FISHERY BULLETIN: VOL. 79, NO. 3 



70-1 



60- 



50- 



40- 



1978 1979 



Figure 7. — Monthly modal progressions for yellowfin tuna in 

 1978 and 1979, .showing apparent movement through the Papua 

 New Guinea region from the eastern Bismarck Sea in a north and 

 northwest direction through the New Hanover areas (line), and 

 no apparent movement out of the area north of New Hanover 

 (dashed line). 



the north of New Hanover, or sometimes in the 

 New Hanover area in general, and terminating in 

 the eastern Bismarck Sea (Figure 8, which shows 

 those portions of data in Figure 4, indicating the 

 reverse movement offish). A possible northward 

 movement of yellowfin tuna recruited into the 

 eastern Bismarck Sea in July and August 1977 is 

 also indicated from the 1977 data (Figure 4). 



80- 



70- 



60 



X 50- 



40- 



""T" 



1979 



Figure 8. — Monthly modal progressions for yellowfin tuna in 

 1978 and 1979 showing apparent movement through the Papua 

 New Guinea region from the New Hanover north area, south and 

 southeast into the eastern Bismarck Sea area (line), and no 

 apparent movement out of the Bismarck Sea (dashed line). 



The periods of northward movement (of yellow- 

 fin tuna recruited to the eastern Bismarck Sea 

 fishery in April and May 1978 and between August 

 and October 1979) coincide with the appearance of 

 small fish in the catch. However, since groups of 

 fish of similar size appeared to move both north 

 and south during 1978 and the first half of 1979, 

 the direction of movement cannot be explained on 

 the basis of size or age alone. During these periods 

 of northward movement, all other groups of fish 

 recruited to the fishery were restricted to the New 

 Hanover area, almost exclusively to the north 

 (Figure 4). These observations imply either a situ- 

 ation similar to that occurring in skipjack tuna 

 where some fish penetrate no farther south than 

 the New Hanover fishery or the northward move- 

 ment of groups offish that for some reason are not 

 apparent in the Bismarck Sea catch. Groups offish 

 recruited at the same time as those moving in a 

 southward direction (November 1978- June 1979) 

 appeared to be restricted to the Bismarck Sea 

 (Figure 4). 



The results indicate extensive emigration and 

 immigration of some of the yellowfin tuna stock, 

 while other parts of the stock show little move- 

 ment during their brief period of persistence in the 

 fishery. The path taken appears similar to that 

 shown by skipjack tuna stocks, and that proposed 

 by Inoue (1969) for yellov^^n tuna, not a surprising 

 result in view of the geographical constraints of 

 the region and the distribution of fishing effort. 

 There is no evidence for emigration soon after 

 recruitment, nor of source of recruitment, and 

 groups of yellowfin tuna show mass movement 

 either clockwise or anticlockwise through the 

 Bismarck Sea with no indication of any retracing 

 of their route. 



Fundamental differences between the skipjack 

 and yellowfin tuna stocks seem to lie in the long- 

 term persistence of groups of skipjack tuna in the 

 fishery (Figures 3, 4), probably an important func- 

 tion of their slower growth rate (Josse et al. 1979; 

 present paper), while, in contrast, yellowfin tuna 

 stocks remain in the exploitable size range for a 

 few months only, since their faster growth rate (Le 

 Guen and Sakagawa 1973; present paper) soon 

 takes them out of the exploitable size range. 



Estimated Length-At-Age 



The von Bertalanffy parameters estimated from 

 all data and all areas combined were k = 0.0429 

 andLx = 74.8 cm for skipjack tuna, and /j = 0.0243 



526 



