THE RATE OF GROWTH OF FOOD-FISHES. 99 



very little, if any, growth in size takes place after the attain- 

 ment of sexual maturity, whereas, in the latter case, growth 

 continues indefinitely after that event 1 . 



We may take an example to make this clear. Let us 

 suppose that a certain fish of species A first comes to sexual 

 maturity at a size of 6 inches, and that it grows 1 inch a year, 

 and continues to reproduce its kind annually up to a length of 

 20 inches. In this case there is an active reproductive period 

 of 14 years. It is a well-known physiological fact that the 

 period of reproduction is not constant in intensity from its 

 inception to its end, but both in quantity of eggs laid and 

 vigour of offspring it varies, attaining a maximum in the 

 middle and minima at each end. In this way the fish although 

 attaining maturity and producing sexual products at a length 

 of 6 inches will then only produce smaller and weaker offspring, 

 whereas by the time of reaching, let us assume, some 19 inches, 

 offspring of greatest vigour will be the result. In a state of 

 nature a balance is struck and both kinds are found living 

 together, or rather, to be more accurate, every gradation, from 

 those maturing from the first time at 6 inches and those 

 maturing at 10 inches. Now let us consider the effect of over- 

 fishing. Assume that fishing has been carried to such an ex- 

 treme that an inappreciable quantity of fishes reach the length 

 of 10 inches, so that only those are left which commenced 

 breeding at 6 inches and somewhat upwards, and their offspring- 

 will have the same tendency to commence breeding at the 

 earlier age and shorter size. The first effect will therefore 

 be that of reducing the average size of the fish. 



It is a well-known fact that a far greater number of fry are 

 annually produced than will ever attain sexual maturity, and 

 the numbers surviving in any given district till sexual maturity 

 will bear a constant relation to the amount of nutriment 

 supplied by that area. Returning to our example let us 

 suppose that in a given district at a given time there are of 

 species A (1) 50,000,000 adults past their maximum reproduc- 

 tive capacity and of 12 to 24 inches in length, (2) 100,000,000 



1 A. T. M. 'Rate of Growth of Marine Food-Fishes.' 13th Ann. S.F.B. 

 Report, p. 289. 



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