Le GUEN and SAKAGAWA: APPARENT GROWTH OF YELLOWFIN TUNA 



tributed to lack of data on older fish as was dis- 

 cussed earlier. The range of mean length is 50 

 to 150 cm. 



Estimated Length at Age 



An estimate of L« --= 194.8 cm, and K = 0.035, 

 on a monthly basis, was derived from data for all 

 year classes and regions combined (Table 3). 

 We believe these estimates are the "best," on the 

 average, for yellowfin tuna of the eastern At- 

 lantic Ocean, because they were based on data 

 from a broad geographic area ofl^ Africa and a 

 wide range of sizes. The estimated growth 

 curve is quite similar to that for Abidjan, Gulf 

 of Guinea, and Pointe-Noire (Figure?), For in- 

 dividual year classes, however, estimates of L^ 

 and K can be expected to deviate from the av- 

 erage, since there is an apparent diflference in 

 apparent growth among year classes (Table 3) 

 and considerable scatter of observed mean 

 lengths around the average curve (Figure 8). 



Data on growth of tagged yellowfin tuna in 

 the eastern Pacific and on the time of spawning 

 in the Atlantic were used to estimate U in months. 

 The above best estimates of parameters of the 

 von Bertalanfl'y equation were then used to esti- 

 mate the length of yellowfin tuna at particular 

 ages. A tacit assumption of this method of esti- 

 mating length at age is that the von BertalanflFy 

 function is a valid growth model for yellowfin 

 tuna, and the date of birth is constant. 



Schaefer, Chatwin, and Broadhead (1961) re- 

 ported that yellowfin tuna of 40-49 cm long at 

 tagging grew at a rate of 33 cm /year. They in- 

 dicated that growth was probably adversely af- 

 fected by tagging, implying that their estimate 

 was too low. 



Le Guen et al. (1969) reviewed the literature 

 on time of spawning of yellowfin tuna in the At- 

 lantic Ocean. They concluded that spawning oc- 

 curred primarily at temperatures greater than 

 26°C and salinity of about 33. 5^;^. From seasonal 

 distributions of temperature, salinity, and tuna 

 larvae captured off" Africa, they estimated that 

 spawning peaked on about March 1 off Pointe- 

 Noire and on about July 1 oflF Dakar. 



From the above information together with the 

 fact that recruitment into the surface fishery is 



APMRENT AGE IMOejTHS) 



Figure 8. — Mean length of year classes of yellowfin tuna 

 as a function of age. The curv^e is for all year classes 

 and regions combined and is estimated by a von Bert- 

 alanffy growth function. Average of observed mean 

 lengths (circles) and the range of mean lengths (vertical 

 line) at various estimated ages are shown. 



generally from June to December, we estimated 

 that yellowfin tuna were, on the average, 18 

 months old at recruitment, about 60 cm long, and 

 ^0 = 7.48. Estimates of length at age were cal- 

 culated with Zoo = 194.8, K = 0.035, and to = 

 7.48, employed in the von Bertalanflfy function 

 (Table 4). The estimates are graphed in Fig- 

 ure 8, together with monthly mean lengths of 

 individual year classes of each region. There is 

 considerable scatter of the data about the line 

 and an indication that lengths at age 50 months 

 and older are overestimated. 



Estimated Weight at Age 



Length can be converted to weight with a 

 weight-length relation. Lenarz (see footnote 4) 

 reported that the weight-length relation for yel- 

 lowfin tuna from the eastern Atlantic is W = 

 0.0000214 L/^•«■^^ where W = weight in kilo- 

 grams and Lf = fork length in centimeters. This 

 equation was employed to convert estimates of 

 length at age to weight at age (Table 4) . 



ANALYSIS WITH APPARENT 

 KNOWN AGE FISH 



METHODS 



The method of analysis with apparent known 



181 



