254 A HISTORY OF FISHES 



and finally, there are the abyssal fishes such as the Grenadiers 

 {Macruridac) which spend their lives in the ocean depths and 

 live on or near to the sea bottom. The pelagic fishes, and those 

 dwelling in the upper layers of the ocean, are mostly found in 

 the warm tropical and temperate regions, few penetrating into 

 the colder waters of the Arctic and Antarctic. As far as the 

 bathypelagic and abyssal forms are concerned, knowledge of 

 their distribution is still incomplete, but there can be little doubt 

 that many of them have an almost world-wide range, being little 

 aflfected by the physical barriers that hmit the wanderings of 

 pelagic and coastal forms. Many seem to have a wide vertical 

 range, spending part of their time comparatively close to the 

 surface and part in the deeper layers of water. The contour 

 of the ocean bed may play an important part in restricting the 

 range of abyssal forms, and the submarine ridge, less than one 

 thousand metres in depth, extending from Scotland to Iceland 

 and Greenland, represents the northern limit of the Grenadiers 

 {Macruridae) . 



Coastal fishes may be described as those forms that live 

 comparatively near to the shore, dwelling either at or near the 

 surface Hke the Herring {Clupea) and Mackerel {Scomber), or 

 close to the sea-floor Hke the Gurnard {Trigla) or Plaice 

 [Pleuronectes) , the latter being found both in the shallow inshore 

 waters and on the Continental Shelf. This plateau or shelf, 

 varying greatly in width in the different regions, surrounds all 

 the great land masses or continents, and is formed either by 

 the erosion of the land by the waves or by the extension into 

 the sea of deposits of mud or silt carried down from the land 

 by rivers. This Continental Shelf slopes gradually downwards, 

 its outer edge being about two hundred metres below the 

 surface of the sea. Beyond this edge is the Continental Slope, 

 with a much steeper declivity, extending to a depth of nearly 

 two thousand metres, and below this is the true abyssal region. 

 The coastal fishes generally present a far greater abundance 

 and diversity in the shallower waters of the shelf, and as the 

 abyssal depths are approached, the number of species and of 

 individuals becomes progressively less and less. This relative 

 abundance of fishes on the Continental Shelf and upper part 

 of the Continental Slope is an important factor in the develop- 

 ment of the sea fisheries, the prominence of the Atlantic and 

 North Sea industries being due to the presence of large areas 

 of sea-floor at a depth of five hundred metres or less in these 

 regions. 



