Onshore-offshore distribution and abundance of tuna larvae (Pisces: Scombridae: Thunnini) in near-reef waters of the Coral Sea.
Coral reefs and islands
Tunas (Fishes) (Behavior)
Tunas (Fishes) (Distribution)
Fowler, Ashley M.
Leis, Jeffrey M.
Suthers, Iain M.
|Publication:||Name: Fishery Bulletin Publisher: National Marine Fisheries Service Audience: Academic Format: Magazine/Journal Subject: Zoology and wildlife conservation Copyright: COPYRIGHT 2008 National Marine Fisheries Service ISSN: 0090-0656|
|Issue:||Date: Oct, 2008 Source Volume: 106 Source Issue: 4|
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|Geographic:||Geographic Scope: Australia Geographic Code: 8AUST Australia|
Abstract--The on-offshore distributions of tuna larvae in near-reef
waters of the Coral Sea, near Lizard Island (14[degrees]30'S,
145[degrees]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
Barrier 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 species, and are referred to as Thunnus spp. We found
species-specific distributional patterns. Thunnus spp. and T. alalunga
(albacore) larvae were most abundant (up to 68 larvae/100 [m.sup.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.sup.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 distributions revealed a greater
proportion of small larvae inshore compared to offshore 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
distributions 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.
Large-scale (100s km) distributions of tuna larvae (family Scombridae), particularly of the commercially important genera Thunnus and Katsuwonus, have been extensively investigated because of the need to identify spawning 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.sup.3], Leis et al., 1991) and abundances of tuna larvae (2-8 larvae/10 [m.sup.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. albacares 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 larvae has been conducted in the open ocean because of the oceanic distributions of adults; however relatively high concentrations of tuna larvae have been found in previously undersampled tropical near-reef (<5 km offshore) locations in three studies (Miller, 1979; Leis et al., 1991; Boehlert and Mundy, 1994). Concentrations of larval T. albacares were up to two orders of magnitude greater within 2 km of leeward (west) Oahu Island, Hawaii, than published concentrations 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 Polynesia than in oceanic waters of the central Pacific Ocean (Leis et al., 1991). Because adjacent oceanic locations were not simultaneously sampled in these near-reef studies, a direct comparison of these two habitats was not possible. Furthermore, comparisons between the near-reef values of larval tuna concentration determined by Miller (1979) and Leis et al. (1991), and those determined for oceanic habitats in the central Pacific Ocean in other studies, may also be confounded because of differences in sampling methods employed between the two habitats. Near-reef concentrations were determined by using short (<15 min.) horizontal tows at or near the surface (Miller, 1979; Leis et al., 1991), whereas oceanic sampling in the region has generally involved tows of longer duration ([greater than or equal to] 30 min.), with nets towed either horizontally at the surface or obliquely from depths reaching 200 m (Matsumoto, 1958; Nakamura and Matsumoto, 1966). Because distributions of tuna larvae are vertically stratified, and Thunnus spp. larvae are rarely found at depths greater than 50 m (Davis et al., 1990b; Boehlert and Mundy, 1994), oblique tows to depths in excess of 100 m most likely underestimate concentrations in shallower strata. Because of this situation, it is appropriate to use the measure of abundance (i.e., the number of larvae below a standard surface area of water) rather than concentration (numbers of larvae per standard volume) to compare numbers of larvae among sites where tows were taken to different depths.
Abundances of tuna larvae were determined at both near-reef (2 km offshore) and oceanic sites (~30 km offshore) around the Hawaiian island of Oahu by Boehlert and Mundy (1994) by oblique sampling to 200 m depth. The abundance of Thunnus spp. larvae decreased with increasing distance offshore, but only on the leeward side of Oahu Island. Interestingly, the abundance of K. pelamis larvae actually increased with distance from the reef on both sides of the island (Boehlert and Mundy, 1994), indicating the possibility of important taxa-specific differences in near-reef distributions of tuna larvae.
The possibility that tuna larvae are generally concentrated near islands and reefs, and not in the open ocean, has important implications for the protection of these vulnerable life stages. Large numbers of tuna larvae near shore have been found in only two regions in the central Pacific Ocean, and only near oceanic islands; therefore further investigation of the generality of this phenomenon is required. Considering that patches of highly concentrated (10,945 larvae/500 [m.sup.3]) T. maccoyii (southern bluefin tuna) larvae in the North East Indian Ocean were only 5-15 km in diameter (Davis et al., 1990a), further investigation of fine-scale patterns of larval tuna distribution is also warranted.
