5 



u 



I 



IX XI 



VII IX XI 

 MONTHS 



III 



VII 



IX 



Figure 9. Curve of linear growth of young bluefin tuna (caught off the Atlantic 

 coast of Morocco) up to the beginning of their third winter (Fumestin and 

 Dardignac 1962). 



May and June and from Nova Scotia 

 in July to September. He estimated a 

 7.5% per month increase in weight 

 of these fish during their sojourn in 

 Nova Scotia waters. 



Butler (1974) supported Rivas' 

 findings for the northern fish. He ex- 

 amined giants taken near Prince Ed- 

 ward Island between July and Octo- 

 ber 1974. By calculating weekly 

 mean weights during this period But- 

 ler showed an approximate weight 

 gain of 70 kg in 12 weeks. He refined 

 these data and calculated a 30 kg per 

 month gain between August 19 and 

 October 4, 1974, or about 10% per 

 month. 



The present authors have calcu- 

 lated length-weight relationships for 

 various monthly periods for bluefin 

 tuna taken during the fishing seasons 

 in two parts of the western North 



growth rate during the remainder of 

 the year must have been much slower 

 (about 0.8 cm per month)(Figure 8). 

 Fumestin and Dardignac (1962) 

 were able to collect material of ages 

 0-2 throughout most of the year along 

 the Atlantic coast of Morocco. Their 

 data for winter sizes fitted very well 

 with corresponding data for January 

 obtained by extrapolation from 

 Mather and Schuck's (1960) results. 

 It also showed that growth virtually 

 ceased from January to March at age 

 0, and at the end of November, when 

 the fish were about 63 cm long, at 

 age 1. Data for age 2 were incom- 

 plete, but suggested that in autumn 

 they had practically reached their 

 winter length of about 85 cm 

 (Figure 9). 



4. Seasonal Changes in the 

 Length- Weight Relationship of 

 Large Bluerm Tuna 



Extensive seasonal changes in 

 the length-weight ratio of large blue- 

 fin tuna have been found by many 

 investigators working in different ar- 

 eas which collectively represent a 



considerable part of the coastal habi- 

 tat of the species. 



In the North Sea area Bahr 

 ( 1 952), Tiews (1957) and Luhmamn 

 ( 1 959) observed an increase in weight 

 in relation to length during the fish- 

 ing season of August to October. 

 Tiews (1957) showed that tunas of 

 215-240 cm increased their weight 

 during their 2-3 month stay in the 

 North Sea by about 1 1.0 kg in 1954 

 and 17.4 kg in 1955. This worked 

 out to about 34% to 54% of their 

 yearly weight increase. Luhmamn 

 (1959) pointed out that variations in 

 the length-weight relationship in dif- 

 ferent years might be directly associ- 

 ated with variations in feeding con- 

 ditions. 



Rodriguez-Roda (1964) ob- 

 served that during the spawning sea- 

 son, from May to August along the 

 south Atlantic coast of Spain, mature 

 fish lost about 1 4.73% of iheir weight 

 between the pre- and post-spawning 

 state. 



In the western Atlantic. Rivas 

 ( 1 955) studied records of the weights 

 of giant tuna from the Bahamas in 



1.4 



1.2 



r 1.0 



0.8 



0.6 



Fork Length / Moi Girth 

 — "i^ Ftmoltt 



Fork Length/ Total Weight 

 Femolet 



0^ L 



July Aug Stpt Oct No» 



Figure 10. Seasonal ratiosolfork length 

 to maximum girth and fork length to 

 total weight by sex for all areas 

 combined. (Caddy et al. 1976) 



12 



