WANKOWSKI: ESTIMATED GROWTH OF SURFACE-SCHOOLING TUNAS 



long-range pole-and-line fisheries in the equato- 

 rial western Pacific take fish 30-100 cm FL (but 

 mainly <70 cm), harvested well before they enter 

 the deeper water longline fishery that exploits fish 

 mainly >100 cm FL. 



However, Wright (1980) indicated that a propor- 

 tion of Japanese longline catches consist of yel- 

 lowfin tuna of 70-100 cm FL which are discarded 

 on capture owing to their unsuitability for the 

 Sashimi (raw fish) market that this fishery 

 supplies. The equatorial western Pacific purse 

 seine fishery exploits fish from 30 to 100 cm FL 

 (Kikawa and Warashina 1972; Warikowski and 

 Witcombe^''), about half the catch consisting offish 

 <100 cm. The absence of significant numbers of 

 yellowfin tuna >70 cm in the Papua New Guinea 

 pole-and-line catch (Figure 5) therefore indicates 

 selectivity for smaller fish (confirmed by the ab- 

 sence of suitable gear, on Papua New Guinea- 

 based vessels, for poling larger fish on board) and 

 possible recruitment of 70-100 cm size class from 

 the surface fishery into the longline fishery (yel- 

 lowfin tuna in this region presumably spending 

 less time at the surface with increasing age). How- 

 ever, that this size class of yellowfin tuna does not 

 wholly enter deeper waters is indicated by obser- 

 vations of their presence in surface schools in late 

 1977 and late 1979, and by the fact that Japanese 

 longliners set shallow lines when their target 

 species is yellowfin tuna ( Wright^ ^). 



A similar point of note is the low incidence of 

 skipjack tuna >65 cm FL (Figure 5) in the catch. 

 This is in contrast to the situation in the central 

 and eastern regions of the Pacific where larger 

 skipjack tuna are common (Rothschild 1965; 

 Doumenge^^), but is similar to that in the western 

 Indian Ocean (Marcille and Stequert 1976b). 

 Again, gear selectivity in the different regions 

 might account for this difference. However, skip- 

 jack tuna >65 cm FL appear to compose a portion 

 (discarded) of the longline catch in the equatorial 

 western Pacific (Wright footnote 11). Barkeley et 

 al. (1978) concluded that skipjack tuna >4.5 kg 

 (about 60 cm FL) would be unable to inhabit the 



i°Wankowski, J. W. J., and D. W. Witcombe. 1979. Fish as- 

 sociated with floating debris in the equatorial western Pacific 

 purse-seine fishery. Unpbul.manuscr.,13p. Kanudi Fisheries 

 Research Laboratory, RO. Box 2417, Konedobu, Papua New 

 Guinea. 



"Wright, A., Fisheries Biologist, Kavieng Fisheries Research 

 Laboratory, PO. Box 101, Kavieng, Papua New Guinea, pers. 

 commun. 1979. 



^^Doumenge, F 1973. The development of tuna and skip- 

 jack fisheries in French Polynesia and experience in live-bait 

 technique. South Pac. Comm. Fish. Newsl. 10:27-30. 



warm surface water of the tropics, unless they 

 were able to make frequent incursions into cooler 

 water, for example below a shallow thermocline. 



Stock Movements 



A stock is defined as the exploitable group offish 

 existing in a particular area at a particular time 

 (Anonymous footnote 9). If it is assumed that a 

 serial progression of length-frequency modes with 

 time represents the progress of one group of fish 

 through the fishery, then it should be possible to 

 follow the movements of that group among the 

 four fishing areas. While it is possible to do this, 

 the analysis of skipjack tuna movements indicates 

 only the complexity of the situation: groups offish 

 apparently moving freely and rapidly (often 

 v/ithin 1 mo) between areas. No pattern nor 

 periodic movement can be inferred from the pres- 

 ent modal data. However, conclusions from Lewis 

 (1980a, b, footnotes 3, 8) from the Papua New 

 Guinea skipjack tuna tagging program are sum- 

 marized below. 



Skipjack tuna appear to be recruited from east 

 of Papua New Guinea and from north of the 

 Equator, and to move clockwise around the eastern 

 and southern parts of the Bismarck Sea. Some fish 

 appear to retrace this route up to 2 yr later, while 

 others emigrate northward out of this area soon 

 after recruitment. At least part of the stock, how- 

 ever, undergoes little translocation, remaining in 

 one area for a considerable period of time. Most 

 entries and exits appear to be through the north- 

 ern Bismarck Sea between New Hanover and 

 Manus. Some fish do not penetrate as far south as 

 the eastern Bismarck Sea fishery, remaining in 

 the New Hanover area only. While these results 

 indicate extensive emigration and immigration, 

 the skipjack tuna stocks cannot be considered 

 purely transient since only a small portion of the 

 tagged fish was recovered outside the Papua New 

 Guinea region (although the variable distribution 

 of fishing effort outside Papua New Guinea waters 

 precludes definitive conclusions). 



The present modal progressions indicate two 

 types of movement of the yellowfin tuna: one com- 

 mencing in the eastern Bismarck Sea, entering 

 the area south of New Hanover and sometimes 

 progressing to north of New Hanover (Figure 7, 

 which shows those portions of the data in Figure 4, 

 indicating movement of fish in the directions 

 under discussion), and the reverse movement, in- 

 dicated by the first appearance of a modal group to 



525 



