152 



Now Fig. 18 apparently shows that the plaice caught in 1911 

 were markedly bigger, on the whole, than the plaice caught in 

 1909, and one might incautiously infer that the rate of growth 

 was greater in 1911 than in 1909. But the comparisons of the 

 composition of the shoals, both in the periods 1908-16, 1914-16, 

 1920 (Groups II and III), and in the periods 1909, 1911 

 (Groups I, II, and III) show clearly that the prevalence of 

 bigger fish in some years is due to the fact that older {and therefore 

 bigger) fish are more abundant in those years. 



Causes of Fluctuations in Abundance and Size. 



Why, then, are plaice bigger, on the whole, and on a certain 

 ground, in one year than they are in another ? It is because 

 there are more older fish in the shoals in those years when the 

 plaice run bigger, not because there is a greater rate of growth. 

 No doubt the rate of growth varies to some extent, but not much 

 — not enough, we think, to account for the differences that are 

 to be observed from year to year. 



Why are plaice more abundant on a certain ground in one 

 year than they are in another ? It is because more fish have 

 passed successfully through the critical periods of metamor- 

 phosis and have managed to settle down on the nurserv 

 grounds, there to grow rapidly and safely. If there are more 

 plaice of three years old on the Liverpool Bay grounds in the 

 summer of 1920 than there were in 1919 (say) it is because 

 more little fish came on to the nurseries in the summer of 

 1916. And so with each age-group. 



The abundance of plaice of any particular age-group, on 

 a fishing ground, depends, then, on a number of conditions, 

 all of which have, by some happy chances, been in existence 

 and have been correlated. 



(1) There must have been, so many years previouslv, an 



unusually large production of spawn. 



(2) An unusally large proportion of the larvae hatching 



out from this spawn must have metamorphosed 

 successfully. 



