ESTIMATED GROWTH OF SURFACE-SCHOOLING SKIPJACK TUNA, 



KATSUWONUS PELAMIS, AND YELLOWFIN TUNA, 

 THUNNUS ALBACARES, FROM THE PAPUA NEW GUINEA REGION 



J. W J. Wankowski' 



ABSTRACT 



The study was undertaken on length-frequency data collected from the Papua New Guinea pole-and- 

 line fishery between June 1977 and December 1979. 



Both skipjack and yellowfin tunas are recruited to all areas of the fishery at between 30 and 46 cm fork 

 length. Skipjack tuna remain in the exploited phase up to an average 69 cm fork length and yellowfin 

 tuna up to an average 85 cm fork length. Periods of time during which the greatest range of fork length 

 occurred in the catch correspond with periods of low abundance, as inferred from catch per unit of effort 

 indices. The estimated von Bertalanffy parameters are k = 0.0429 and L ^. = 74.8 cm for skipjack tuna; 

 and^ = 0.0243 and Lx = 180.9 cm for yellowfin tuna (^ on a monthly basis). Estimated growth over the 

 observed range of modal values corresponds closely with that estimated from Papua New Guinea 

 tagging data for skipjack tuna and from studies in other regions for yellowfin tuna. 



Modal progressions indicate a 12-month periodicity in mass movement of yellowfin tuna stocks in 

 northerly and southerly directions. The presence of two skipjack tuna spawning groups, one spawning 

 during the northern summer and the other during the northern winter, is indicated by back calculation 

 to date of birth of all length-frequency modes using an estimate of growth derived from the tag and 

 recapture data. 



With the recent expansion of surface fisheries for 

 skipjack tuna, Katsuwonus pelamis , and yellowfin 

 tuna, Thunnus albacares , in the Pacific (Bour and 

 Galenon 1979; Kearney^) the need for information 

 on this resource is becoming increasingly impor- 

 tant for rational management. One method that 

 might provide the quantitative information neces- 

 sary is that of estimating yield per recruit 

 (Schaefer and Beverton 1963), for which an esti- 

 mate of growth is necessary. 



Three techniques of obtaining growth estimates 

 are in common use: the analysis of tag and recap- 

 ture data, the analysis of data from the examina- 

 tion of hard parts of the fish for growth marks, and 

 the analysis of modal progressions in length- 

 frequency distributions. All three techniques have 

 been used, with varying success, throughout the 

 world for both yellowfin and skipjack tunas. The 

 results of these studies have recently been re- 

 viewed by Le Guen and Sakagawa (1973) and 



'Kanudi Fisheries Research Laboratory, Department of Pri- 

 mary Industry, PO. Box 2417, Konedobu, Papua New Guinea; 

 present address: Ministry for Conservation, Marine Science 

 Laboratories, PO. Box 114, Queenscliff, Victoria 3225, Australia. 

 Kearney, R. E. 1979. An overview of recent changes in the 

 fisheries for highly migratory species in the western Pacific 

 Ocean and projections on future developments. South Pac. Bur. 

 Econ. Co-op., Fiji, SPEC ( 79)17, 99 p. 



Josse et al. (1979). Both studies recalculated pub- 

 lished growth estimates using standardized pro- 

 cedures and indicated that, since the variances of 

 the estimates were so wide, calculated growth 

 rates for each species were not dissimilar among 

 geographical areas. 



Studies of growth of tunas in the western Pacific 

 have been few Yabuta et al. (1960) investigated 

 growth of longline-caught yellowfin tuna, Lewis^ 

 reported the results of aging studies of skipjack 

 tuna using readings of "daily" growth increments 

 on otoliths, and Josse et al. (1979) analyzed skip- 

 jack tuna growth from the results of tagging 

 studies conducted in Papua New Guinea in 1971- 

 74. 



This paper presents the results of a program of 

 length-frequency data collection carried out from 

 June 1977 through December 1979 from the pole- 

 and-line (baitboat) fishery operating in Papua 

 New Guinea waters. The results are presented as 

 length-frequency modal progressions from which 

 an estimate of growth is compared with estimates 

 available from published sources. Since the 



^Lewis.A.D. 1976. The relevance ofdata collected in Papua 

 New Guinea to skipjack studies in the western Pacific. Unpubl. 

 manuscr., 5 p. Kanudi Fisheries Research Laboratory. PO. Box 

 2417, Konedobu, Papua New Guinea. 



Manuscript accepted February 1981. 

 FISHERY BULLETIN: VOL. 79, NO. 3, 1981. 



517 



