112 THE RATE OF GROWTH OF FOOD-FISHES. 



proportion of the whole bulk, and this is not so. The curves, 

 for instance, of the gadoids may be compared amongst them- 

 selves with a fair degree of accuracy, but a glance at the shape 

 of a pleuronectid will at once remind the reader that the 

 length is a less predominant factor of the total bulk than is 

 the case with a gadoid. 



It is well also to bear in mind that the length is not a 

 constant factor in the bulk of a single fish throughout its life ; 

 and it bears a greater proportion to the other dimensions as a 

 rule, in the younger stages, so that the curves cannot in any 

 way be said to represent accurately the increase in bulk or 

 total growth of the species in question. 



Any rhythmical disturbing factor in the growth under 

 natural conditions should appear in these growth-curves as 

 a constantly repeated deflection in the curve, so that the 

 seasonal variation in temperature should, if the curves were 

 quite accurate, cause a recurrent series of secondary curves. 



The method of investigating rates of growth, as described 

 and advocated here, depends, as already stated, upon a know- 

 ledge of the spawning-period, and the inverse to this holds. 

 Thus, given a sufficient number of specimens of a certain 

 species and their date of capture, the curves of maximum and 

 minimum size at every stage may be defined, and these, 

 produced back to the base line, will give the duration of the 

 spawning-period. This method has been pursued recently for 

 the herring 1 and the sand-eel 2 and shows that both these forms 

 have two spawning-periods in a year, besides indicating other 

 important facts, such as the difference in size and rate of 

 growth of the spring and autumn ' races ' of herring, and the 

 habitat of the young fishes at each stage, as mentioned under 

 the sections devoted to the herring and the sand-eel respec- 

 tively. Table A will give a good idea of this graphic method 

 of representing the rate of growth and life-history of the 

 herring; the gradual progression of the young forms as they 



1 A. T. M. ' Rate of growth of the Herring.' lth Scot. Fish. Board 

 Report. 



2 A. T. M. 'Life-history and rate of growth of the Lesser Sand-eel. 

 Annals and Magazine of Natural History, Sept. 1895. 



