CHARACTERISTICS AND HABITS OF FISHES 39 



sea and are never seen again. It is probable that the eel 

 only spawns once in its life and then dies. The spawn 

 floats to the surface and hatches out into curious little 

 transparent, leaf -shaped larvae. These larvae develop 

 rapidly into elvers and commence the return journey 

 to the shores and rivers. In the spring, the young eels 

 ascend the rivers in enormous swarms. Many of them 

 leave the rivers and travel over damp ground and grass 

 to isolated pools and lakes. It is probable that the 

 eels that are found in the Thames travelled overland 

 from the Severn. 



The Baltic flounder migrates in winter from rivers and 

 estuaries to the open sea, and spawns in spring in deep 

 water. It returns in the summer when the spawning 

 is over. By observing the movements of marked fish, 

 it has been shown that the fish move at an average rate 

 of from three to four miles per day. During its sea- 

 ward migration, the flounder takes no food, but uses the 

 material stored up in its tissues for the development of its 

 reproductive organs. 



In addition to these spawning migrations, there are 

 migrations that are prompted by a search for food, or 

 for warmer or colder water. 



In northern ana temperate seas, the surface water 

 grows warmer with the spring. This warming influence 

 spreads northwards from the equator, producing what is 

 known as the annual wave of sea temperature. A 

 direct result of the rise of temperature and the increased 

 sunshine is a rapid increase in the amount and quality 

 of the plankton. It is not surprising, therefore, that 

 fish migrate in the wake of this annual wave of sea 

 temperature, attracted by the increased food supply, 

 and possibly, also, by the warmer water. 



The mackerel is a southern fish, and prefers the warm 

 water of the Mediterranean and West African coast. In 



