AGE AND GROWTH OF YELLOWFIN TUNA 



135 



bidding had begun. As the fish are sohl indi- 

 vidually, it is necessary for the company to keep 

 accurate records of the weight of each fish sold. 

 AYeights as determined by the auction company 

 were taken from the auction records whi<'h were 

 available beginning vvith November 1947. Weights 

 of tunas caught by the live-bait fishery during 1949 

 were recorded by Fish and Wildlife Service 

 scientists at the cannery of Hawaiian Tuna 

 Packers, Ltd. This study includes only the data 

 of 1948 and 1949. 



The data for the 2 years consist of 4,793 indi- 

 vidual weights of yellovvfin tuna ranging from 5 

 to 265 pountls. Of the total number of weights 

 taken, 124 are of small fish most of which were 

 representative of four mixed schools caught by 

 live-bait methods. A few of this group were taken 

 incidentally by trolling or hand-lining. The re- 

 mainder of the data were obtained from the auc- 

 tion records. 



Since small yellowfin and big-eyed tuna are 

 likely to be confused, a check of the reliability of 

 species determination by the auction company was 

 made during October 1949. During this period 

 95 yellowfin and 272 big-eyed tuna were identi- 

 fied by various Fish and Wildlife Service scientists. 

 In no case was there found to be an auction record 

 in disagreement with our identifications. It was 

 concluded that the assignment of species as shown 

 by the auction records was accurate. 



The auction records provided an excellent source 

 of weight-frequency data for several reasons. Be- 

 cause Honolulu is the center of population in the 

 Hawaiian Islands, most of the long-line catch is 

 landed there, and most of this long-line catch 

 passes through the auction of the Kyodo Fishing 

 Co., which supplied the auction records. Fish 

 taken by long-line gear are generally few in num- 

 ber per day's fishing, which would suggest that 

 either the fish tend to be solitary in habit or, if 

 they are schooled, only a few fish from several to 

 many schools are caught during a fishing trip. 

 Since tunas tend to school by size (Brock, in un- 

 published ms.; Schaefer 1948), samples of this 

 sort which are composed of a few fish from each 

 of many schools will tend to be more nearly 

 representative than large samples drawn from 

 only a few schools as are the samples from the 

 cannery. 



Weights of fish in the round, that is, the entire 



uncleaned fish as landed at the dock, were weighed 

 on the auction company's scales or on those of the 

 Hawaiian Tuna Packers. Weights were recorded 

 to the nearest pound for long-line fish and to the 

 nearest quarter pound for small fish taken by 

 live-bait fishing. 



ANALYSIS OF WEIGHT FREQUENCY DATA 



The initial step in processing the raw data (see 

 the appendix) was to plot the weights of individual 

 fish as frequency distributions for monthly 

 periods. A class interval of 10 pounds was arbi- 

 trarily chosen, with midpoint values of 4.5, 14.5, 

 and so on. Because the monthly catches varied 

 considerably in numbers of fish, they were made 

 comparable by converting the class frequencies 

 into percentages of the total for the month. The 

 average frequency distribution for each year was 

 calculated by averaging the 12 monthly -percentage 

 curves. The results are plotted in figures 1 and 

 2 for 1948 and 1949. In order to show more 

 clearly the presence of modes, positive de^nations 

 from the mean curve for the year are shaded on 

 the graph for each month. The scale at the 

 bottom of each graph is in terms of both weight 

 in pounds and length in centimeters. The length 

 scale was derived from the equation log L= 

 1.45660 + 0.33290 log IF which was calculated from 

 a sample of 200 length-weight measurements of 

 yellowfin tmia collected during 1949 by Fish and 

 Wildlife Service scientists. 



Because there were many irregularities evident 

 in the frequency curves of each month's catch in 

 both 1948 and 1949, and because the 2 years were 

 similar in monthly frequency distributions, it was 

 convenient to combine the 1948 and 1949 data. 

 The combination of the data for the 2 years was 

 then treated in the same manner as that of the 

 the individual years with the exception of a process 

 of first smoothing the data by the formula 



-7 1 where a, b, and c, are actual values for 



consecutive class intervals. After smoothing, the 

 data were transformed into percentages of montlily 

 catch. The resulting monthly distribution curves 

 of the combined data with the superimposed 

 mean-percentage curve for the 2 yeai-s calculated 

 in the same manner as for individual years is 

 shown in figure 3. 



