LEIS ET AL.: DISTRIBUTION AND ABUNDANCE OF BILLFISH LARVE 



Table 3. — Day-time vertical distribution of sailfish and 

 blue marlin larvae in the vicinity of Carter Reef in Febru- 

 ary and March 1983. N refers to number of vertical 

 sets (i.e., a tow in each stratum) that contained at least 

 one larva of that species. 



Sailfish (A/ = 7) 



Depth 

 stratum 



Concentration 

 (larvae/400 m3) 



Median 



Number of 

 positive 

 Range hauls (of 7) 



Neuston 

 Bongo net 

 0-6 m 

 6-13 m 

 13-20 m 







1.7 











0-0 



0-7.8 

 0-1.6 

 0-1.0 







6 



1 

 1 



Blue marlin {N = 3) 



Depth 



stratum 



Concentration 

 (lan/ae/400 m3) 



Median 



Number of 

 positive 

 Range hauls (of 3) 



Neuston 

 Bongo net 

 0-6 m 

 6-13 m 

 13-20 m 







1.8 











0-0 



1.8-4.3 

 0-0 

 0-1.0 



exception of a single larva from 13 to 20 m from 

 one of the interreef channel sets. In the three 

 positive sets, blue marlin larvae were always 

 most concentrated in the 0-6 m stratum, but there 

 were too few data for rigorous testing. 



Blue marlin and sailfish larvae occurred in one 

 vertical set taken on the windward side of Lizard 

 Island in January 1980 (see Leis 1986). One larva 

 of each species was taken in each of the 0-1 m and 

 the 3-4 m tows, while none were taken in the 6-7 

 m tow. 



Istiophorid larvae from our neuston samples 

 were developmentally more advanced (older) 

 than those from bongo net samples. In all our 

 samples, the bongo net captured 160 istiophorid 

 larvae (black marlin, blue marlin, sailfish), three 

 of which were postflexion stage, while the neu- 

 ston net captured 17 istiophorid larvae (black 

 marlin and blue marlin), 13 of which were post- 

 flexion stage (chi square, P < 0.001). 



During the day preflexion blue marlin and sail- 

 fish larvae inhabit the upper 6 m, and possibly 

 the upper half of that, but not the neuston. It 

 appears that once the caudal fm is formed, istio- 

 phorid larvae move upward even more and enter 

 the neuston. 



DISCUSSION 



Distribution of istiophorid larvae over such a 

 small scale has not been studied previously, nor 



have such high concentrations of larvae been re- 

 ported. Our results were surprising. Highest con- 

 centrations and abundances of istiophorid larvae 

 in our study area were consistently found in the 

 Coral Sea very close to the windward side of the 

 ribbon reefs at the outer edge of the Great Barrier 

 Reef The size-frequency data (see below) suggest 

 that this near-reef environment was a spawning 

 area or just down wind of one for the three types 

 of billfishes considered here. 



Concentration and abundance of istiophorid 

 larvae in the Great Barrier Reef Lagoon (here- 

 after referred to as the Lagoon) were always 

 lower than in block A when both areas were sam- 

 pled, but lagoonal numbers were generally not 

 different from those further offshore in the Coral 

 Sea. We cannot exclude the possibility that some 

 istiophorid spawning takes place within the 

 Lagoon, but believe it is more likely that the lar- 

 vae were advected into the Lagoon through the 

 interreef channels, as are larvae of many other 

 oceanic fishes (Leis 1986; Leis and Goldman 

 1987). Still, concentrations of istiophorid larvae 

 were high at times in the Lagoon (e.g., February- 

 March 1983), and the relative survival of the lar- 

 vae in the Lagoon vs. the Coral Sea is an open 

 question. 



The marginally significant difference between 

 areas in size frequency of black marlin larvae 

 suggests that hatching of the eggs takes place 

 very near the windward face of the reefs. This also 

 suggests that black marlin larvae found else- 

 where were largely the result of dispersal away 

 from the near-reef area, and these dispersed lar- 

 vae had grown somewhat during their dispersal. 

 Spawning could either be concentrated in the 

 near-reef area or more widely spread, in which 

 case the eggs would have become concentrated in 

 the near-reef area through wind-induced surface 

 drift and forereef down welling (see below). Alter- 

 natively, larval growth rates could be higher or 

 mortality lower in the areas further from the reef. 

 Our data do not allow us to distinguish between 

 these alternatives, but we believe the first is the 

 most likely. 



The data on blue marlin larvae gave no indica- 

 tion of differences in size frequency between 

 areas. The lack of difference in size-frequency dis- 

 tribution could indicate that spawning in blue 

 marlin was more evenly spread than in black 

 marlin. If so, the increase in numbers nearest the 

 windward side of the reef would be attributable to 

 concentration and retention of larvae there. We 

 cannot differentiate between this possibility and 



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