WIKBE ET AL.: AVOIDANCE OF TOWEL) NETS BY NEMATOSCELIS MEGALOPS 



IOOO.Of 



100.0 



^ 



^ 



10.0 



1.0 



DC = Day Catch 



NC= Night Catch 



Xn = Day Reaction Distance 



Xn = Night Reaction Distance 



A - True Abundance 



— o — o 



2DC = NC 



— I0DONC 



— 100 DC - NC 



~ U D =Day Escape Speed 

 Un= Night Escape Speed 



100.0 



t 



^ 



10.0 



°-° 2 DC = NC 

 — 10DC=NC 

 x— 100DC = NC 



NC/A 



Figure 8.— Relationships between the ratio of night catch to 

 true abundance (NC/A) and a) the ratio of day and night reac- 

 tion distances (x,,/x s ), and b) the ratio of day and night escape 

 speeds ( u„/u v ). 



predicts that the ratio of the number of individ- 

 uals caught per size class at night (NC) to those 

 caught during the day (DC) will increase with in- 

 creasing individual size (the inverse of Equation 



3). 

 This relationship is illustrated in Table 4 where 



u N and Ho are assumed to be equal and 10 body 

 lengths/s, Xd — 175 cm, xn = 150 cm, R = 50 cm, 

 and U = 100 cm/s. This ratio increases dramatic- 

 ally with individual size until at the largest size, 

 the model predicts all individuals avoid capture. 

 No such pattern emerges if we compute the ratio 

 NC/DC for each size class in our paired day/ 



Table 3.—Nematoscelvi megalops reaction distances (xo) for 

 the 1 m 2 and 10 m* MOCNESS nets derived from the plots like 

 those in Figure 7. 



'Not sufficient points to derive an estimate, Station 2 omitted for this 

 reason. 



Table 4.— The ratio of night catch to day catch as a function of 

 individual swimming speed (it*) as predicted by Barkley's 

 avoidance model (inverse of Equation 3). w e is assumed to be a 

 function of body size as described in the text. 



'Catch units are proportion of individuals present per unit volume 



night MOCNESS 1 or MOCNESS 10 tows 

 (Table 5). Thus, the assumption of increasing 

 swimming speed with increasing size does not 

 appear to be valid, i.e., for the size range of indi- 

 viduals used in this study, avoidance swimming 

 speeds are essentially the same. One implication 

 of this finding is that the size-frequency distribu- 

 tions evident in the field data may not be seri- 

 ously biased by the avoidance although the esti- 

 mates of average density clearly are. 



87 



