96 FISHERIES OF THE NORTH SEA 



This movement is occasioned by rise or fall 

 of temperature in the ocean currents, but 

 more often predatory instinct is the chief 

 cause. For instance, cod-fish migrate in 

 huge quantities and follow the herrings, 

 on which they feed. The haddock " sets 

 in " in numbers where the herrings leave 

 their spawn, and on these huge masses 

 of fish eggs, often covering the sea bottom 

 for hundreds of square miles, they live 

 almost entirely at this season. Plaice at 

 different ages move to different depths, 

 the young always preferring the shallow 

 waters, such as the coastal banks off 

 Holland and Denmark. On reaching larger 

 sizes they move farther into the North 

 Sea, where they mature and are in the best 

 condition for the market. The wintry 

 weather is often very severe in the North 

 Sea. The temperature of the air is then 

 lower than that of the water, and the sur- 

 face waters, by proximity to the cold air, 

 become cooler in turn until the deep water 

 is warmer than the shallow, and the small 

 bottom fish, such as the sole, then creep into 

 lower depths of the pits to the south of the 

 Dogger Bank. Hence fishermen know that 



