405 
Onshore-offshore distribution and abundance 
of tuna larvae (Pisces: Scombridae: Thunnini) 
in near-reef waters of the Coral Sea 
Email address for A M. Fowler: ashley.m.fowler@student.uts.edu.au 
1 Fisheries and Marine Environmental Research Laboratory 
School of Biological, Earth, and Environmental Sciences 
The University of New South Wales 
Kensington, New South Wales, 2052, Australia 
Present address for A. M. Fowler: Department of Environmental Sciences 
University of Technology, 
P. O Box 123 Broadway 
Sydney, New South Wales 2007 Australia 
2 Ichthyology, Australian Museum 
6 College Street 
Sydney, New South Wales, 2010, Australia 
Abstract — The on-offshore distri- 
butions of tuna larvae in near-reef 
waters of the Coral Sea, near Lizard 
Island (14°30'S, 145°27'E), Australia, 
were investigated during four cruises 
from November 1984 to February 1985 
to test the hypothesis that larvae of 
these oceanic fishes are found in 
highest abundance near coral reefs. 
Oblique bongo net tows were made in 
five on-offshore blocks in the Coral 
Sea, ranging from 0-18.5 km offshore 
of the outer reefs of the Great Bar- 
rier Reef, as well as inside the Great 
Barrier Reef Lagoon. The smallest 
individuals (<3.2 mm SL) of the genus 
Thunnus could not be identified to spe- 
cies, and are referred to as Thunnus 
spp. We found species-specific distri- 
butional patterns. Thunnus spp. and T. 
alalunga (albacore) larvae were most 
abundant (up to 68 larvae/100 m 2 ) in 
near-reef (0-5.5 km offshore) waters, 
whereas Katsuwonus pelamis (skipjack 
tuna) larvae increased in abundance 
in the offshore direction (up to 228 
larvae/100 m 2 , 11.1-18.5 km offshore). 
Larvae of T. albacares (yellowfin tuna) 
and Euthynnus affinis (kawakawa) 
were relatively rare throughout the 
study region, and the patterns of their 
distributions were inconclusive. Few 
larvae of any tuna species were found 
in the lagoon. Size-frequency distribu- 
tions revealed a greater proportion of 
small larvae inshore compared to off- 
shore for K. pelamis and T. albacares. 
The absence of significant differences 
in size-frequency distributions for 
other species and during the other 
cruises was most likely due to the 
low numbers of larvae. Larval dis- 
tributions probably resulted from a 
combination of patterns of spawning 
and vertical distribution, combined 
with wind-driven onshore advection 
and downwelling on the seaward side 
of the outer reefs. 
Manuscript submitted 8 January 2008. 
Manuscript accepted 23 June 2008. 
Fish. Bull. 106:405-416 (2008). 
The views and opinions expressed or 
implied in this article are those of 
the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Ashley M. Fowler (contact author)’- 2 
Jeffrey M. Leis 2 
lain M. Suthers’ 
Large-scale (100s km) distributions of 
tuna larvae (family Scombridae), par- 
ticularly of the commercially impor- 
tant genera Thunnus and Katsuwonus , 
have been extensively investigated 
because of the need to identify spawn- 
ing locations and the possibility of 
estimating spawning stock biomass 
from surveys of larvae (Strasburg, 
1960; Richards, 1976; Scott et al., 
1993). Despite this effort, and the 
apparent abundance and fecundity of 
T. albacares (yellowfin tuna) and K. 
pelamis (skipjack tuna) in the western 
and central Pacific Ocean, relatively 
low mean concentrations (2-4 larvae/ 
100 m 3 , Leis et al., 1991) and abun- 
dances of tuna larvae (2-8 larvae/10 
m 2 , Leis et al., 1991) have been found 
during most sampling programs. Low 
numbers of tuna larvae in any one site 
or region may simply reflect a broadly 
distributed, but sparse, spawning 
pattern of adults, as indicated by the 
wide geographical ranges of T. alba- 
cares and K. pelamis larvae. Other 
possible explanations, however, are 
that previous sampling scales of 100s 
km between samples were too coarse 
to account for spatial variability in 
abundances (Davis et al., 1990a), and 
that tuna larvae are more abundant 
in undersampled near-reef areas (Leis 
et al., 1991). 
Nearly all sampling of tuna lar- 
vae has been conducted in the open 
ocean because of the oceanic distri- 
butions of adults; however relatively 
high concentrations of tuna larvae 
have been found in previously under- 
sampled tropical near-reef (<5 km 
offshore) locations in three studies 
(Miller, 1979; Leis et al., 1991; Boe- 
hlert and Mundy, 1994). Concentra- 
tions of larval T. albacares were up 
to two orders of magnitude greater 
within 2 km of leeward (west) Oahu 
Island, Hawaii, than published con- 
centrations in surrounding oceanic 
waters (Miller, 1979), and Thunnus 
spp. and K. pelamis larvae were up to 
100 times more concentrated within 
200 m of coral reefs in French Polyne- 
sia than in oceanic waters of the cen- 
tral Pacific Ocean (Leis et al., 1991). 
Because adjacent oceanic locations 
were not simultaneously sampled in 
these near-reef studies, a direct com- 
parison of these two habitats was not 
possible. Furthermore, comparisons 
between the near-reef values of lar- 
val tuna concentration determined by 
Miller (1979) and Leis et al. (1991), 
and those determined for oceanic hab- 
itats in the central Pacific Ocean in 
other studies, may also be confounded 
because of differences in sampling 
methods employed between the two 
