86 DISCOVERY REPORTS 



Besides the evidence to be got from nets, studies of fishes in relation to deep-scattering layers 

 (Marshall, 1951; Tucker, 1951; Hersey and Backus, 1954; Kanwisher and Ebeling (1957); Backus 

 and Barnes (1957); Johnson, Backus, Hersey and Owen (1956) and Barham (1957)) indicate that 

 bathypelagic species, particularly those with swimbladders, are a conspicuous constituent of these 

 layers. Furthermore, it seems likely that the most prominent of the fish sound-scatterers will prove 

 to be myctophids. Clearly, when surer identification becomes possible, echo-sounders will provide 

 the biologist with a valuable record of these changing events. 



In conclusion, a beginning has been made on observing vertical migrations from bathyscaphes. 

 After dives off Villefranche, Tregouboff (1958) wrote as follows: 'La plongee de nuit a permis 

 egalement d'observer la migration nocturne vers la surface de divers autres animaux, tels que les 

 Crevettes Euphausiaces, lesquelles, ayant quitte leur zone habituelle de 500 m. de profondeur environ, 

 ont apparu en nombre a partir de 100 m. de profondeur. A leur cote chassaient activement, egale- 

 ment a ce niveau, des petits Argyropelecus, qui se sont maintenus aussi nombreux jusqu'a 250 m., 

 tandis qu'au jour on ne les capture au filet fermant qu'a partir d'au moins de 500 m. Enfin, des petits 

 Myctophidae et quelques Cyclothone ont effectue egalement un deplacement notable vers la surface 

 et se sont montres, surtout les premiers, en grande quantite deja entre 200 et 300 m. de profondeur.' 



The evidence to be got from nets, sound exploration and bathyscaphes also suggests that the 

 vertical migrations of deep-water fishes differ in extent. However, these findings will best be left 

 until the third part of this section, when the physical problems facing fishes as they move up and 

 down will be considered. 



The physical and biological environment 



The headquarters of bathypelagic fishes are in the tropical and temperate parts of the ocean. They live 

 below the mixed surface zone, which contains all or the greater part of the actively assimilating 

 phytoplankton, and has an average depth of about 75 m. The lower limit of this zone is marked by 

 a distinct thermocline, which tends to be permanent in subtropical and tropical regions, but is seasonal 

 in temperate waters. Temperatures range from about io° to 30 C. and salinities from about 32 % 

 to 4 i% . 



Below this thermocline comes a transition zone separating the surface mixed layer from the cold, 

 deep water. Iselin (1936) calls this the thermocline layer, for in the warmer parts of the ocean, the 

 temperature drops quite rapidly from about 20 C, its value just below the near-surface thermocline, 

 to about 5 C. at about 1000 m. Taking the level of the lower thermocline to be the depth at which 

 the rate of change of temperature is greatest, in the North Atlantic it is in this lower transition zone 

 that the values of dissolved oxygen begin to fall towards the minimum value. But over much of the 

 eastern tropical Pacific north of the equator, the fall in quantity of dissolved oxygen begins below the 

 upper thermocline, and a thick oxygen minimum layer (containing less than 0-25 ml./l.) is found 

 between depths of 100 and 1000 m. (Wooster and Cromwell, 1958). Conditions appear to be similar 

 over the central area of the equatorial Indian Ocean, where there is an equally thick oxygen minimum 

 layer (with values i-o ml./l. or less) between much the same depth-intervals (Sverdrup, Johnson and 

 Fleming, 1942). In the Atlantic, the lowest values (1-5 ml./l. or less) are centred at a depth of about 

 400 m., below the North and South Equatorial currents (Riley, 1951). 



Turning to submarine light, the thermocline layer contains the twilight zone of the ocean. Using 

 a sensitive photomultiplier tube, Clarke and Wertheim (1956) were able to measure the penetration 

 of sunlight down to depths of about 600 m. in clear water off the Western North Atlantic coast. 

 However, the threshold of light for the eyes of many bathypelagic fishes is likely to be well below this 

 level (Denton and Warren, 1957). 



