersal fish.Previous work on the biological effects of china-clay waste includes that of Miss N. Sproston (1945, unpublished), who examined the effects of suspended clay upon some species of pelagic fish. This worker was unable to demonstrate any recognizable effect of mica particles (present in clay waste) upon the gill tissues of fish taken from polluted water at Pentewan. However, Herbert et al. (1961) recorded cases of gill damage in Salmo trutta from china-clay-polluted reaches of the River Fal. These workers also showed that ‘the bottom fauna in the control streams was on average 3.3 times greater than in the polluted part of the Fal and 19 times greater than in the polluted part of the Par’.

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