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FISHERY BtlLLETIN OF THE FISH AND WILDLIFE SERVICE 



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FORK LENGTH-INCHES 



60 



Figure 12. — Frequency distribution by 1-inch groups of 

 lengtiis of bluefin tuna for age groups 0-IV as deter- 

 mined by length frequencies (solid lines) and by counts 

 of annuli (dotted lines). 



The fish less than 20 inches long listed in table 4 

 obviously form a distinct age group. Figure 15 

 shows the averages of these measurements with a 

 curve drawn by inspection to fit the points and 

 also to fit in with the curve for 1-year-olds from 

 figure 14. Although our unpublished studies of 

 the distribution of the bluefin tuna indicate that 

 all these fish belong to one population, samples 

 from different areas have been designated by 

 different symbols. Figure 16 shows a curve of 

 estimated growth of bluefin tuna for the first 

 4}i years of life, and table 6 lists the average sizes 

 at the middle of each month, as indicated by 

 this curve. 



Figure 16 indicates extremely rapid initial growth 

 and distinct seasonal variations in growth rate. 

 Bluefin tuna spawn during an undefined period in 

 spring (Rivas, 1954; Bullis and Mather, 1956). 

 Assuming, as we did in drawing figure 16, that 

 hatching occurs in mid-May, the young may 

 grow at a rate of nearly 6 inches per month to 

 reach a size of 8)^ inches by July 1. In the 

 ensuing discussion, however, we shall consider 

 July 1 as the date of birth and shall refer to the 

 period July 1-October 16 as "summer" and the 

 remainder of the year as "winter." The growth 

 rate diminishes rapidly during the first summer, 

 but the average rate is estimated at 2 inches per 

 month. The rate continues to decrease during 

 most of the first winter, averaging about four- 



70 100 



200 



270 



WEIGHT-POUNDS 



Figure 13. — Weight frequencies by 10-pound groups of 

 bluefin tuna from 70 to 270 pounds, taken in Cape Cod 

 Bay and ofT Nova Scotia in 1948-51, by years. 



fifths of an inch per month. For the remainder 

 of the period studied, ages 1-4, the growth rate 

 does not change greatly with age, averaging about 

 iy2 inches per month in summer and about one- 

 third of an inch per month in winter. 



ESTIMATED ANNUAL GROWTH OF BLUE- 

 FIN TUNA THROUGH 10 YEARS 



The average sizes at each age, determined by 

 length frequency analysis for ages 0-4, and by 

 counts of annuli for older ages, are plotted in figure 

 17. The curve shown was derived graphically, 

 starting from the point for 3-year-olds, from the 



