BARKLEY: SELECTIVITY OF TOWED-NET SAMPLERS 



PC 



I.CXX3 



0.100 



0.010 



0.001 



NUMBER 



1,000 



100 



10 



Figure 6. — Catches of Bathylagus stilbius with an Isaacs-Kidd midwater trawl 

 at two speeds. Length frequencies converted to speed frequencies are compared 

 with theoretical curves for P^, showing fairly good fit at Xq/R =i 3.8 for part of 

 data. No overall fit is possible for these observations, indicating that length-fre- 

 quency curves for this species were not determined primarily by avoidance. Data 

 from Aron and Collard (1969). 



a minor role, accounting only for the difference 

 between the 35-fold range in apparent abund- 

 ance and the observed 48- to 57~fold differences 

 in catch rates for the 62- and 97-mm size classes. 

 The most important effect of avoidance in the 

 B. stilbius catch data is the nearly constant 5- 

 f old difference in catch rates at the two different 

 speeds, for all animals larger than 62 mm. 



Figure 7 shows sample results for only two 

 class intervals. Table 1 shows results for all 

 class intervals of Aron and Collard's B. stilbius 

 catch data: Catches (as read from length-fre- 

 quency curves) , ratios of catches at two speeds, 

 and values of Xo/R, No, and Pc obtained by fit- 

 ting the catch data to the theoretical curves of 

 Figure 3. Obviously, "fitting" two points to a 



curve is a trivial exercise, which is only justi- 

 fied by the apparently consistent results obtained 

 for larger fish in Table 1. Data from three or 

 more sets of tows, at as many different speeds, 

 should be used for such analyses. 



Figure 7 also shows the ranges of values of 

 Ue/U, and the extreme values for Xo/R and iVo, 

 which result when the full range of net speeds 

 reported by Aron and Collard (1969) is used 

 instead of the modal speeds. The numerical re- 

 sults, like the net speeds, vary by factors of about 

 two — an indication of the uncertainty inherent 

 not only in the B. stilbius data but also in the 

 data for other species considered here, since var- 

 iations in net speed comparable to those mea- 

 sured by Aron and Collard are doubtless present 



811 



