BATHYPELAGIC SQUID BATHYTEUTHIS 165 



Vertical and Regional Distribution 



Calculation of Maximum Depth of Capture and Vertical Range 



The sliallowest depth of the vertical range of a species is easily de- 

 fined by the shallowest tows that took specimens, when an adequate 

 number of samples have been taken. The depths where most of the 

 population lives are indicated by the frequency of captures and num- 

 bers of specimens at particular depth increments. The lower limit of 

 the vertical range of deep-sea, pelagic species is extremely difficult to 

 establish with open nets because of the possibility (or probability) 

 that the specimens in deej) tows were captured while the nets were be- 

 ing set or retrieved. Often the lower limit of a species is set near the 

 depth below which a sudden, sharp decline occurs in numbers of cap- 

 tures antl specimens. Frequently, however, no sharp break exists and 

 in general this approach is unsatisfactory. 



Bruun (1943, p. 21) devised a method for determining the vertical 

 range of Sphula spirilla taken during the Dana expeditions. First he 

 calculated the number of Spinila that were caught during a standard- 

 ized tow (S-2()0 hours) in the depth-layers (in terms of meters of wire 

 out) where Splnda appeared to be abundant. To determine if the 

 s])ecimens that were taken in nets fished at depths greater than the zone 

 of abundance actually came from those depths, Bruun first calculated 

 the number of Spimla that would be expected to be caught during the 

 time that the deep-fishing nets passed through the layer of abundance; 

 then he compared this figure with the actual catch per standard tow. 

 When the actual catch was smaller than the number that would be 

 expected while the net passed through the zone of abundance, the 

 specimens probably were not taken at the set-depth but at shallower 

 depths while the net was being set or, more likely, hauled. Pickford 

 (1946) and Thore (1949) used the same method for determining the 

 vertical distributions of Vampyroteuthis and pelagic octopods. 



Since the specimens of B. ahyssicola taken on Eltanin have been cap- 

 tured in open 3-meter Isaacs-Kidd Midwater Trawl nets, some of 

 which fished at depths in excess of 3000 m, a means of determining 

 actual dei)ths of capture and true vertical range must be applied. More 

 than one hundred 3-meter IKMT tows (and numerous 1-meter IKMT 

 tows) were made at depths of less than 500 m during the cruises cur- 

 rently being studied (through Cruise 15) ; no specimens of B. ahyssi- 

 eoJn. have been taken in these tows. This is a good indication that the 

 species does not normally occur shallower than 500 m in the Antarctic. 

 (A survey of captures through Cruise 24 has revealed that a total of 

 only three tows shallower than 500 m have captured B. ahyssicola. 



