90 THE SEAS 



species. It is, so to speak, a m^^eting ground of the two 

 great areas and certain fishes common to both occur. 

 Besides the herring and the cod, both northern repre- 

 sentatives, we have, for instance, the pilchard, and occasion- 

 ally the red-mullet and the anchovy, fishes which are typical 

 of the warm waters of the Mediterranean and Atlantic. 



Within the large areas, also, it is to be noticed that there 

 are further sub-divisions ; certain fishes which live in the 

 very cold part of the Norwegian Sea, and in the Arctic 

 waters, are quite characteristic of those regions. 



In the tropics, again, the fish population is quite character- 

 istic, and here are to be found many of the most brilliantly 

 coloured fishes in the world. 



In the geographical distribution of fishes, apart from 

 these differences caused by temperature, there are also 

 differences that are occasioned by the depth of the water. 

 There are shallow-water fishes, deep-water fishes, and 

 abyssal fishes, that is, fishes who live on the very flat plains 

 in the deepest parts of the oceans known as the abyss. 



It is natural that those fishes which will be limited to 

 certain areas by depth boundaries are those that live most 

 of their time swimming on, or close to, the sea bottom itself. 

 Fishes such as herring and mackerel (so-called pelagic 

 fishes), can have a much wider area over which to roam, 

 because, swimming as they always do in the upper water 

 layers, they can keep to the depths they most prefer, 

 irrespective of at what depth the actual bottom of the sea 

 may be. 



As exam.ples of fish whose distribution is limited by 

 depth, we can name the plaice, which lives in the com- 

 paratively shallow flat areas such as the North Sea ; the 

 hake, that roams along the deep-water shelf from one to five 

 hundred fathoms, that constitutes the continental slope ; 

 and that curious fish of the cod family known as the 



