36 THE FISHING INDUSTRY 



two or three weeks, during which time they subsist upon 

 the contents of the yolk sac, which they carry attached 

 to their ventral surface. When this is exhausted, they 

 feed upon the microscopic plankton which abound in 

 the surrounding water. 



The characteristic forms of the different species of 

 flat fish are gradually assumed by the young fish during 

 the period of their larval development. The appearance 

 of a newly-hatched young plaice exhibits little change 

 during the first week or so, other than that due to the 

 gradual disappearance of the yolk sac. The young fish 

 grows very slowly, and, twenty-one days after hatching, 

 is only of an inch in length. For thirty days the 

 development of the young fish is entirely symmetrical. 

 During the succeeding fifteen days, the shape and appear- 

 ance of the fish become profoundly modified. The left 

 eye gradually moves upwards and forwards, until it 

 attains its final position above and in front of the right 

 eye. At the same time, the fish gradually acquires a 

 new swimming position, finally swimming on what is 

 really its left side. This left side becomes colourless. 

 With these changes in form and habit, there proceeds a 

 transformation in the diet of the fish. At twenty-one 

 days it feeds upon the young stages of various Crustacea. 

 Gradually it acquires a taste for copepoda and the 

 larvae of molmsca and Crustacea. After its metamor- 

 phosis is complete, it feeds upon various worms, small 

 shrimps and small, bottom-living Crustacea. The 

 adult plaice feeds upon mollusca of the cockle and 

 mussel families. 



The Migration of Fishes. Fishes, like birds, migrate 

 over great distances at certain seasons of the year. In 

 most cases, this migration occurs just before spawning, 

 and is evidently connected directly with the spawning 

 instinct. True marine fishes, such as the herring, 



