Fowler et al.: Distribution and abundance of tuna larvae in near-reef waters of the Coral Sea 
415 
than Thunnus spp. larvae (Boehlert and Mundy, 1994; 
Hare et al., 2001), and migrate into deeper water during 
the day, at which time larvae of Thunnus spp. move into 
surface layers (Richards and Simmons, 1971; Davis et 
ah, 1990b). It is therefore possible that while Thunnus 
spp. and T. alalunga larvae in the present study were 
advected shoreward by wind-driven surface currents, 
and accumulated there by a tendency to remain near 
the surface, larvae of K. pelamis were advected offshore 
by deeper return flow originating from downwelling 
near the outer reefs. At the least, K. pelamis larvae 
would not accumulate near the reef front because they 
would not be expected to counter the putative down- 
welling at those locations. 
The larger size and greater abundance of K. pelamis 
larvae offshore indicate that the larvae of this species 
likely accumulated there, providing support for the 
hypothesis of offshore physical transport at depth for 
this species. And although T. albacares and T. alalunga 
larvae were not larger inshore, as would be expected 
if larvae were transported onshore and accumulated 
near the reef, larvae of these species >3.5 mm SL were 
common near the reef, which was not the case for K. 
pelamis. It is possible that the size distributions of T. 
albacares and T. alalunga larvae in the Coral Sea may 
have been affected by greater mortality of larger size 
classes in the inshore zone than in the more offshore 
waters. It has been hypothesized that predation rates 
of larval fish are higher in near-reef waters than in the 
open ocean (Johannes, 1978), and direct observations 
of late-stage reef fish larvae have shown that larvae 
near reefs feed less and are preyed upon more often 
than larvae farther offshore (Leis and Carson-Ewart, 
1998). 
The patterns of on-offshore distribution of tuna lar- 
vae documented here support the hypothesis that at 
least some tuna species have high larval abundances 
near reefs in the Tropical Pacific Ocean. We conclude 
that fine-scale (1-10 km) on-offshore distributions of 
tuna larvae found in the Coral Sea were most likely 
the result of relatively near-reef spawning patterns 
of adults (<10 km offshore) subsequently modified by 
wind-driven onshore currents and presumed down- 
welling in front of the outer reefs of the Great Barrier 
Reef. To account for different horizontal distributions of 
larvae among taxa, we suggest that putative opposing 
flow directions between the surface layers and deeper 
water may have interacted with the taxa-specific verti- 
cal distributions of larvae. An investigation of physical 
and biological factors, vertical distributions of larvae, 
and the abundance and distribution of spawning adults 
near reefs is required to further our understanding 
of the primary causes of on-offshore distributions of 
tuna larvae. 
Regardless of how distributions occurred, near-reef 
areas may generally be more important than offshore 
areas for the production of T. albacares and T. alalunga 
larvae, and possibly other large pelagic species. It is 
now evident from four studies that larvae of T. alba- 
cares and T. alalunga are abundant in near-reef (<5 km 
offshore) waters, and in the two studies where larval 
tuna abundances near a reef were compared with larval 
tuna abundance in offshore areas, higher abundances of 
Thunnus spp. larvae were found near the reef (the pres- 
ent study; Boehlert and Mundy, 1994). These studies 
also indicate that K. pelamis may, at least, spawn close 
to shore, although their larvae are not most abundant 
there. Larvae of other large pelagics, such as billfishes, 
may also be generally more abundant near reefs, as 
indicated by the near-reef abundance of larvae of three 
species in our study area (Leis et al., 1987). Near-reef 
areas have not received much attention in studies of 
distribution and abundance of larvae of large pelag- 
ic predators like tunas and billfishes. If the patterns 
found thus far are a general occurrence in tropical 
regions, larval abundance surveys that do not include 
these areas may underestimate true abundances. It 
must be kept in mind, however, that near-reef areas are 
much smaller than oceanic areas. Therefore, in spite 
of higher abundances (per unit of area) of larvae near 
reefs, the offshore areas may provide the bulk of the 
recruits to adult populations, because of the vast areas 
involved. As yet, there are no data on the survival rates 
of larvae near reefs compared to the survival rates of 
larvae offshore, or on their relative contributions to 
spawning populations. 
Acknowledgments 
We would like to thank M. McGrouther for providing 
access to the samples held in the Australian Museum 
collection. We also thank T. Trnski and A. Hay for help 
with sorting and identification of the tuna larvae. We 
are grateful for A. Poore’s advice on appropriate statisti- 
cal analyses. 
Literature cited 
Boehlert, G. W., and B. C. Mundy. 
1994. Vertical and onshore-offshore distributional pat- 
terns of tuna larvae in relation to physical habitat 
features. Mar. Ecol. Prog. Ser. 107:1—13. 
Collette, B. B., and C. E. Nauen. 
1983. Scombrids of the world: an annotated and illus- 
trated catalogue of tunas, mackerels, bonitos and 
related species known to date, 137 p. FAO Fish. Synop. 
125. Food Agr. Organ. U.N., Rome. 
Davis, T. L. O., G. P. Jenkins, and J. W. Young. 
1990a. Patterns of horizontal distribution of the larvae of 
southern bluefin (Thunnus rnaccoyii ) and other tuna in 
the Indian Ocean. J. Plankton Res. 12:1295-1314. 
1990b. Diel patterns of vertical distribution in larvae of 
southern bluefin Thunnus rnaccoyii, and other tuna in the 
East Indian Ocean. Mar. Ecol. Prog. Ser. 59:63-74. 
Fritzsche, R. A. 
1978. Development of fishes of the Mid-Atlantic Bight, 
an atlas of egg, larval, and juvenile stages, vol 5. Chae- 
todontidae through Ophidiidae, 340 p. Fish. Wildl. 
Serv. -Off. Biol. Serv. no. 78/12. U.S. Government 
Printing Office, Washington, DC. 
