170 



U.S. NATIONAL MUSEUM BULLETIN 291 



2.0 4.0 6.0 80 



40 60 



Figure 63. — Antarctic, a, Average number of specimens captured per tow for total tows 

 (hatched area) and percentage of successful tows (clear area) by 250-meter increments. 

 B, Total number of 3-meter IKMT tows (clear area) and number of successful tows 

 (hatched area) by 2S0-meter increments. 



1750 m.) Figure 63a gives the average number of specimens captured 

 per total tows grouped by 250 m intervals. The average catch, as well 

 as the total catch, drops off between 1500-1750 m then reaches a peak at 

 1750-2000 m. The percent of successful tows taken at each depth incre- 

 ment is also plotted on the graph. The plot generally follows the plot 

 for captures ; that is, the greater the catch and the greater the average 

 per tow, then the greater the number of successful tows (or vice versa) . 

 Even the tows below 2500 m were relatively successful, undoubtedly 

 because they passed through the thick layer of abundance. Figure 63b 

 shows the relationship between total number of tows at each depth 

 interval and the number of successful tows. A great number of tows 

 was made between the surface and 1000 m, but only a few below 500 m 

 caught B. dbyssicola. Fewer tows were made below 1000 m but they 

 were considerably more successful (also compare with figure 63a for 

 percent of successful tows) . 



