REPORT ON THE DEEP-SEA FISHES. xxxiii 



VERTICAL AND HORIZONTAL DISTRIBUTION OF DEEP-SEA FISHES. 



Before the voyage of the Challenger scarcely thirty deep-sea fishes were known ; this 

 number is now increased to about 370, if we include a few species which are, in fact, 

 surface fishes, but descend occasionally or regularly to or even beyond the 100-fathoms 

 limit. In a table appended to these introductory remarks, I have endeavoured to show 

 in a graphic manner the bathjanetrical range of each species, so far as it has been 

 ascertained at present. Unfortunately, some uncertainty prevails as regards the depth 

 at which certain specimens were obtained, I formerly assumed that the fishes of the 

 open sea were living either near to the surface or at the bottom, but I think now that 

 Mr. Murray is right in supposing that certain fishes live habitually in intermediate 

 strata, without ever coming to the surfiice or descending to the bottom.' The function 

 of the pneumatic apparatus with which many deep-sea fishes are provided is to regulate 

 their specific gravity, so that a fish is able to maintain its position at a certain 

 depth, which would vary only within certain limits, comparable to those which we 

 observe in surface fishes. As the mouth of the dredges or trawls used by the 

 Challenger was open during their descent or ascent, it is within the limits of proba- 

 bility that sometimes fishes were accidentally enclosed whilst the apparatus was 

 traversing the strata intermediate between the surface and bottom. And this lias 

 actually happened more than once ; for it is quite certain that common surface fishes, 

 like Sternopti/.r and Astronesthes, never ranged to the depth of 2500 fiithoms, the 

 depth to which the dredge had descended on the occasion of the capture of these 

 specimens. On the other hand, many of the fishes obtained by the Challenger off"er 

 suflicient evidence, from their own organisation, that they live at the bottom, and are 

 unable to maintain themselves suspended in the water ; and, consequently, that they 

 actually were obtained at the depth to which the dredge descended. Besides, the 

 statements of the depths of the Challenger fishes have been confirmed in many instances 

 by the observations made during the more recent deep-sea explorations. 



However, it may be assumed that of those mid-water Jishes, as the forms may be 

 called which inhabit intermediate strata, comparatively much fewer specimens are 

 captured than of the bottom fishes. It must be of rare occurrence that fishes accidentally 

 enter the narrow mouth of the dredge whilst it is passing to the bottom or to the surface, 

 and it is more likely to happen when the larger trawl is used. But such free-swimming 

 fishes are much more agile in their movements, and escape more easily on jDerceiving the 



■ The habit of some fishes of living at a distance from the surface and bottom is a fact which has been known for 

 a long time among fresh-water tishes. Salmo lacustris, of the Lake of Constance, has received its vernacular name 

 from this habit ; it is called " Schwebe-forelle," that is, the trout which is suspended in mid- water. 



(ZOOL. CHALL. EXr. — FAHT LVII. — 1887.) Lll 6 