The aim of the present study was to investigate the near-reef abundance and on-offshore distributions of tuna larvae in the Coral Sea, near the Great Barrier Reef, Australia, over a fine (1-10 km) scale. Although the sampling design was originally intended for investigation of distributions of reef fish larvae (Leis, 1986; Leis and Reader, 1991), the sampling scale and gradation of habitat from near-reef to oceanic in the offshore direction also made it appropriate for investigating the near-reef distributions of tuna larvae.
Materials and methods
Study area and experimental design
Larval fish samples were collected in an area of the Coral Sea between Lizard Island (14[degrees]30'S, 145[degrees]27'E) and 19 km seaward of the outer ribbon reefs of the Great Barrier Reef, Australia (Fig. 1). Lizard Island is situated approximately halfway across the Great Barrier Reef Lagoon (hereafter "lagoon") where water depths range from 25 to 40 m. The outer reefs lie on the continental shelf break, beyond which depth increases rapidly reaching 2000 m within 12 km. There is an abrupt change from shallow, protected waters of the lagoon to oceanic conditions in the offshore direction. Winds were usually from the E to SE during the study period (common for this region); therefore the near-reef waters that we sampled were on the windward side of the outer reefs.
Quantitative, double-oblique plankton tows were made from a 14-m catamaran with a bongo net (cylinder-cone mesh design) with 0.85 m mouth diameter and 0.5-mm mesh. The net was towed at approximately 1 m/s and was fitted with both a calibrated mechanical flow meter and a calibrated mechanical depth and distance recorder. Tows usually filtered 1000-2000 [m.sup.3] of water with a mean volume (and standard deviation) of 1554 (585), 1644 (629), 1637 (306), and 1348 (278) [m.sup.3] for each cruise, respectively. All tows were completed during daylight, between one hour after sunrise and one hour before sunset. Tows were taken to a target depth of 200 m on the first cruise and to 120 m thereafter, except in the lagoon and block A where they were taken as close to the bottom as considered safe. The net hit bottom occasionally in block A because of great variation in water depth in this area. Samples were fixed in formalin (5-10% in seawater) in the field.
[FIGURE 1 OMITTED]
Larvae from both the port and starboard sides of the bongo net were sorted, except for those in the lagoon samples from the first and second cruises where the catch from only one randomly chosen side was sorted because of high plankton volumes. Samples from block D were not sorted because of funding cuts to the research program. Larvae were removed with the aid of a dissecting microscope and transferred to 70% ethanol for storage. Tuna larvae (family Scombridae) were identified to species, when possible, by using the descriptions of Fritzsche (1978) and Nishikawa and Rimmer (1987). Larvae of T. albacares and T. alalunga (albacore) <3.2 mm standard length (SL) could not be separated and were identified as Thunnus spp. larvae. For larger larvae, Richards et al. (1990) advocate also using an osteological character, rather than relying solely on pigment, when identifying Thunnus larvae to species. However, their study was primarily concerned with T atlanticus (blackfin tuna), a species that is not found in our study area. Further, the osteological character in T alalunga and T. albacares seems to vary about as much as does the pigment character, and the former cannot be used in specimens <6 mm SL (Richards et al., 1990). Therefore, we relied on the pigment character to separate our T alalunga and T. albacares larvae [greater than or equal to] 3.2 mm SL. We note, however, that one-third of the Thunnus larvae >6 mm SL that we cleared and stained had pigment inconsistent with the osteological characters listed in Table 1 of Richards et al. (1990). Therefore, according to the criteria of Richards et al. (1990, Table 1), at least 67% of our Thunnus larvae are correctly identified to species, and between 0 and 33% may be misidentified to species. Given the variability in osteological features noted by Richards et al. (1990, their Tables 2 and 3), we cannot be more precise about this "uncertain" 33%. Larvae of the genus Auxis cannot currently be identified to species (Nishikawa and Rimmer, 1987), and adults of both A. thazard (frigate tuna) and A. rochei (bullet tuna) inhabit the region of the study area (Collette and Nauen, 1983). Auxis larvae were therefore all identified as Auxis spp. Larvae of Auxis spp. and Euthynnus affinis (kawakawa) <2.3 mm SL could not be separated and were identified as Auxis-Euthynnus larvae. Notochord length and SL were measured to the nearest 0.1 mm for preflexion and postflexion larvae, respectively, by using a calibrated ocular micrometer. No correction was made for shrinkage of the larvae.
