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ON THE FAILURE OF THE PLYMOUTH

In the winter of 1930-31 it was noticed that the winter maximum of phosphorus at the International Station E I was less than the average of the previous 10 years. The change which has persisted ever since was associated with certain biological events (Kemp, 1938) of which the most important was the failure of the Plymouth herring fishery between 1931 and 1935, due to a failure of recruitment. Other changes included the replacement of Sagitta elegans by S. setosa; this change of a conspicuous and long-lived plankton indicator species was accompanied by changes of other indicators, implying a change of communities. At the same time a reduction was found in the quantity of macroplankton, as sampled by the wide-meshed ring-trawl. Certain species of medusae were no longer caught and the numbers of late summer medusae were reduced by four times (Russell, 1938b). A hint of a further change can be found in Russell (1939b) where he noticed that 'the western end of the Channel has been characterized by a great number of pilchard eggs and a general poverty of plankton indicators, including both' species of Sagitta'. From re-examination of the evidence available, including some new material hitherto unpublished, it is clear that a significant increase in pilchard occurred. The evidence for this increase is found in the regular ring-trawl catches taken off Plymouth, in marked contrast to the decline in the Plymouth herring fishery, which had no detectable effect on the numbers of young clupeids caught during the winter. The relative importance of the increase in pilchard leads to the hypothesis that the Plymouth herring fishery failed because the stock became reduced in competition with the pilchard population of the Western Channel, and that the change in the winter maximum of phosphorus was the effect of an increase in the number of pilchards.