vii. 21 MIGRATION OF EELS 227 



their own continental waters. The populations of so-called European 

 eels {Anguilla anguilld) would then be maintained by reinforcements 

 of larvae of the American A. rostrata, the differences between the 

 two being due to temperature and other factors (Tucker, 1959). 



Social behaviour is marked in many species and shoals of some 

 fishes may contain many thousands of individuals. The animals are 

 presumably kept together in most species by visual stimuli, though 

 sounds may play a part. Shoaling gives protection to small fishes, and 

 in some species the animals come together in shoals to breed (her- 

 rings). There may also be some advantage for the finding of suitable 

 feeding conditions, but on all these points we can do little more than 

 speculate and hope for further information. 



