WIEBE ET AL.: AVOIDANCE OF TOWED NETS BY NEMATOSCELIS MEGALOPS 



To make estimates of lie, we have followed 

 Barkley (1972) and used generalized swimming 

 speed-body length relationships because there is 

 no direct information about u t for N. megalops. 

 However, as discussed below, inconsistencies be- 

 tween model expectations and the field data de- 

 velop when this assumption is applied. Our basic 

 size-frequency data are in units of wet weight. 

 However, as noted under "Methods," a subset of 

 individuals of N. megalops from MOCNESS 10 

 tows were used to establish the relationship be- 

 tween wet weight and body length (Fig. 2). Mean 

 body wet weight of individuals in each size class 

 was converted to body length (L) and then to rela- 

 tive escape speeds ( u e / U) assuming initially that 

 u t = 10 L. This assumption is supported by work 

 of Kils (1979) and Semenov (1969). Relative 

 escape speeds were plotted versus the catch/1,000 

 m 3 in each size class on semilog graph paper 



scaled so that each plot could overlay a reproduc- 

 tion of the upper panel of Barkley's figure 3 

 (1972:805). These plots were adjusted vertically 

 to obtain a "best" fit with the Xo/R curves such 

 that a maximum number of points fell between 

 any two of the Xo/R curves (Fig. 7). This produces 

 an estimate of Xo if it is assumed that the shape of 

 the size-frequency distribution is entirely pro- 

 duced by improvement of avoidance capability 

 with increasing size. 



Estimates of reaction distance (xo) for each net 

 system (Table 3) are between 1.7 and 2.3 for 

 the 1 m 2 MOCNESS and between 4.9 and 6.6 m 

 for the 10 m 2 MOCNESS. No significant differ- 

 ences between night xo's and day aso's for either 

 net system are observable. Our initial conclusion 

 was that this was an unreasonable result since in- 

 tuitively one would expect an increased night 

 catch to be related to reduced nighttime Xo values 



M0C-1-63N 

 Ue/U 



.05 .1 .15 20 .25 30,, 



M0C-10-28N 

 Ue/U 



.05 



.15 20 25 .30, 



c^ 



M0C-1-62D 

 Ue/U 



1.0- 



c^ 



.10 



.01 



100 



10"! 



M0C-10-27D 



Ue/U 



.05 .1 .15 20 25 .30 



1.0 E 





.01 E 



10 



1 T> 



4 3 



4 3 



.01 



Figure 7.— Examples of relative es- 

 cape speed of Nematoscelis megalops 

 individuals versus the catch per 1,000 

 m 3 . Superimposed on this plot are the 

 theoretically derived curves of Xr>/R as 

 a function P c and uj U adjusted to give 

 a "best" fit of the observed points. 



85 



