90 SECTIONAL ADDRESSES 



This change in the constitution of the herring shoals was not immediately 

 reflected in the size of the catches, which for some years were maintained 

 at a good level by the considerable stocks of older fish. But as these 

 passed out they were not replaced by any adequate numbers of the younger 

 year classes and in recent years the fishery has been profoundly affected. 

 Formerly the number of Lowestoft drifters which visited Plymouth for 

 the herring season rarely fell below 75 and was sometimes well over 100 ; 

 during the past season only one came. And in similar fashion the weight 

 of fish landed has fallen from a figure which sometimes reached 80,000 cwt. 

 to one of under 30 cwt. 



It is interesting and perhaps significant to note that as Mr. G. P. Farran 

 has shown ^ the stock of herring on the north coast of Donegal has shown 

 a pronounced decline in recent years. The decline began in 1930, some 

 eighteen months before the change in the constitution of the Plymouth 

 shoals was first seen, and the industry based on this fishery has suffered 

 greatly. Conditions at Plymouth and on the Donegal coast are not 

 identical, for the successful spawning seasons in the latter area were 1920, 

 1924 and 1925, whereas at Plymouth they were in 1920, 1923 and 1925. 

 The annual fluctuations have thus not operated in exactly the same way. 

 Mr. Farran tells me, however, that the shortage of herring in recent years 

 has been accompanied, just as at Plymouth, by a great reduction in the 

 numbers of the earlier year classes, and it is thus possible that the same 

 long period fluctuation is affecting both areas. 



Since 1931, when the depression in the Plymouth area began, there 

 has been a marked change in the amount of phosphate in the offshore 

 waters. Records made by Dr. W. R. G. Atkins and Dr. L. H. N. Cooper 

 show that the phosphate is at its maximum in the winter, in December 

 and January, and since the phytoplankton crop is limited by the amount 

 of phosphate in the water, the winter records give a good indication of 

 the quantity of food which will be available for fish larvae. The records 

 show a heavy decrease in phosphate beginning in 1931, and, as seen in 

 Table II, there is an evident relation between the amount of phosphate 

 and the abundance six months later of the larvae of summer spawning 

 fish. If the average phosphate values for the two four-year periods 

 1924-27 and 1934-37 are compared we find that the decrease has been 

 about 35 per cent. The fact that the larvae of summer spawning fish were 

 the first to feel the adverse conditions, and that those of the spring spawn- 

 ing fish were not seriously affected until 1935, can in theory at least be 

 explained in terms of nutrient salts ; a reduced crop of phytoplankton 

 will mean a smaller supply of zooplankton, and this will mostly be con- 

 sumed by the spring larvae, leaving little or none for those that come later 

 in the year. 



The herring on which the Plymouth fishery depends are mature fish 

 running up Channel to their breeding places on the Cornwall and Devon 

 coasts. On this migration they are not feeding and, presumably, they 

 are unaffected by plankton conditions. It is possible that the disastrous 

 change which has occurred is due to a long series of unproductive spawning ' 

 seasons caused by the abnormal conditions of the Channel water and lack 



' G. P. Farran, ' The Herring Fisheries off the north coast of Donegal,' Journ. 

 Dept. Agriculture for Ireland, XXXIV, no. 2 (1937). 



