Table 2. — Catch rates (catch per 100 tows) of billfish larvae in 1-m plankton net and 1- x 2-m 



neuston net. 







Species 









Short- 







All species 





bill 







including 



Blue marlin 



spearfish 



Sail fish 



Swordfish 



unidentified 



No. of tows 









larvae 



Type of tow- 



Day Night Day 



Night Day 



Might Day Night Day Night Day Night 



Surface 

 Oblique 

 Neuston 



201 1,119 50.0 



1,280 1,570 0.7 



15 160.0 



3.7 14.8 

 1.0 0.4 

 — 0.0 



3.5 1.0 0.0 2.0 I.I 74.2 8.9 

 0.5 0.0 0.0 0.2 0.1 1.5 2.1 

 — 0.0 — 0.0 — 160.0 — 



water, the catches at night at the three depths sam- 

 pled were approximately equal. 



The neuston net catches (Table 2) provide further 

 information on the vertical distribution of these lar- 

 vae. The net was normally towed with part of the net 

 above the surface, so that on an average it only 

 sampled the upper 0.5 m of water. The catch per tow 

 was more than three times that of the 1-m net towed 

 fully submerged at the surface. Since the neuston net 

 strained roughly twice the volume of water as the 

 1-m net, the catch per unit volume of water strained 

 was about 1.5 times that of the 1-m net. The higher 

 catch rate of the neuston net thus suggests that bill- 

 fish larvae could be concentrated not only in the 

 upper 1-m of water but even closer to the surface. 



DISTRIBUTION OF 

 ISTIOPHORID LARVAE 



Howard and Ueyanagi (1965) have plotted the 

 occurrence of istiophorid larvae in the Pacific 

 Ocean. Outlines drawn of their plots by species (Fig. 

 2) show that catches of most species were largely 

 confined to the western Pacific. Our data of larval 

 captures fill the gaps in the distribution given by 

 Howard and Ueyanagi (1965), particularly around 

 the Hawaiian Islands and in the central Pacific south 

 of the equator. The northern limits of distribution of 

 the four species of Istiophoridae in the western 

 North Pacific are notably similar (Fig. 2, panels A 

 and B). The southern limits of distribution for all 

 species cannot be defined, since sampling for the 

 larvae on all cruises east of long. 180° did not extend 

 far enough southward. Judging on the basis of the 

 close relationship between larval distribution and 

 the 24°C surface isotherm (Ueyanagi, 1964; Jones 

 and Kumaran, 1964) and on the configuration of the 

 surface temperature isotherms across the South 



Pacific (U.S. Hydrographic Office, 1948), it seems 

 that the southern limits of distribution of these larvae 

 should not extend much beyond lat. 25°S. 



Blue Marlin 



Blue marlin larvae, which comprised 60.8% of all 

 billfish larvae collected by us, occurred in both the 

 North and South Pacific. In the North Pacific they 

 were distributed heavily around the Hawaiian Is- 

 lands and in waters to the west between lat. 7° and 

 24°N. This distribution seems to be contiguous with 

 that shown by Howard and Ueyanagi (1965). In the 

 South Pacific the larvae occurred in a band between 

 lat. 0° and 24°S from the New Hebrides through the 

 Tuamotu Archipelago. The western end of this band 

 ties in with the southwestern outline of the distribu- 

 tion of Howard and Ueyanagi (1965). The interven- 

 ing area (lat. 5°-10°N and long. 140°W-180°) appears 

 to be devoid of blue marlin larvae, but this could be 

 due to inadequate sampling; only a few surface day 

 tows were made there. Sampling especially for bill- 

 fish larvae would likely change this distributional 

 picture and provide us with better information in the 

 area east of long. 140°W and in equatorial waters 

 westward to long. 180°. 



Seasonal Distribution. — Seasonal changes in the 

 distribution of blue marlin larvae were observed only 

 in the Hawaiian Islands area, where enough sea- 

 sonal sampling was done (Fig. 3). The blue marlin, as 

 well as some other billfishes, spawn throughout the 

 year in warm tropical and subtropical waters. At 

 both the northern and southern fringes of distribu- 

 tion, however, spawning occurs only during the 

 warm seasons (Howard and Ueyanagi, 1965). In the 

 Hawaiian Islands area, the northern fringe of larval 

 blue marlin distribution lies roughly parallel to the 



241 



