FISHERY BULLETIN: VOL. 70, NO. 3 



NUMBER 



.1 



1,000 



"•/u 



.3 



.5 



too 



10 



Figure 7. — Alternative analysis of Bathylagus stilbius data from Figure 6. Estimates 

 of population (intercept of x^/R curves for 1.8 and 2.8 with ordinate) by class in- 

 terval show wide variations: from 900 animals at 62 mm to 26 animals at 97 mm. 

 This range is comparable to the observed range in catch for these class intervals (57 

 to 1), showing that the length-frequency curves are primarily determined by popu- 

 lation structure. Horizontal bars show measured ranges of towing speeds, and the 

 effect of different speeds on the analysis. Data from Aron and Collard (1969). 



during any normal towing procedure. This un- 

 certainty does not affect the qualitative conclu- 

 sions obtained from theoretical analyses; it is 

 quite clear, for example, that anchovy react 

 earlier to the IKMT than they do to the 1-m net. 

 It is also quite clear that the population structure 

 of B. stilbius determines the shape of the catch 

 size-frequency curve, whereas differences in 

 overall levels of catch for this species at two 

 ranges of towing speed are due to avoidance, 

 at least for those animals effectively retained 

 by the meshes. In any case, numerical estimates 

 of parameters such as Pc and population density 

 which are correct to within a factor of two or 



less are entirely adequate for many purposes. 

 Simultaneous solutions for Pc as a function of 

 length have been obtained from the theoretical 

 equations for three pairs of samples from as 

 many populations. The results, while not de- 

 finitive, appear encouraging. 



It should therefore be possible to apply the 

 theory to a more typical set of data, a single com- 

 posite sample of larval and juvenile stages of 

 Katsuwonus pelamis, the skipjack tuna. These 

 animals were sampled with a large midwater 

 trawl, the Cobb pelagic trawl (Higgins, 1970), 

 during a series of five equatorial cruises made by 

 the RV Town-send Cromwell in 1969-70. Repli- 



812 



