GROWTH RATES OF NORTH PACIFIC ALBACORE, 

 THUNNUS ALALUNGA, BASED ON TAG RETURNS 



R. Michael Laurs and Jerry A. Wetherall 



ABSTRACT 



Estimates of growth parameters for North Pacific albacore, Thunnus alalunga, were based on tag- 

 recapture statistics, using the standard von Bertalanffy model and an extended model. Sequential 

 estimation of Lx and K allowed us to test hypotheses concerning variation in growth rate between 

 tagged albacore recaptured in different ocean regions. 



Significantly lower growth rates were found in albacore recaptured off Japan and the United States 

 north of latitude 40° north compared with those recaptured off the United States south of latitude 40° 

 north. The differences in estimated growth rate were generally consistent with differences in length- 

 frequency distributions of albacore taken off the United States north and south of latitude 38° north 

 during the period when most recaptures were made. The findings add to a growing body of evidence 

 that the North Pacific albacore population is not homogeneous; rather, at least two different 

 subpopulations may exist. 



Growth rates of North Pacific albacore, Thunnus 

 alalunga (Bonnaterre), have been estimated by 

 counting vertebral rings (Uno 1936; Aikawa and 

 Kate 1938; Partlo 1955), examining scale circuli 

 (Nose et al. 1957; Bell 1962; Yabuta and Yukinawa 

 1963), tracing progressions of length modes (Brock 

 1943; Suda 1954), and by measuring tagged fish at 

 release and recapture (Otsu 1960; Clemens 1961). 

 Of these techniques, only tagging provides direct 

 estimates of growth rate, and the tagging results 

 of Otsu and Clemens are reasonably consistent 

 with the conclusions of Yabuta and Yukinawa's 

 scale analysis and Suda's modal progression work. 

 However, as Shomura (1966) noted in a review of 

 tuna growth studies, comparisons are complicated 

 by the biases and uncertainties peculiar to each 

 method. For example, in the case of tagging we 

 assume that the growth rate is unaffected by 

 stresses resulting from capture, handling and 

 tagging, and from the burden of carrying the tag 

 itself. Conclusive results will require that the 

 basic assumptions of any particular method be 

 tested and verified. 



In this paper, we present new estimates of 

 growth parameters based on recent tag-recapture 

 experiments conducted jointly by the National 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, RO. Box 271, La Jolla, CA 

 92038. 



^Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, RO. Box 3830, Honolulu, HI 

 96812. 



Marine Fisheries Service (NMFS) and the Amer- 

 ican Fishermen's Research Foundation (AFRF). 

 We use the standard von Bertalanffy growth 

 model, but also briefly explore an extension. 

 Sequential estimation of the parameters Ly^ and 

 K allows us to test hypotheses concerning varia- 

 tion in growth rate between tagged fish recap- 

 tured in different ocean regions. 



Transpacific recaptures of albacore tagged in 

 the eastern North Pacific off North America and 

 in the western North Pacific off Japan have 

 established the interdependence of the United 

 States and Japanese North Pacific albacore fish- 

 eries, and have also fostered the hypothesis of a 

 single, common stock (Ganssle and Clemens 

 1953; Clemens 1961; Otsu and Uchida 1963). How- 

 ever, our results add to growing evidence (Laurs 

 and Lynn 1977) that the North Pacific albacore 

 population is not homogeneous, as usually as- 

 sumed, but is composed of at least two subgroups 

 with different migration patterns and growth 

 histories. 



METHODS 



lagging Procedures 



Albacore were caught in the eastern North 

 Pacific and tagged aboard U.S. commercial jig and 



■■'AFRF administers revenues derived from a landing assess- 

 ment paid by the U.S. albacore industry on U.S. -caught albacore. 



Manuscript accepted December 1980. 

 FISHERY BULLETIN: VOL. 79, NO. 2, 1981. 



293 



