GROWTH OF BLUEFIN TUNA 



47 



20 30 40 



FORK LENGTH-INCHES 



I 



Figure 10. — Frequency distribution by 1-inch groups of 

 lengths of bluefin tuna taken October 1-18, by years. 



and the 6-year-olds in Nova Scotia in 1949. 

 Split modes show a preponderance of 7-year-olds 

 in both areas in 1950, suggesting the progression 

 of the year class of 1943 through the Cape Cod 

 Bay fishery 1948-50 and the Nova Scotia fishery 

 1949-50. 



Two successful tagging experiments with bluefin 

 tuna yielded approximate data on their actual 

 growth. One fish, tagged off Cape Cod, Mass., 

 July 27, 1954, was recaptured by French fisher- 

 men in the Bay of Biscay August 16, 1959. When 

 tagged, the fish measured 72.5 cm., and its weight 

 when recaptured was reported as approximately 

 65-70 kilograms (143-154 pounds equivalent to 

 about 150-154 cm.). These sizes are near the 

 lower limits for ages 2 and 7, respectively, from 

 table 1. The other was tagged August 11, 1957, 

 off Chatham, Mass., and recaptured August 30, 

 1959, off Gloucester, Mass. Its weight when 

 tagged was estimated as 65 pounds (equivalent to 

 about 114 cm.) by an experienced fisherman, and 

 it weighed 130 pounds (equivalent to about 150 

 cm.) when recaptured. These lengths are in good 

 agreement with those listed in table 1 for ages 4 

 and 6, respectively. Hence the results of these 

 experiments are in reasonable agreement with our 

 age determinations by counts of annuli. 



GROWTH OF YOUNG BLUEFIN TUNA 



As length measurements are several times as 

 numerous as counts of annuli, and permit us to 

 trace growth during each summer as well as from 

 year to year, we shall base our discussion of 

 growtli of young tuna on length frequency 

 analvsis. 



10 20 30 40 



FORK LENGTH-INCHES 



Figure 11. — Frequency distribution by 1-inch groups of 

 lengths of bluefin tuna taken in 1941 off Long Island 

 and in 1950-57, mostly off Xew England, by periods. 

 Three very small tuna taken off New Jersey in 1938-40 

 are also included. 



Table 5 lists the average lengths of the fish in 

 each age group of fish more than 20 inches long, 

 as demarcated by vertical lines in figures 4-11. 

 It identifies year classes and also shows average 

 lengths for all years combined. We plotted these 

 lengths by periods in figure 14 and fitted curves to 

 them empirically, taking into account the number 

 of measurements represented by each point, ex- 

 cept in 2 or 3 where tlie preponderant samples 

 were not, in our opinion, composed of average- 

 sized fish. 



