ESTIMATION OF AGE AND GROWTH OF YELLOWFIN TUNA (NEOTHUN- 

 NUS MACROPTERUS) IN HAWAIIAN WATERS BY SIZE FREQUENCIES 



By Harvey L. Moore, Fishery Research Biologist 



With a commercially important species, such as 

 tlie yellowfin tuna (A'eothunnes macropterus Tem- 

 niinck and Schlcgel), knowledge of age and growth 

 is essential in both the management and develop- 

 ment of a fishery. To be able to assign ages and 

 to determine the rate of growth makes it possible 

 to determine the number and strength of the year 

 classes that comprise the fishable stock. A 

 fishery dependent on a few age groups or year 

 classes is greatly affected by the marked success 

 or failure of the brood produced in any one year. 

 The reduction or increase in numbers is strongly 

 evident in the total catch when that particular 

 year class enters the commercial fishery. If, 

 however, the fishery is composed of many age 

 groups, the success or failure of spawning in any 

 one year will have little effect on the total stock. 

 It is only when there are several consecutive years 

 of marked failure or success that any appreciable 

 differences in numbers are evident. 



The vital statistics necessary for quantitative 

 study of fish populations are based on knowledge 

 of the age composition of the stock. It would be 

 difficult indeed to determine such statistics as 

 rates of increase, decrease, fishing, and natural 

 mortality without some knowledge of age and 

 growth. These vital statistics are fundamental in 

 the management of a fish stock. 



The age and growth rate of tunas may also be of 

 value in the study of migrations, since it seems 

 logical to expect, in general, that short-lived, fast- 

 growing fish travel shorter distances than fish 

 which are long lived and slow growing. 



Since Petersen's first application of the method 

 of size-frequency study to age and growth de- 

 termination of plaice (1922) many such studies of 

 different species have been made. Much im- 

 provement in the original method has been made, 

 and the application of mathematical formulae to 

 describe the growth of fishes has contributed much 

 toward its refinement. 



Application of length- or weight-frequency 

 analysis to study of growth of tunas has been 

 limited. Kimura (1932) calculated growth curves 

 for bluefin (Thunnus orientalis) and yellowfin 

 (Neothunnus macropterus) from weight frequencies 

 of fish taken in Japanese waters from 1924 to 1931. 

 Although the data were collected over a long 

 period of time, those for yellowfin were based on a 

 few specimens if all data were included in the 

 graphs. An examination of the data, as presented, 

 shows that the values plotted in the graphs are 

 based on a few specimens of yellowfin. 



Westman and Gilbert (1941) employed length- 

 frequency distributions in their study of the 

 Atlantic bluefin (Thunnus fhynnus). The ages of 

 bluefin as determined by this work were based 

 primarilj' on scale readings although the conclu- 

 sions were correlated with the results of the length 

 frequencies. Westman and Xeville (1942), in 

 another study of the Atlantic bluefin tuna, used 

 length frequencies of tuna samples from chum- 

 ming and trolling catches made during August and 

 September 1941. The results of this study were 

 also correlated with scale readings. Brock (1944) 

 applied the method of length frequencies in a 

 study of albacore (Oerino alalunga) taken in the 

 North Pacific and was able to demonstrate the 

 growth of size groups through the albacore season. 

 Partlo (1950) has produced weight-frequency dis- 

 tributions of albacore {ThunnuN alalunga) taken 

 in the waters of British Columbia during 1949. 

 Sampling was not sufficient to show changes in 

 length throughout the albacore season, but the 

 frequency distributions show the definite size 

 groups which make up the fishery. Okamoto 

 (1940) apphed Petersen's method to weight data 

 of skipjack (Katsuwonus vagans) taken in Japanese 

 waters. It was possible to follow definite modal 

 groups through 5 montlis of the fishing season. 

 The question whether modes represented age 

 groups or whether they represented different 



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