40 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



two former coasts I can see no evidence that the increase 

 (which did occnr in 1919) was due to the saving-up of plaice 

 uncaught because of restrictions on fishing during the war years. 

 It seems " reasonable " to assume that, because a large 

 quantity of plaice were not caught in 1915-18, the fishery had 

 improved in 1919 and there is no doubt that the reduction of 

 fishing during the war did have some effect. But how much ? 

 Since the catch made by the North Sea trawlers was less in 1920 

 than in 1919 it does not appear that the effect was a notable 

 one. Now, one has to accept the general conclusion arrived 

 at by the English and German fishery zoologists — -that the 

 effect of the war restrictions was a recovery of the plaice fishery 

 from the process of depletion that was in progress from 1908 

 to 1914 — but, all the same, one does so without much con- 

 viction ! In that attitude of mind, then, it is up to the doubter to 

 seek for some other explanation of the facts. 



Is the egg-production per plaice of mean size always the 

 same from year to year ? This is unlikely (though I know of 

 no satisfactory observations on this matter). In some years, 

 for instance, in 1914, and the years before and after, there 

 may have been an egg-production well above the average. 

 In that case the numbers of plaice of four to six years old ought 

 to have been greater in 1919 (or in the years before and after) 

 than it would have been if the egg-production had been an 

 average one in 1913-15 — provided that the proportion of post- 

 larval fish resulting from the development of the eggs had been, 

 at least, as good as the average. 



That leads to the suggestion that the proportion of eggs 

 that successfully develop and become young plaice may be 

 greater or smaller in some years than it is, on the average. 

 This is the idea underlying much fine work by Dr. Johann 

 Hjort. Obviously either it, or the idea that egg-production 

 rules the abundance of plaice in a fishing region some three to 

 five years later, or both, may be used to explain such variations 

 in the productivity of the North Sea plaice fisheries as those 



