Fowler et al.: Distribution and abundance of tuna larvae in near-reef waters of the Coral Sea 
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Size (mm) 
Figure 8 
Size-frequency distributions for Thunnus albacares 
(yellowfin tuna) larvae from inshore and offshore 
zones during the early February cruise in the 
Coral Sea. Standard length (mm) of larvae was 
measured to the nearest 0.1 mm. The P-value 
refers to the significance of a Kolmogorov-Smirnov 
(K-S) test conducted between the inshore and 
offshore zones. 
Inshore zone 
n= 33 
P<0.02 
m 
Offshore zone 
n=1 6 
-I ‘l I 
This finding is remarkable, considering T. albacares and 
K. pelamis are considered to be truly oceanic species 
with similar adult distributions, whereas adult E. af- 
jin is and Auxis spp. have coastal distributions (Collette 
and Nauen, 1983). Because of potential distributional 
differences, further research on the near-reef larval 
distributions of other tuna species is required; however 
this may be difficult considering the relative rarity of 
the larvae of some species (e.g., T. tonggol). 
The greater abundances of small Thunnus spp. and 
Auxis -Euthynnus larvae within 5.6 km of the outer 
Great Barrier Reef indicates that these species may 
have spawned more intensely or more frequently (or 
both) in this area, than farther offshore, during the 
study period. Larvae of Thunnus spp. were all <3.2 
mm SL because of the limits of our ability to identify 
small larvae, and therefore their near-reef distribution 
observed on at least three cruises was most likely the 
result of near-reef spawning activity of T. albacares 
(which likely comprised most of the Thunnus spp. lar- 
vae). In support of this conclusion, there was a greater 
proportion of small T. albacares larvae within 1.85 km 
of the outer Great Barrier Reef than farther offshore 
during the early February cruise. Auxis spp. or E. af- 
finis (or both) may have also spawned near the reef in 
early February, as indicated by the greater abundance 
of small (<2.3 mm SL) Auxis-Euthynnus larvae within 
5.6 km of the outer GBR, which approached significance 
(ANOVA, P=0.06, Fig. 5). Their narrow size range (1.9- 
2.2 mm SL) did not, however, allow for a comparison of 
sizes between inshore and offshore zones. Although it is 
likely that initial spawning distributions of the larvae 
of these two taxa would have been modified to some 
degree by subsequent physical or biological processes, 
or both (see below), their small size (and likely young 
age) would have minimized the time between spawn- 
ing and capture and therefore would have reduced the 
potential effect of subsequent modification on their ob- 
served distributions. 
The greater abundance and size of K. pelamis larvae 
offshore indicates that observed distributions of this 
species most likely arose from considerable modifica- 
tion of initial spawning distributions. Like T. albacares, 
K. pelamis likely spawned more intensely or more 
frequently, or both, within 1.85 km of the outer Great 
Barrier Reef during the study period because there was 
a greater proportion of small larvae within the inshore 
zone than in the offshore zone, on two, possibly three, 
cruises. Larval abundance of this species increased with 
increasing distance from the outer Great Barrier Reef, 
however, indicating that larvae may have accumulated 
in the offshore area. A similar pattern of increasing 
abundance offshore, combined with smaller larvae near 
the reef, was found for K. pelamis on the leeward side 
of Oahu Island, Hawaii (Boehlert and Mundy, 1994); 
however no mechanism was suggested to account for 
these patterns. Differential growth or mortality, or 
both, for K. pelamis larvae may have occurred between 
near-reef and offshore areas; however we believe that 
offshore transport by means of physical mechanisms 
(see below) provides the best explanation of observed 
distributions, at least in the Coral Sea. 
The scarcity of larvae of any tuna taxon in the Great 
Barrier Reef Lagoon, even when offshore abundances 
were quite high, indicates that little, if any, spawning 
occurred there. The moderate abundances of K. pelamis 
(13.1 larvae/100 m 2 ) and T. alalunga (10.3 larvae/100 
m 2 ) larvae in the lagoon on the late November cruise 
were most likely caused by advection of larvae through 
the inter-reef passages from spawning locations on the 
seaward side of the outer reefs, either by onshore winds, 
or by tidal movement (Leis et al., 1987). We cannot 
exclude the possibility that some individuals of these 
species spawned inside the lagoon during the study 
period, but such spawning would have represented only 
a small proportion of total spawning effort. 
The high abundances of Thunnus spp. and T. alalunga 
larvae found near the reef in the present study likely 
resulted, at least in part, from onshore advection due 
to wind-driven currents interacting with the surface- 
orientated distribution of the larvae. The relatively 
consistent light to moderate onshore (E-SE) winds dur- 
ing the study period in the Coral Sea most likely re- 
sulted in shoreward advection of surface water layers. 
Onshore advection of surface water and subsequent 
downwelling on the windward side of the outer reefs, 
combined with a shallow vertical distribution of larvae, 
was suggested as a possible mechanism resulting in 
