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 o 



O 



X 



6 7 8 9 10 



STANDARD LENGTH (mm) 



Figure 1. — Snout to orbit (horizontal diameter) ratios of blue and striped marlins. Growth 

 stanzas fitted by Bartlett's best-fit line. 



character needs clarification: it is the shape of the 

 orbital crest as well as the extent of protrusion that 

 sets the blue marlin larvae apart from those of 

 striped marlin. In the blue marlin the anterior part 

 of the orbital crest, beginning slightly ahead of the 

 anterior naris, rises sharply and the anteriodorsal 

 part is high and angular. In other istiophorid larvae 

 the orbital crest slopes up and back more gradually 

 (Ueyanagi, 1963, Plate 3). 



A more useful character by which larvae of these 

 two species can be separated is the snout to orbit 

 ratio. Ueyanagi (1959) has used this character to 

 show the difference between larvae of sailfish and 

 blue marlin, except that his snout measurement in- 

 cluded the distance from the tip of snout to center of 

 eye with the orbit measured vertically. We have 

 used snout length as measured from the tip to the 

 anterior edge of the orbit and the orbit as measured 

 horizontally. Regardless of which snout length or 

 orbit measurement is used, the separation of the 

 curves is similar. 



Figure 1 shows the snout to orbit ratios of 138 

 blue marlin larvae from the central Pacific and 10 

 striped marlin from the western Pacific (seven 

 measurements from Ueyanagi, 1964 and three 

 measurements from specimens sent to us by 

 Ueyanagi) plotted against standard length. 

 Bartlett's (1949) best-fit lines were drawn through 

 points representing growth stanzas for each 

 species. Despite the small number of points shown 

 for striped marlin, the separation of the species, at 



least in the larger size range, appears to be valid. 

 Among the smaller stages (below 6 mm), however, 

 the points approach each other close enough to 

 make separation more difficult. 



The scatter of points about the curve shown for 

 blue marlin above 6 mm (Fig. 1) and the absence of 

 snout to orbit ratios falling near the curve shown for 

 striped marlin suggest that larvae from the central 

 North Pacific without pigmentation on the posterior 

 half of the lower jaw and branchiostegal membranes 

 are all of blue marlin. 



COLLECTION OF SAMPLES 

 AND CATCHES 



The samples of billfish larvae were obtained 

 mainly from 1-m plankton net tows taken from ves- 

 sels of the HL and other organizations from 1950 

 through 1970, and from 1- x 2-m neuston net tows 

 in 1971. The plankton net was usually towed for 30 

 min, either horizontally at the surface or obliquely 

 to depths ranging from 40 to 200 m. The neuston 

 net, constructed entirely of 1-mm mesh netting, was 

 used only on one cruise to the western Pacific. 

 Owing to operational difficulties, this net was 

 towed at the regular plankton net speed of 3.7-5.5 

 km/h for 30 min. Catches by the plankton and neus- 

 ton nets included juveniles as large as 20 mm. A 

 12. 2-m mouth diameter Cobb pelagic trawl, made of 

 19.0-mm stretch mesh netting lined with 6.4-mm 

 netting at the cod end, was used on several cruises 



239 



