tions of T. albacares show different regressions, V)ut 

 in adults of this species the fins always become higher 

 than in any other species of Thunnus. In the ecjua- 

 torial Pacific the fin heigiits vary in clinal fashion 

 from highest in the west to lowest in the cast. 

 Western Atlantic specimens have very higli fins, 

 therel)y rcsemhUng those from the western eriuatorial 

 Pacific; eastern Atlantic (Angola) specimens have 

 lower fins, as do those from the eastern Pacific. 



At sizes between 350 and fiOO mm., T. tonggol 

 appears to have higher fins than any other species of 

 Tluninus. From aliout oOO to SOO mm., T. alahtriga 

 has tlie lowest fins. The otlier five species are diffi- 

 cult to distinguish imtil al)out 700 mm. Beyond 

 800 mm., T. albararcs clearly develops the highest 

 fins, T. Ihynnus and T. alaliiugo remain the shortest, 

 and T. obesus and T. tonggol are intermediate. 



The pectoral fin shows a growth pattern that is 

 probably similar in all species, but which iliffers 

 among the species in size of fin and fork length at 

 times of inflection. Simply stated, the pectoral fin 

 imdergoes a period or stanza of increase in length 

 relative to fork length in juveniles, followed by an 

 isometric period that leads into a final and contin- 

 uous stanza of relative decrease in length (fig. 2G). 

 Size range of the stanzas is shown in table 3 for each 

 ade(iuately represented species. The smallest T. 

 tonggol for which data were available are probably 

 in the size range at which the distinction between 



increasing relative fin size and isometry is difficult 

 to distinguish; hence the stanzas could not be 

 determined. 



5 »-^ 



Figure 26. — Relative length of pectoral fin in Thunnus 

 species. Data, in addition to our own, include Dung and 

 Royce (1953: tables 12, 21, 27, 28, 42, 45-50), Godsil and 

 Holmberg (1950: 54-55), and Serventy (1956b). 1—7'. 

 atlanlicus; 2 — T. tonggol; 3 — T. albacares; 4 — T. obesus; 

 5 — T. alalunga; 6 — T. thynnus; 7 — T. maccoyii. 



T.\Bi,E 3. — Approximate range in fork length of growth stanzas 

 of pectoral fins in species of Thunnus. Smallest size limited 

 by available data 



At less than 500 mm., only T. thynnus is distinct, 

 with pectorals 21 percent of fork length or less. 

 Data for T. maccoyii are lacking. All other species 

 overlap more or less in the 25-31 percent range, 

 although between 400 and 500 mm. species distinc- 

 tions begin to be apparent (viz. Pacific T. obcsun 

 pectorals become relatively longer, those of T. tonggol 

 shorter). 



The marked positively allomctric growth of the 

 pectorals of T. alalunga and Pacific T. obesus makes 

 these two forms clearly distinctive from all others 

 between 500 and 1,200 mm. (fig. 20). Most speci- 

 mens have fins 34-46 percent of fork length. I'p to 

 700 mm., T. alalunga has slightly shorter fins than 

 Pacific T. obesus, but from 700-1,200 mm. they are 

 virtually identical. Atlantic T. obesus in the 050- 

 1,200 mm, range (no data were available for smaller 

 specimens) appear to have significantly shorter pec- 

 torals than Pacific specimens : 29-35 percent at sizes 

 of 0.50-1,000 mm., then gradually decreasing until 

 no suggestion of difference is seen above 1,300 mm. 



In T. albacares a gradual negative allometry after 

 600 mm. keeps the pectorals shorter than in T. 

 alalunga or either Atlantic or Pacific T. obesus until 

 about 1,100 mm., wli(>n overlap with T. obesus 

 begins to increase. 



Whereas the pectorals of T. allanticus are, at first, 

 very similar in length to those of T. albacares, the 

 more rapid decline in relative length makes them at 

 sizes above 000 mm. even shorter than in 7'. albacares. 



The greatest negative allometry is seen in T. 

 tonggol, which, at 500 mm., already shows the trend 

 that brings the pectoral length into the ranges of 

 T. maccoyii and T. Ihynnus between 650 and 900 mm. 

 Th(> fins are the shortest of all the species at fork 



94 



U.S. FISH .\ND WILDMKK SKKVICE 



