FISHERY BULLETIN: VOL. 81. NO. 1 



MONTH 

 JFMAMJJASOND 



6,000 



<25 



25-300 



>300 



Figure 4. — Mean monthly catches of bluefin tuna (^_) for the 



13 

 period 1957-69 in metric tons per latitude. Totals are for areas 

 between a given parallel and the next higher parallel. 



Table 3.— Mean catch of bluefin 

 tuna (metric tons (t)) and mean effort 

 (boat-days) per latitude for the years 

 1957-69 and 1974. 



season there are catches both in northern and 

 southern parts of the bluefin range, whereas 

 there are relatively few catches late in the season 

 in the southern part of the range, thus indicating 

 northward movement. This shift is also apparent 

 in the number of occurrences of recorded bluefin 



1957 1960 1963 1966 1969 1972 1975 

 YEAR 



Figure 5. — Logged annual effort (boat-days/year) for bluefin 

 tuna north and south of lat. 32°N for 1957-69 and 1974. 



catch per month and latitude during the 1957-69 

 period (Table 4). The northward shift in location 

 of the largest catches does not reflect a shift in 

 fishing effort, since effort remains high in the 

 south throughout the season (Fig. 6). Apparently, 

 bluefin move northward or there is a shift in 

 bluefin vulnerability towards the north during 

 the fishing season. 



Catch and effort data for 1957-69 summarized 

 by vessel size indicate that seiners of 101-300 ton 

 capacities accounted for more than 70% of blue- 

 fin landings and that smaller vessels tended to be 

 phased out of the fishery and replaced by larger 

 ones (Table 5). 



CATCH-PER-UNIT-EFFORT 

 ANALYSIS 



Catch per unit effort (CPUE) is calculated for 

 each year as total catch divided by total effort 

 (Table 6). The relationship between the CFG 

 data and the IATTC data (Bayliff and Calkins 

 1979) is expressed as a ratio which includes the 

 origin. The ratio estimator 2,y/%x, obtained from 

 years for which both CFG and IATTC measures 

 of CPUE are available (1966-74), yielded a value 

 of 1.01, by which the CFG values (1954-65) were 

 multiplied to obtain IATTC equivalents. These 

 equivalent CPUE values were then plotted, and 

 a regression line fit to them reveals a decline in 

 CPUE with time (Fig. 7). This observed decline 

 is probably conservative because fishing effort, 

 which was not standardized, has most likely be- 

 come more effective with time (Pella and Psaro- 

 pulos 1975). 



CPUE values were highest in the northern 



110 



