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 their 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 
