For convenience in estimating the limits of catches of several 

 stations or single stations (up to 80 baskets in size of set) figure 6 

 has been prepared, following the procedures outlined in this section. 

 Although it is primarily designed to furnish a guide to the 95-percent 

 limits of catches made by POFI vessels, it can serve as a rough 

 estimate of these limits for other similar exploratory longline in- 

 vestigations, provided the construction and method of handling the 

 gear are not too dissimilar. 



SUMMARY 



A study of the variability of longline catches of yellowfin tuna 

 was conducted (1) to establish a means of estimating the variance of 

 catches made with different announts of gear in a single set or station, 

 and (2) to suggest a method of estimating the variance of a catch rate 

 derived from the average catches of several stationso Pursuant to 

 this, the existence of schooling was investigated, and the suitability 

 of two transformations was tested ennpirically. 



The evidence for schooling of subsurface yellowfin is from two 

 sources: (1) An analysis of runs indicating that they are clumped or 

 grouped on the line; and (2) the markedly greater day-to-day variation 

 in yellowfin catches as compared with the catches of a non-schooling 

 fish, the black marlin. 



The skewness of the data and the linear relation of the mean and 

 (T occurring in the catch data make it desirable that they be transformed 

 before applying statistical analysis. A logarithmic transfornnation 

 appears to be the most appropriate. 



The variances of 18-, 37-, and 74-basket sets were investi- 

 gated by analysis of variance on the logarithmically transformed 

 catches. The square root of the discrepance., interpreted as cr, was 

 found to be inversely pr'oportional to the square root of the nunnber ©f 

 baskets in the sampling unit, when the construction of gear and nnethod 

 of operation were held constant. Analysis of two groups of Japanese 

 commercial fishing catches, based on 350-basket sets, gave estimates 

 of (T considerably larger than would be expected if the cr calculated from 

 the smaller sets were adjusted to the 350-basket level. This suggests 

 that there was a lack of standardization in the conduct of the commercial 

 fishing, 



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