768 DEPTHS OF THE OCEAN chap. 



herring stock, one single annual class may thus be enormously 

 prolific, the individuals exceeding in number those of all other 

 annual classes taken together. 



These facts naturally lead to the following conclusions 

 touching questions of interest to general biology as well as to 

 practical fisheries. The age-composition of a fish-stock varies 

 exceedingly ; there are good and bad years, producing annual 

 classes rich or poor in individuals. Favourable and unfavour- 

 able conditions must thus vary in nature, and seem to affect 

 specially the earlier phases in the life of the fish, inasmuch as 

 we perceive that in advanced years the numerical preponderance 

 of an annual class is equally perceptible for a number of years. 



The variations caused by the influence of one year will 

 therefore not always perceptibly influence the number of 

 individuals of the total stock, and in practical fishery its influence 

 will as a rule only be felt some years later, when the annual 

 class in question plays an important part in the catches of 

 fishermen. If favourable years have occurred just before or 

 after the birth of the class in question its influence may perhaps 

 not be felt at all. All this of course applies only to species 

 with many annual classes of spawners, for where few annual 

 classes spawn (or perhaps only one) conditions will be diff"erent. 



The influence of one year may, however, appear in the 

 quality of the whole stock, for instance in the fat-contents (see . 

 Fig. 556 representing the growth of the sprat). 



Wherever there is a good opportunity of obtaining repre- 

 sentative samples showing the age-composition of a fish-stock, 

 it should be possible to predict the composition of that stock 

 for the following years. We may thus, for instance, count upon 

 the possibility of annual classes containing a marked abundance 

 of young individuals reappearing, after the lapse of a definite 

 time, as equally abundant shoals of older and more valuable fish. 



The results here mentioned have been obtained through 

 laborious investigations occupying many years, involving the 

 study of the fishes at all seasons, in order to prove that the 

 various growth-rings of the scales really correspond to seasonal 

 changes. 



As far as I know, no systematic investigations as to growth 

 have ever been made in the open ocean, but I may point out 

 that in tropical waters and at all depths in the ocean the same 

 biological problems, which we have just described from boreal 

 waters, present themselves for study and solution. In this 

 connection I consider it interesting to cite some instances from 



