basket were separated by only 30 fathoms. (3) The hooks were not 

 all fishing at the same depth and this decreases the chance of a 

 school's leaving more than one of its nnembers on the line unless 

 there is a considerable vertical component to the shape of a school. 

 (4) Whenever a tuna or other fish takes a bait without becoming hooked 

 a potential sequence of yellowfin is broken. It seems almost certain 

 that had these four factors been absent the number of runs would have 

 deviated even more from the expected number. This lends qualitative 

 support to the conclusion that the subsurface yellowfin are aggregated 

 rather than randomly distributed in space. 



Providing an additional test of schooling or aggregation of 

 yellowfin is a comparison of the variability of black marlin and 

 yellowfin tuna catches made on the same gear at the same tinne and 

 place. The basis for this comparison is the assumption that the de- 

 gree of variability of the catch of any species increases with the degree 

 of aggregation of that species. Black marlin are not a schooling species 

 while at the surface, whereas yellowfin are usually aggregated into com- 

 pact schools. If these contrasting behaviours are retained when the 

 fish occupy deeper levels, there should be a difference in the relative 

 variability of the catches, 



A comparison of the variability of black marlin and yellowfin 

 catches is given in figure 1, where each point is based on a catch from 

 the approximately 2, 000 hooks comprised in one set of longline gear. 

 It is at once apparent that the yellowfin catches are much more variable 

 than those of the marlin, though the catch rates per 100 hooks are nearly 

 equal. 



Numerical expression of this difference in variability has been 

 obtained by testing the frequency distribution of the catches against a 

 Poisson distribution appropriate to the mean catch rates, following the 

 method of Snedecor (1948). The test of the yellowfin catches for the 

 period October 2-10 (selected for relative homogeneity in catch rate), 

 with a catch rate of .454 fish, yielded a chi-square of 47 with a P of 

 less than .000001. On the other hand a similar test of the marlin catchgs 

 for the period October 2-12, with an average rate of .6565 yielded a 

 chi-square of only 11, P of .03. Thus the marlin catches differ only 

 slightly, though significantly, from the Poisson distribution, whereas the 

 yellowfin catches do not even approach conformance with the Poisson, 

 indicating that some factor is introducing great variability. 



