52 MARVELS OF FISH LIFE 



line of its body shows the wriggling movement of the 

 fish as it endeavoured to escape. 



At first the alevin, exhausted with the exertions of 

 escaping from its shell, lies panting on his side, but 

 soon he takes up a comfortable position in the gravel, 

 and keeps very much to this one spot, until his yolk 

 sac is almost absorbed. Alevins are not gregarious, and 

 avoid each other ; they also dislike the light, and when 

 covered with insufficient gravel they burrow deeper 

 among the stones. 



I have already alluded to the continual fanning 

 movement of the pectoral fin, which commences even 

 before the alevin is completely hatched. The result of 

 these continual fin movements is to make a current 

 round the little fish as he lies buried deep in the gravel, 

 thus removing the water vitiated by his breathing. 



When trout are bred artificially, large numbers of 

 alevins are kept in hatching boxes, and here the fish, 

 when healthy, crowd together or " pack " in the dark 

 corners. This packing is due to the instinct which 

 makes them seek shelter, and as there is no gravel for 

 them to hide in, they hide amongst themselves. An 

 incidental result of this packing is, that a steady current 

 of water is maintained round the fish, for every alevin 

 as he lies shoulder to shoulder with his neighbour, keeps 

 up the continual fanning of his two large pectoral fins 

 as already explained, so that the polluted water is swept 

 on in a steady stream, to be replaced by a fresh oxygen- 

 laden supply. 



