Le GUEN and SAKAGAWA : APPARENT GROWTH OF YELLOWFIN TUNA 



I50r 



1969 



JFMAMJ JASONDJFMAMJ JASONDJFMAMJJASOND 

 1968 1969 1970 



Figure 4. — Mean length of size groups of yellowfin tuna 

 as a function of sampling date at Dakar. Growth of the 

 1965-69 year classes are indicated. 



I75r 



150- 



E 



■« 125 



100 



75 - 



50 



JFMAMJJASONOJFMAMJJASONDJFMAMJJASOND 

 1968 1969 1970 



Figure 5. — Mean length of size groups of yellowfin tuna 

 from the Gulf of Guinea. Growth of the 1965-69 year 

 classes are indicated. 



in some years. Recruitment is assumed to be 

 completed in the second year of life (Le Guen 

 et al, 1969). 



RESULTS 



Recruitment 



Yellowfin tuna are recruited into the surface 

 fishery when about 60 cm long. Recruitment is 

 year-round but most pronounced during June to 

 December. Two groups of yellowfin tuna appear 

 to be recruited in some years. For example, in 

 1968 at Pointe-Noire (Figure 6) one group en- 

 tered in January and another in August-Septem- 

 ber. The January group was of low relative 

 abundance and persisted up to a length of about 

 90 cm, while the August-September group was 

 of high relative abundance and discernible up to 

 a length of about 140 cm. A similar phenomenon 

 was described by Hennemuth (1961) and later 

 verified by Davidoff (1963) for yellowfin tuna 

 of the eastern Pacific Ocean. Hennemuth sug- 

 gested that sampling bias, differential growth 

 in a year class, and multiple spawning were some 

 possible causes of the phenomenon. Variation 

 in the seasonal distribution of fishing effort can 

 be added as another possible cause. 



Year Class Difference in Apparent Growth 



Estimates of apparent growth for individual 

 year classes for each region are shown in Table 3. 



75 



^ -OI963 



^ 



o &■ 



0-' o 



' I I I I I I I I I I I I I I I I I I I I I I I  I I I I I I I I I I I I I I I I ■' I I I I I I I I I I I [ I I 



^I96S 



JFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASONOJFMAMJJASONO 

 1965 1966 1967 1968 1969 1970 



Figure 6. — Mean length of size groups of yellowfin tuna as a function of sampling date at Pointe- 

 Noire. Growth of the 1963-69 year classes are indicated. 



179 



