Le GUEN and SAKAGAWA : APPARENT GROWTH OF YELLOWFIN TUNA 



Table 6. — Size classes (cm) of yellowfin tuna identified 

 in samples from Sao Tome to southern Angola. Year 

 classes are separated by horizontal lines. 



1 Data from Le Guen et 



(1969). 



that there is no appreciable difference in the es- 

 timate of apparent growth of yellowfin tuna from 

 the region of the eastern Atlantic, illustrated in 

 Figure 1 or from a smaller region within that 

 part such as off Pointe-Noire. 



COMPARISON OE GROWTH 

 ESTIMATES 



Studies on growth of yellowfin tuna have 

 largely been based on two types of data: length- 

 frequency distributions and scale readings. For 

 comparative purposes we chose two studies that 

 were based on scale readings — one each from 



the Pacific (Yabuta, Yukinawa, and Warashina, 

 1960) and Atlantic (Yang et al., 1969)— and 

 three studies that were based on modal progres- 

 sion of length-frequency distributions — two from 

 the Pacific (Davidoff, 1963; Moore, 1951) and 

 one from the Atlantic (Le Guen et al., 1969) — 

 for comparison with our best estimates for the 

 eastern Atlantic. The procedure of estimating 

 the parameters of the von Bertalanffy function 

 was standardized with the use of the Fabens' 

 (1965) procedure whenever appropriate data 

 were available. 



ESTIMATES FROM SCALE READINGS 



Lengths at mark formation from interpreta- 

 tion of marks on scales were reported by Yabuta 

 et al. (1960). They indicated that mark forma- 

 tion occurs twice a year, in March-April and in 

 September-October, or 6 months apart in the 

 western Pacific. An estimate of growth was 

 calculated from their data with 6 months between 

 marks (Table 7). Growth appears to be sub- 

 stantially slower in the western Pacific than in 

 the eastern Atlantic (Figure 9) . Either growth 

 is indeed slower in the western Pacific or the in- 

 terpretation of scale marks by Yabuta et al. is 

 in error. The latter possibility is suggested by 

 the absence in their data of fish greater than 

 119 cm long with a designated mark, although 

 fish as large as 161 cm long were reportedly 

 sampled. Moreover, only about 42% of their 

 scales were readable. Other studies made in the 

 western Pacific (see Shomura, 1966; Suzuki, 

 1971) suggest that growth was underestimated 

 by Yabuta et al. 



Loo = 222.8 cm and K ^ 0.023, on a monthly 

 basis, were estimated by Yang et al. (1969). 

 Their estimates were based on scale readings of 

 296 yellowfin tuna caught by the Atlantic long- 

 line fishery. Since Yang et al. used the Walford 

 (1946) procedure to estimate growth, we re- 

 calculated growth with the Fabens' procedure 

 using the data of Yang et al. and the assumption 

 that the scale marks formed every 6 months. 

 The results (Table 7) were not markedly dif- 

 ferent from the estimates by Yang et al. Com- 

 pared to our best estimate of growth rate (K), 

 on the other hand, their estimate is substantially 



183 



