THE SEA-FISHERIES 307 



the fish. In an assemblage of fish caught together in the 

 trawl, we generally find fish of several different ages, and 

 are able from the sizes to separate them into age-groups. 

 It is believed that we can also determine exactly the age 

 of many of the bony fish by examining the otoliths, or the 

 scales, where the successive annual rings of growth show 

 more rapid increase in summer and much less in winter ; 

 and even the growth in different summers is found to vary 

 according to the temperature. It is curious to think we 

 may be able to pronounce upon the cUmate of past years 

 by examining with a microscope the scales of a fish caught 

 to-day. 



Amongst examples of movements due to spawning needs, 

 an extraordinary case is that of the common eels, which live 

 in fresh-water streams and lakes and other shallow fresh 

 waters for years without breeding, and then towards the 

 end of their lives change their appearance (acquiring a 

 silvery sheen and large eyes), and, giving up all their previous 

 habits, migrate to the deep sea and spawn in mid- Atlantic, 

 west of the Azores, beyond the 2,000-fathom line. From 

 there the leptocephalus-larvse are carried by the Gulf 

 Stream drift to the coasts of North-West Europe, taking 

 three years to the journey, and as elvers they migrate up 

 the rivers to people the inland waters, where no sexually 

 mature individuals, no eggs, and no larvae have ever been 

 found. The herring furnishes another good example of 

 spawning migrations, and comes into shallow water at 

 various points on our coast and in the Baltic, etc., to deposit 

 its eggs on suitable ground. 



Feeding migrations or movements may be local and smaU 

 in amount and more or less irregular, as when a shoal of 

 plaice invade a bed of young mussels and move off again 

 when they have exhausted the food ; or may be greater 

 in amount, and periodic, as in the case of the mackerel 

 and the herring following their planktonic food. 



Scientific investigations bearing on sea-fisheries questions 



