20 LIFE IN THE SEA [CH. 



that they have the greatest difficulty in again going 

 down to the sea-bottom, and for a time they struggle 

 helplessly at the surface belly upwards. This is 

 because of the expansion of the gas contained in 

 their swim-bladders, and it is only after this has 

 been absorbed by the blood stream that the fish 

 regains its former specific gravity and is able to go 

 below the surface. It used to be the habit of the 

 Grimsby trawlers, who often brought cod alive into 

 the markets, to prick the fish behind the shoulder 

 with a long needle and so allow r the gas in the swim 

 bladder to escape. On the other hand fish like the 

 plaice and sole, which have no swim bladders, are able 

 to be transferred directly from water of moderate 

 depths to quite shallow tanks without any apparent 

 bodily disturbance, even although the difference in 

 pressure may be more than one atmosphere. Pelagic 

 fish are able to live both at the surface of the sea 

 and in the depths say of ten to twenty fathoms; 

 thus the herring must go down to the sea-bottom in 

 order to spawn, and they are often caught by means 

 of the trawl-net in the same manner as the proper 

 demersal fishes are captured. But we may be sure 

 that these fishes are able by means of some control 

 over the blood vessels of the swim bladder to alter 

 their specific gravity. 



In the case of the fishes of moderately shallow 

 water say down to twenty fathoms there is little, 

 if any essential difference in their form: a herring, 



