Il6 MARKETABLE BRITISH MARINE EISHES chap. 



method consists in measuring each individual of all the fish that 

 can be collected of a particular species, taking care to collect 

 samples of all sizes and from every different region it inhabits 

 at different stages of its growth. It will then be found that the 

 individuals group themselves in largest numbers about certain 

 lengths, while there are comparatively fewer of the intermediate 

 lengths. The groups of which the members thus approximate 

 to certain lengths are the products of successive spawning 

 seasons, and when the lengths and numbers arc registered in the 

 form of curves the whole growth and development of the species 

 is exhibited. We have seen that the shoals of }'oung fish 

 derived from the preceding season can often be recognised, and 

 also sometimes those which are in their second )-ear. The 

 statistical method not only gives more precise information about 

 these, but enables us to distinguish the two-year-old and three- 

 year-old fish. When the condition of the fish with regard to 

 spawning is also registered, we see at what age and size maturity 

 is attained. The full application of this method is work for the 

 future, since it obviously involves considerable labour: to apply 

 it properly the collections for a single investigation ought to be 

 made within a brief period, as otherwise the measurements are 

 not comparable.^ 



The migrations of adult fishes depend principally on three 

 causes or conditions — the development of the breeding 

 organs, the need for food, and the season of the year : 

 most fishes have their customary breeding grounds, feeding 

 grounds, and winter and summer quarters. With regard to the 

 herrings their visits to their spawning beds are very regular, the 

 same beds are visited annually for centuries or longer. But 

 although the movements and positions of the young at successive 

 stages have been followed with some success we cannot say with 

 much certainty where are the adults in the intervals between the 

 spawning periods. They are taken occasionally in small numbers 

 in these intervals, and in all probability retire to some distance 

 from land. If we consider the multitudes of herrings in the North 

 Sea in summer from July to November, it seems certain that 

 they cannot remain there from November to July without being 

 caught ; the winter herring are not the same, and we must there- 



^ See Fourth Report of the Danish Biological Station, 1893. 



