POLACHECK: YELLOWFIN TUNA CATCH RATES 



estimates within years. If a normal distribution 

 is assumed, most of the differences among the 

 different stratifications within a year would not 

 be significant at the 0.05 probability level. As with 

 the purse seine data, the lack of differences in 

 the annual estimates should not be interpreted 

 to mean that area and temporal effects do not 

 exist. 



Fine-Scale Relationship Between 

 Purse Seine and Longline Catch Rates 



Comparison of catch rates by longliners and purse 

 seiners in the same area and during the same time 

 period suggests that there is little relationship be- 

 tween them (Fig. 9). Thus, for all the rectangular 

 areas in which there were at least five quarters with 

 a reasonable amount of effort by both gear types, 

 the correlation coefficient between the catch rates 

 for the two gear types ranges from -0.37 to 0.89 

 (Table 3). When the variances associated with the 

 individual catch rates are taken into account (e.g.. 

 Figure 9), there is nothing to suggest that these cor- 

 relation coefficients are not zero. 



Table 3— Estimates of the correlation coefficients 

 for ttie quarterly yellowfln tuna catch rates between 

 Japanese longliners and purse seiners within rec- 

 tangular areas of 2.5° of latitude by 10° of longi- 

 tude. 



Changes in Longline Catch Within 

 Areas Relative to Purse Seine Catches 



A comparison of the percentage change in the 

 1984-85 yellowfin tuna hooking rate from the 

 1979-81 rate within an area suggests that the ob- 

 served changes are not related to the magnitude of 

 the purse seine catches (Fig. 10). In Figure 10, the 

 percentage changes are compared with the purse 



seine yellowfin tuna catch from 1979 to 1983 in 

 order to allow for a time lag due to the differential 

 size or capture in the two gears. Similar results are 

 obtained if different time frames are used for the 

 purse seine catches. The spatial distribution of these 

 percentage changes suggests that the largest decline 

 in longline catch rates has occurred in the western 

 and northern borders of the area fished by longliners 

 (Fig. 11). This area overlaps, but tends to be out- 

 side of the areas of major Japanese purse seine 

 catches (Fig. 12). 



DISCUSSION 



Effect of Different Stratifications 



For both the longline and purse seine fisheries for 

 yellowfin tuna, the different stratifications yielded 

 relatively consistent patterns for the annual changes 

 in catch rates. For the purse seine rates, the fact 

 that different temporal stratifications had little ef- 

 fect on the overall annual averages is not surpris- 

 ing given the relatively equal temporal distribution 

 of effort within a year (i.e., any seasonal differences 

 in catch rates will be given approximately even 

 weight in the pooled estimates). 



Stratifications by area could have been expected 

 to have a large effect on the annual purse seine catch 

 rates given the highly clustered distribution of ef- 

 fort during any given month (Fig. 13; unpubl. re- 

 sults). The ratio of an unstratified catch rate esti- 

 mate to a stratified estimate has been defined as a 

 concentration index by Gulland (1955). A value near 

 1 for this ratio is usually interpreted to mean that 

 fishermen are not concentrating their fishing effort 

 in area and time strata where fish are most abun- 

 dant. Values for this index based on the values in 

 Table 1 range from 0.82 to 1.39. While there is some 

 tendency for the annual estimates of catch rates 

 which include stratification by area to be less than 

 the unstratified estimates, the lack of any large 

 and significant differences is due to the fact that 

 there is almost no effort outside of these areas 

 of high concentration. Thus, the data even when 

 stratified, adds little information on catch rates out- 

 side the specific areas being fished at any given 

 time. 



For the longline results, the value of Gulland's 

 concentration index ranges from 0.9 to 1.18 when 

 calculated from the values in Table 2. In this case 

 the lack of any large differences between the strati- 

 fied and unstratified estimates in Table 2 is not due 

 to effort being concentrated in only a few strata, 

 but may be related to the multispecies aspect of the 



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