THE RATE OF GROWTH OF FOOD-FISHES. 101 



in a finite series to less than 2. A study of the average sizes 

 of fishes shows that the annual increase is practically distinctly 

 appreciable. There is no reason, therefore, to believe that 

 there is any definite limit to the size of a fish, as used in the 

 sense in which we speak of a definite size attained by mammals 

 and birds, beyond which growth in bulk does not proceed. 



Exceptions to this rule have been put forward from time 

 to time ; for example, some observers hold that the herring 

 forms one of these, so that after attaining a definite size it 

 entirely ceases to grow. It seems, however, that these state- 

 ments, though handed on from one authority to another, do not 

 rest upon sufficient basis of fact. 



We may next enquire consecutively into the various factors 

 of environment which are known to affect the rate of growth 

 of fishes. 



Nutrition. The direct effect of nutrition upon growth is 

 well known, even in the case of the higher animals, so that 

 the result is probably intensified in the case of fishes. This 

 factor obviously cannot come into force till the post-larval 

 stage is reached, for before this, the larva is supplied with 

 a copious quantity of yolk-material, which though doubtless 

 varying in slight degree may be reckoned as constant for all 

 practical consequences. 



Dr Meyer in 1878 made experiments with young herrings, 

 and found that those reared artificially grew at a slower rate 

 than those living under natural conditions, but that later they, 

 so to speak, made up the leeway. This is shown as follows : 



Length in millimetres attained by fish under 



Artificial Natural 



Age from impregnation. conditions. conditions. 



One month 1011 1718 



Two months 1719 2436 



Three months 3035 4550 



Four months 48 54 55 61 



Five months 6570 6572 



From this table it can be seen that the two months' herring 

 brought up in captivity are some of them less than half the 



