[19] scientific examination of the german seas. 543 

 4. What causes the herrings to migrate and to gather in 



SCHOOLS ? 



Profitable herring fisheries can only be carried on in places where the 

 herrings gather in schools. 



In February, 1872, 240,000 herrings were caught every day in seines 

 in the inner Bay of Kiel. To take so many fish out of the water in one 

 day would have been an impossibility if they had been scattered over 

 the whole bay ; for even the largest seine is small compared with the 

 area of the Bay of Kiel. 



The principal causes why the scattered herrings gather in schools and 

 migrate to certain parts are the desire for food and the desire to propa- 

 gate the species. 



The herrings which live outside the bays of Kiel and Eckernforde and 

 the Schlei cannot know whether they will find sufficient food in these 

 bays, not even when they have been there during their early youth; 

 but they enter these bays, gradually proceeding towards the inner parts, 

 because there they find more food than outside. Whenever these bays 

 contain a great quantity of good food as, for example, in February and 

 March, 1872, when the waters of the Bay of Kiel were literally swarm- 

 ing with small crustaceans (Fig. VII), an unusually large number of 

 herrings will enter the bay. So many herrings had not been caught 

 before in the Bay of Kiel within the memory of the oldest inhabitant 

 as were taken during the winter of 1871-72. During the same period 

 an extraordinarily large number of large codfish were also caught. The 

 codfish followed the schools of herrings, and feeding on them soon grew 

 large and fat. 



The second cause why the herrings gather in schools is the desire to 

 propagate the species. The rapid growth of the sexual organs must 

 necessarily awaken hitherto unknown desires which mutually draw the 

 milters and spawners together, and keep them near each other even 

 after the desire for food no longer unites them. 



In the Western Baltic the mature herrings gather in spring, especially 

 in shallow places where the bottom is covered with sea-weeds and other 

 plants, and where the water contains but little salt and is easily warmed 

 by the rays of the sun. 



It would be a mistake to suppose that the herrings go into these 

 waters because they have some knowledge of their condition, and be- 

 cause they think that they are best suited for receiving and developing 

 the spawn. Without having the slighest knowledge of the purpose of 

 their gathering, they nevertheless gather in schools, because tbey are 

 all animated by the same desires and instincts. 



It is probable that at the time when the sexual organs approach 

 maturity, the less degree of saltness and the greater warmth of the 

 shallow coast waters is pleasanter for them than the cold and strong salt- 

 ness of the deep waters ; and they consequently swim in the direction in 



