POLACHECK: YELLOWFIN TUNA CATCH RATES 



However, for estimates stratified by both area and 

 season, only the 1984 estimate would be significantly 

 different from any of the other stratified means 

 within a given year. A lack of consistent differences 

 among the various stratifications within years does 

 not mean that siginificant area and seasonal 

 differences may not exist, but only that whatever 

 effects may exist tend to balance in the present 

 data. 



Among the various annual estimates in Table 1, 

 the estimates for 1983 tend to be the lowest (perhaps 

 reflecting the large El Nifio of that year), while those 

 for 1979 and 1986 tend to be the highest. While the 

 length of the time series is short, there is no indica- 

 tion within any of the stratifications of an overall 

 temporal trend in the annual estimates. 



Relationship Between 

 Purse Seine Catch and Effort 



A production plot of total monthly catch versus 

 total monthly effort suggests that monthly catch 

 rates can be highly variable and that months with 

 the highest effort tend to have lower catch rates 

 (Fig. 3). Thus, the catch rates in the 6 months in 

 which the total effort exceeded 500 days of effort 

 are all below the overall mean catch rate (Fig. 4) 



12 



11 - 

 10 



9 



8 



> 

 o 



■D 7 |- <h 



o o 



o o o oo 



o o 



—9 O- o 



O O D 



O 



_1 L_ 



100 200 300 400 500 

 Effort - days fished plus searched 



600 



Figure 4.— The relationship between monthly catch rates (metric 

 tons per day) and fishing effort of yellowfin tuna by Japanese purse 

 seiners. The horizontal lines represent the mean catch rate for each 

 100 day range of effort. 



and the average of the catch rate for these 6 months 

 is 38% below the mean catch rate for all 77 months. 

 However, the results in Figure 3 should not be in- 

 terpreted in terms of a general catch curve because 

 the changes in catch rates associated with change 

 in effort appear to be too large to be a reflection 

 of the overall population dynamics (see Discussion). 

 These lower catch rates at higher effort levels 

 are not as apparent when a quarterly stratification 

 of the data is considered (Fig. 5), and there is no 

 evidence for these lower rates with an annual 

 stratification (Fig. 6). Caution is warranted in in- 

 terpreting any of these figures as general catch 

 curves since they are not based on total catch and 

 effort statistics for the yellowfin tuna surface 

 fisheries (most significantly, the lack of information 

 from the United States and other eastern Pacific 

 vessels). Also, note that for all of the catch curves, 

 statistics from 1986 are not included because of 

 current incompleteness of currently available 

 data. 



Longline Catch and Effort 



Effort by Japanese longliners has been relatively 

 constant, but with some decline in recent years. 

 However, a large amount of monthly variation oc- 

 curred, with a suggestion of seasonal periods of 

 reduced effort during the second half of the year 

 (Fig. 7A). Catches also exhibit a large amount of 

 monthly variation, but suggest a declining trend 

 since 1982 (Fig. 7B). As with the purse seine 

 statistics, the drop in catch and effort in 1986 is due 

 to the time lag in receiving catch reports. There has 

 been a general decline in the average hooking rate 

 since 1979-80, except for 1983 (Fig. 8). 



Comparison of estimates of the average annual 

 catch rates of yellowfin tuna for various combina- 

 tions of temporal and area stratification shows a 

 consistent temporal pattern (Table 2) which is simi- 

 lar to the pattern shown by the monthly rates in 

 Figure 8. The annual estimates of the average catch 

 rate tend to be highest in 1983, and the lowest esti- 

 mates occur either in 1985 or 1986. The high catch 

 rates in 1983 might be related to a change in vul- 

 nerability as a result of the large El Nino which oc- 

 curred during this year. The 1985-86 estimates are 

 about 33% below the 1979-80 levels. Whether over- 

 all the catch rates in this short time series indi- 

 cate a general decline depends critically upon the 

 interpretation given to 1983 catch rates (see Dis- 

 cussion). 



Similar to the purse seine estimates, there is 

 no consistent pattern among the stratified annual 



129 



