lengths from 900 to 1,050 mm. (the maximum size of 

 T. (onggol for ^\•hich data were availal^le). 



Except for the size range where T. tonggol overlaps 

 it, T. thynniis has pectorals consistently shorter than 

 those of other species, the longest on record being 

 about 23 percent of fork length. 



Although more data are needed for T. maccoyii, it 

 appears that tliis species has a slightly longer pectoral 

 fin than T. Ihynnus. From 650-7.50 mm. fork length, 

 for which a fair amount of data is available, the fin 

 of T. maccoyii is 20-24 percent of fork length, that of 

 T. thynnus 17-21 percent. A few specimens of T. 

 maccoyii between 900 and 1,000 mm., and one of 

 1,445 mm. have pectorals 22-23 percent of fork 

 length, also slightly longer than those of similar sized 

 T. thynnus. The ranges given by Iwai, Nakamura, 

 and Matsubara (1965: 31, 33) of 4.8-6.0 in fork 

 length (=16.7-21.7 percent) for T. thytinus and 

 4.4-4.5 in fork length ( = 22.2-22.7 percent) for 

 T. maccoyii suggest a distinctness of separation that 

 is not upheld by our data, although the basic species 

 differences in pectoral length appear to be real. 



The distance from snout to second dorsal origin 

 relative to fork length (fig. 27) shows a negative 

 regression in all species of Thunnus over 400 mm. 

 When size is taken into account, this measurement 

 provides a reliable separation of some species, but a 

 simple statement of range is inadeciuate. For ex- 

 amjile, T. tonggol appears distinct throughout its 

 size range (maximum around 1,000 mm.) but larger 

 T. albacarcs, T. obesus, and T. thynnus all have a 

 distance that is the same as that of smaller T. tonggol. 



Throughout the size ranges examined by us (fig. 27) 

 the distance is greatest in T. alahtnga and least in 



T. tonggol. Two intermediate groups can be cate- 

 gorized: one with a shorter distance that includes 

 T. atlanticus and T. albacares, and one with a longer 

 distance that includes T. obesus and T. thynnus. 

 The meager data for T. maccoyii fall in the range of 

 T. thynnus. Below a fork length of about 600 mm. 

 there is so much overlap that the usefulness of snout- 

 second dorsal distance in species distinction is doubt- 

 ful, but above 600 mm. it appears to be useful. 

 Greatest body depth is shown in fig. 28. This 



Figure 27. — Relative distance from snout to second dorsal 

 origin in Th minus species. Data, in additon to our own, 

 include Dung and Royce (1953: tables 12, 21, 28, 42, 50-54). 

 1— T. atlanticus; 2-—T. tonggol; 3—T. albacarcs; i—T. 

 obesus; 5 — T. alahinga; 6 — T. thynnus. 



Figure 28. — Relative greatest bod_y depth in Thunnus species. 

 Data, in addition to our own, include Dung and Royce 

 (1953: tables 27, 28, 42). 1— T. atlanticus; 2— T. tonggol; 

 3 — T. albacares; 4 — T. obesus; 5 — T. alahinga;^ — T. thynnus. 



character is so variable that it should not be used by 

 itself. Rather, there are tendencies which, with 

 other characters, can be helpful in species deter- 

 mination. 



In specimens less than about 600 mm. fork length, 

 overlap is particularly evident, but two categories 

 can be based on greatest depth: deep-bodied species, 

 including T. obesus, T. thynnus, and T. atlanticus, 

 with depths usually 26-30 percent of fork length; 

 and slender species, including T. albacares, T. tonggol, 

 and T. alahtnga, with depths usually 23-26 percent 

 of fork length. The few data for T. maccoyii fall in 

 with T. thynnus. 



There appears to be little change in depth relative 

 to fork length from 600 to 1,500 mm. in any species 

 except T. alalunga, in which the relative depth 

 increases gradually until specimens over 1,000 mm. 

 are clearly in the deep-bodied category. 



The two species that commonly become larger 

 than 1,500 mm., T. obesus and T. thynnus, exhibit 



A.NATOMY AND SYSTEM,\TICS OF TUNAS 



95 



