MATSUMOTO ET AL. : LARVAL TUNA IDENTIFICATION 



Once the young tuna has acquired the full 

 complement of spines and rays in all the fins, 

 complete ossification of all the vertebrae, and the 

 relocation of the anus near the origin of the anal 

 fin, it is generally considered a juvenile of the 

 species. Certain characters such as the full num- 

 ber of gill rakers, however, develop much later, 

 when the juvenile has attained a length of 40 

 or 45 mm SL. If we consider juveniles to in- 

 clude all sizes up to the time of full gonad de- 

 velopment signified by initial spawning, the size 

 range of the juvenile stage would extend from 

 about 13 mm SL to 700 mm FL (fork length) in 

 T. albacares (Yuen and June, 1957) and to 860 

 mm FL in T. alalunga (Otsu and Hansen, 1962) . 

 For the purpose of clarifying species identifi- 

 cation of the young, however, individuals beyond 

 200 mm SL need not be included. The term ju- 

 venile, as used here, thus refers to tunas between 

 13 and 200 mm SL. 



EVALUATION OF CHARACTERS 



The greatest difficulty in identifying juveniles 

 of Thunnus is that the most useful characters 

 are located internally. Except for the flattened 

 first elongate haemal spine in T. alalunga, there 

 is no single character that is peculiar to each 

 of the species; but by using a combination of 

 characters it should be possible to identify most 

 of the other species. A summary of the most 

 useful characters discussed is listed in Table 5. 



The size of juvenile at which each of the char- 

 acters can be observed is listed also. Those char- 

 acters whose usefulness in the early juvenile 

 stages has not been shown conclusively are indi- 

 cated by a question mark (?). The general 

 formula of distribution of pterygiophores of the 

 second dorsal fin has not been used before. 



The counts and descriptions given for those 

 characters listed with a question mark gener- 

 ally are those of the adults. These have not yet 

 been substantiated for juveniles as well. 

 Changes in the position of the first haemal arch 

 with growth, for example, have been known to 

 exist in other closely related fish such as the 

 wahoo, Acanthocyhium solandri (Matsumoto, 

 1967). This could be true of the tunas also. 



Comparisons of body parts, particularly of or- 

 bit diameter, body depth at origins of the first 

 dorsal and anal fins, preanal and postanal dis- 

 tances, and snout length, have not been investi- 

 gated sufficiently in the past. The unavailability 

 of specimens in sufficient numbers as well as the 

 nonuniformity of body lengths (fork and stan- 

 dard) used have contributed greatly to this ne- 

 glect. Acceptance of standard length as the 

 standard measure of body length and publishing 

 of actual measurements in the future should help 

 in the accumulation of sufficient data for anal- 

 yses. This has to be done by all workers in this 

 field of study, since the juveniles are not easily 

 taken in large numbers. 



Table 5. — Characters for separating juveniles of Thnnnus species. 



Character 



Useful on 

 juveniles obove 



Thunnus thynnus Thunnus alalunga Thunnus atlanticus Thunnus obesus 



Thunnus albacares 



^ Da refers to second dorsal fin. 



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