412 DEPTHS OF THE OCEAN 



2. The Geographical Distribution of Bottom-Fishes 

 IN the North Atlantic 



The Fishes of the Abyssal Plain ^ 



In Chapter IV. the areas of the ocean-floor at different 

 depths are given, the percentages being as follows : — 



About two-thirds of the sea-floor is thus covered by more 

 than 2000 fathoms (or 3600 metres) of water, forming an abyssal 

 plain 90J millions of square English miles in extent, or nearly 

 half the surface of the earth. 



What organisms inhabit this abyssal plain ? When studying 

 the literature of deep-sea expeditions, we must remember that 

 all the hauls hitherto made in the abyssal area have been effected 

 by means of trawls or dredges, which function not only while 

 being towed along the bottom, but also while being lowered 

 and raised, filtering the immense column of water from bottom 

 to surface. Therefore only organisms like worms, molluscs, 

 holothurians, starfishes, corals, and all sessile forms may safely 

 be considered as having been captured at the bottom. In the 

 case of crustaceans and fishes, however, it may be doubted 

 whether they were really caught at the bottom or in intermediate 

 waters. Lists recording the catches of deep-sea expeditions at 

 great depths cannot therefore be accepted as representing the 

 animal-life on the ocean-floor, for in such lists we often find 

 forms which are now known to live quite close to the surface. 

 Although we have now a much more precise idea of the vertical 

 distribution of pelagic fishes than was previously possible, some 

 surprising facts are occasionally brought to light. Thus, as 

 mentioned in Chapter HI., the "Michael Sars " at Station 48, 

 between the Canaries and the Azores, brought up an Alepo- 

 cephalus in the large trawl towed at the bottom in 5000 metres, 

 just as these fishes have been captured by most deep-sea 

 expeditions ; on the trawl-rope a small tow-net was fixed in 



^ The mean sphere level, which lies at a depth of about 1700 fathoms, has hitherto been 

 regarded as the depth at which the abyssal plain of the ocean commences, but it will be seen 

 that Dr. Hjort places this depth at 2000 fathoms. — J. M. 



