CHARACTERISTICS AND HABITS OF FISHES 37 



haddock, plaice, cod, associate in vast numbers at 

 spawning time, choosing a locality in which the tempera- 

 ture and food supply will be favourable to the develop- 

 ment of the young larvae. Generally, the spawning 

 ground is in deep water. The eggs are buoyant, and 

 drift up to the warm surface water and hatch out 

 amongst the plankton. The herring differs from most 

 other pelagic fish in laying its eggs in relatively shallow 

 water, over a rocky bottom covered with seaweed. 

 The eggs are denser than sea-water and are covered with 

 an adhesive substance, so that they sink to the bottom 

 and become attached to the stones and seaweed. 



It is at the time of this annual migration to the 

 spawning grounds that the fish are most profitably 

 caught, for not only are they gathered together in large 

 numbers, but, just before spawning, the fat content and 

 general condition of the fish, and therefore its food 

 value, reach a maximum. After spawning, the food value 

 of the fish is at a minimum, and remains comparatively 

 low until a few months before the next spawning. 



The plaice migrates in the autumn from the feeding 

 grounds in various parts of the North Sea to the spawn- 

 ing grounds near the Straits of Dover. Spawning takes 

 place between December and March. In the spring 

 and summer it returns northwards to the feeding 

 grounds in the centre of the North Sea. 



In the Irish Sea, there are two distinct annual migra- 

 tions of plaice. The first occurs in summer (from June to 

 September), the larger plaice moving from the warmer, 

 shallow water inshore to the deeper, cooler waters 

 offshore. In winter and spring, (from October to May), 

 the mature plaice migrate from Morecambe and Liver- 

 pool bays to the spawning ground in deep water to the 

 North-East of Douglas (Isle of Man). 



In winter, also (from November to January), a large 



