168 U.S. NATIONAL MUSEUM BULLETIN 291 



the below-2500 m calculation also apply to the below-2250 m calcu- 

 lation. The nets have to pass through 3500 m of productive zone 

 (2250-500 = 1750; 1750X2=3500 m). This requires 2.3 hours. There- 

 fore, 2.99 specimens would be expected to be caught while the nets 

 were traversing the zone where B. ahyssicoJa is abundant. This figure 

 is greater than those representing the total catches below 2250 m and 

 also the limited zone of 2250-2500 m, although the latter catch ap- 

 proaches the theoretical figure. Taken as a total unit below 2250 m, 

 2.15 specimens may be sufficiently fewer than the theoretical 2.99 

 specimens to indicate that B. abyssicola does not live in numbers below 

 2250 m. On the other hand, the 2.70 specimens value for the zone 2250- 

 2500 m may approach too closely the 2,99 value to be significant for 

 establishing the lower limit of vertical distribution of B. ahyssicola 

 within this zone. With the sources of error inherent in any system 

 using open nets, this is as far as the calculations should be pursued. 



A more conservative and perhaps more accurate approach can be 

 taken by assuming the IKMT captures specimens only while being 

 hauled and not while being set. The IKMT probably does not filter a 

 large volume of water while being set because the rate of pay-out of 

 cable may be nearly as great as the speed of the vessel ; water may pass 

 through the net at only one or two knots. This would be sufficient speed 

 to capture the lethargic and less mobile forms, while allowing the 

 more active or more perceptive animals to escape. Based on the 

 morphology of B. abyssicoJa and on observations of living specimens, 

 it is clear that this animal is not an extremely rapid, active swimmer ; 

 neither is it completely lethargic and immobile. Possibly, some speci- 

 mens may be captured even at the low filtering speed during the setting 

 of the net. Therefore, the following calculations represent minimum 

 values. The assumption that the IKMT fishes only while being hauled 

 will give greater significance to the few specimens taken in the deeper 

 tows by reducing by i/^ the number of specimens which are assumed 

 to be captured in the productive levels during set and retrieval of the 

 trawl. At depths greater than 2500 m the time spent fishing in the 

 shallower productive zone would be 1.3 hours instead of 2.6 hours, dur- 

 ing which time 1.71 specimens would be expected to be captured. Since 

 1.57 specimens per 2-hour tow were captured below 2500 meters this 

 value probably still does not indicate that B. ahyssicola regularly lives 

 below 2500 m. 



The result is different when considering the zone below 2250 m and 

 especially the block between 2250 and 2500 m. Assuming that the net 

 fishes only during haul-in, the catch would amount to 1.5 specimens. 

 Since all tows below 2250 m averaged 2.15 specimens, it is possible that 

 0.65 specimens could actually be captured below 2250 meters. In the 



