88 SECTIONAL ADDRESSES 



Some very valuable information on fluctuations in year classes of fish 

 has recently been collected by the International Council for the Exploration 

 of the Sea.^ The object of the Council was to summarise data on good 

 and bad survival years in some of the principal food fishes, and the reports 

 from the specialists who were appointed to undertake the work are of 

 particular interest. For some fish the available information was found 

 insufficient ; but for cod, haddock, herring and plaice the data are adequate, 

 at least for some areas. The results show that in different parts of the 

 north-east Atlantic there are with rare exceptions no coincidences in 

 good or bad spawning seasons, even if one species only is considered, and 

 the evidence thus is that the fluctuations which are observed are regional 

 in their incidence. 



There is, I believe, good reason to hope that with improved knowledge 

 of the spawning areas and more exact information on the environmental 

 factors during the critical period the causes of these annual fluctuations 

 will in due course be discovered. 



But, of recent years, it has become apparent that in addition to the 

 annual fluctuations there are other over-riding influences at work, which 

 not only affect the abundance of marine animals, but may bring about 

 great changes in their distribution. Since I have been at Plymouth I 

 have been impressed with the very marked changes that have taken place 

 in the western half of the Channel during the past seven or eight years, 

 and the evidence points to the existence of long-period fluctuations which 

 are superposed upon the normal annual fluctuations. 



For the past thirteen years Mr. F. S. Russell ^ has been studying the 

 young fish taken in the plankton at Plymouth and has made regular col- 

 lections by standard methods in the neighbourhood of the Eddystone. 

 His observations thus give a picture of what is happening on the offshore 

 grounds in this area. He finds that from 1931 onwards there has been an 

 alarming decrease in the abundance of larval fish. At first this decrease 

 occurred in the comparatively small number of summer spawning fish ; 

 but it has now extended to the spring spawning fish also (see Table H, 

 p. 91). If we compare the average numbers for the four-year period 

 1934-37, with those for the same period ten years ago, 1924-27, we find 

 that the larvae of summer spawning fish have now been reduced to little 

 more than one-fifth of their former abundance, while the numbers of the 

 young of spring spawning fish have dropped to one-third. It is par- 

 ticularly to be noted that all species of fish are similarly affected, and bear- 

 ing in mind the evidence I have already mentioned on good and bad 

 survival years, this fact alone is sufficient to show that the decrease is not 

 due to a chance coincidence in annual fluctuations. 



This change which has come about in recent years is not shown only 

 in larval fish ; it is unfortunately apparent also in the Plymouth herring 

 fishery, which has declined to such an extent that it is now virtually non- 



^ ' Comparative Studies of the Fluctuations in the Stocks of Fish in the seas 

 of North and West Europe.' Conseil Internal, four I'Explor. de la Mer. Rapp. 

 et Proc.-Verb. des Riunions, CI, part 3, 1936. 



^ F. S. Russell, ' The Seasonal Abundance of the Pelagic Young of Teleostean 

 Fishes in the Plymouth Area,' Parts I-V, Journ. Marine Biol. Assoc. XVI, 

 p. 707 ; XX, p. 147 ; XX, p. 595 ; XXI, p. 679 ; XXII, p. 493 (1930, '35- '36, 

 '37. '38). 



