326 



THE SEAS 



like the North Sea it is necessary to be able to tell the age 

 of great numbers of fish. Fortunately the plaice carries 

 its own age indelibly written upon it. Situated in a cavity 

 just behind the brain are two small white bones. These 

 lie loose in the ear sac and enable the fish to regulate its 

 balance in the water, just as the semicircular canals in our 

 ears provide us unconsciously with a means of keeping 

 ourselves upright (Plate 118). Examination of these ear 

 bones, or " otoliths," shows that they possess alternate 

 light and dark concentric rings, similar in a way to those 

 on a cross section of a tree trunk. It has been proved 

 by a study of the earbones of marked fish that these rings 

 correspond to the summer and winter growth of the fish. 



Of great importance in the study of the life-history of a 

 fish is a knowledge of its food and feeding habits. The 

 method of finding out the chief food eaten is a very simple 

 matter and merely consists in opening the stomach and 

 examining the contents. When a very large number of 

 fish have been thus examined over a period of two or three 

 years a knowledge is gained of the most common kind of 

 food for the different sized fish and of any changes in the 

 diet that may occur during the seasons. Having discovered 

 the food the next problem is to find out what its distribution 

 is in the sea, because where the food is there we should 

 naturally expect to find the fish that feed on it. 



Now, the adult plaice is a bottom feeder ; that is, it 

 makes its meals of animals that mostly live on the sea 

 floor and while very many animals are included in its diet, 

 the most important item by far for the adult plaice in the 

 North Sea is a small shellfish known as Spisula (Plate 

 1 1 8). To examine the distribution of this sort of food the 

 grab is used and the actual numbers of animals caught in a 

 given area counted, so that, by making observations at 

 stated intervals over a large region, a chart may be drawn 



