MARINE FOOD FISHES 123 



Off the Norfolk and Suffolk coasts mackerel are caught 

 in May and June, and again for about six weeks during 

 September and the beginning of October. 



The probable explanation of these migrations of the 

 mackerel is as follows : 



In the spring, the fish come in shore to spawn, and 

 then move back into deeper waters off the coast. Their 

 reappearance during late summer and early autumn is 

 due to then: following up the inshore migration of larval 

 fishes, for it has been abundantly proved that though 

 mackerel usually feed on copepods and other free- 

 swimming crustaceans, in late summer and autumn, 

 their food mainly consists of young fish. 



With the approach of winter the sea round our 

 shores cools down, and as a temperature of less than 

 45 F. is uncongenial to the mackerel, they disappear to 

 the warmer waters of the Atlantic. The gradual return, 

 as stated, during the following spring can be traced by 

 the fish being caught nearer and nearer to our shores 

 as the spawning season approaches. 



Thus we see that the spawning instinct, food supply 

 and the temperature of the water all play their part 

 in the migrations of the mackerel. 



But for marvels of migration we have to turn to the 

 life history of the common eel. For many years peculiar 

 forms of fish life, known as leptocephali, have been 

 recognised in the sea. These leptocephali possess per- 

 fectly transparent ribbon-like bodies of considerable 

 depth, and flattened from side to side. The head, as 



