SWIMMING ANIMALS 91 



Macrurus or rat-tail, who spends the greater part of its life 

 in the cold dark depths over the abyssal plain (Plate 33). 



These remarks on the distribution of fishes will help to 

 explain also the distribution of some of our large sea 

 fisheries. Certain fish, such as the mackerel {Scomber 

 scomber), which prefer warm water, will be found around 

 the north coast of Scotland and also in the North Sea. 

 This, at first sight, appears rather contradictory, but the 

 explanation is simple when we realize that there lies the 

 course of the Gulf Stream, and it is to the warm waters of 

 this oceanic current that the mackerel are keeping. 



It is evident that all the fishes that live most of the time 

 on the bottom will be limited by the depth barriers to coastal 

 waters or the regions of comparatively shallow banks. 

 There are however many fishes practically unknown to most 

 people that may be termed oceanic. These fishes roam 

 about all through the water layers out in the open oceans. 

 They are mostly very small, the largest being a few inches 

 in length. Many of them are very bizarre (Plate 34) in 

 appearance and possess most interesting organs for the 

 emission of phosphorescent hght. 



The distribution of these small oceanic fishes is also of 

 great interest. They, like most other fishes, are apparently 

 hmited in their geographical distribution by those unseen 

 temperature barriers. At the same time just as the bottom 

 fishes appear to be restricted to areas within certain depths, 

 so in the open waters of the ocean these fishes are to be found 

 living, each species inside a definite limited range of depth 

 above the ocean floor. There will be, for instance, those 

 that occur mostly between the surface and one hundred 

 fathoms ; others, again, will never be met with in these 

 water layers, but are only found below perhaps two hundred 

 fathoms ; and yet others may only be captured at very 

 great depths, such as a thousand fathoms. 