Abundances (no. of larvae/100 [m.sup.2]) were calculated by 1) calculating larval concentration (larvae/[m.sup.3]), 2) multiplying concentration by the depth (m) sampled, and 3) multiplying the result by 100 to obtain appropriately scaled values. Abundance values incorporate a depth component, and are therefore appropriate for comparing oblique samples taken down to different depths. Statistical analyses of on-offshore patterns of distribution of tuna larvae were performed on natural log-transformed abundances, following inspection of the data for normality and heterogeneity of variance. A count of 1 was added to all data-points before transformation, to allow transformation of zero values. Preflexion and postflexion larvae were combined for on-offshore analyses. For each taxon, analysis was done only for cruises with an average larval abundance >1 larvae/100 [m.sup.2] to avoid problems associated with numerous zeros. Lagoon samples were not included in calculation of the cruise average because few larvae of any tuna taxon were caught in the lagoon. Only K. pelamis larvae were sufficiently abundant for analysis on all four cruises, and their abundance among on-offshore blocks (including the lagoon) and cruises was compared by using a two-factor analysis of variance (ANOVA). For other taxa, abundance among blocks was analyzed with a one-factor ANOVA for each cruise with larval abundance >1 larvae/100[m.sup.2]. When ANOVA tests yielded significant (P<0.05) results, pairwise differences between blocks were analyzed using Tukey's test. Auxis spp. larvae were not sufficiently abundant on any cruise to allow for statistical analysis.
Size-frequency data from blocks A and B (inshore zone, 0-1.85 km from the reef) were pooled and compared with size-frequency data pooled from blocks C and E (offshore zone, >1.85 km from the reef) using the Kolmogorov-Smirnov (K-S) test. Data were pooled to increase n, because few taxa had sufficient numbers of larvae in each block to provide adequate statistical power. K-S tests were done on data from individual cruises. Larvae from the lagoon were excluded from size-frequency analysis because of low abundance. The significance level used for all statistical tests was 0.05. Not enough E. affinis larvae were caught in both zones on any cruise to allow a statistical comparison of size distributions.
Species composition and abundance
Over 1000 tuna larvae were caught, comprising at least five species and four genera (Table 1). Larvae of K. pelamis were the most abundant, making up over one third of all tuna larvae caught. Numerous small (<3.2 mm SL) Thunnus spp. larvae were caught on the early February cruise, coinciding with a peak in abundance of less common T. albacares larvae. In contrast, T. alalunga larvae were most abundant on the November cruises, and only 26 individuals were caught on the early February cruise. It is, therefore, likely that most of the Thunnus spp. larvae were T. albacares. Larvae of other Thunnus species (e.g., T. obesus [bigeye tuna] or T. tonggol [longtail tuna]) were not caught and would have been distinguishable from T. albacares and T. alalunga, even at small (<3.2 mm SL) sizes (Fritzsche, 1978). Euthynnus affinis larvae were most common on the early November and early February cruises, and Auxis-Euthynnus larvae were most abundant on the early February cruise. Only 22 Auxis spp. larvae were caught during the study, therefore Auxis-Euthynnus larvae were most likely primarily E. affinis larvae.
Tuna larvae generally had near-reef distributions, as greatest abundances usually occurred within 5.6 km of the outer reefs of the Great Barrier Reef in the Coral Sea. Patterns of on-offshore distribution differed among taxa, however.
Species of Thunnus had the most consistent near-reef distribution among the taxa of tuna larvae, with highest abundances found within 5.6 km of the outer reefs on most cruises with sufficient numbers of larvae for statistical analysis. Thunnus spp. larvae were more abundant in block C than in either the lagoon or block E on the late November cruise (Tukey's test, P < 0.01, Fig. 2) (blocks A and B being intermediate), and more abundant in blocks A and B than in the three other blocks, which did not differ, on the early February cruise (Tukey's test, P < 0.03, Fig. 2). Thunnus spp. larvae showed a similar inshore distribution on the late February cruise; however the only significant difference was a greater abundance of larvae in block B than in the lagoon (Tukey's Test, P < 0.03, Fig. 2). Thunnus alalunga larvae were more abundant in blocks B and C than in the lagoon on the early November cruise (Tukey's test, P < 0.02, Fig. 3) (the the other blocks being intermediate) and more abundant in block A than in either the lagoon or block E on the early February cruise (Tukey's test, P<0.04, Fig. 3). A similar pattern of abundance of T. alalunga larvae occurred on the late November cruise; however differences among blocks were not quite significant (Tukey's test, P=0.051, Fig. 3). Thunnus albacares larvae were most abundant in block A on the early February cruise; however there were no significant differences among blocks (ANOVA, P = 0.06, Fig. 4).
Distributions of E. affinis larvae and Auxis-Euthynnus larvae were similar to that of T. alalunga, and appeared to be most abundant within 5.6 km of the outer reefs. Differences in larval abundance among blocks were not, however, significant for the early November (ANOVA, P = 0.14, Fig. 5) and early February (ANOVA P = 0.11, Fig. 5) cruises analyzed for E. affinis, or the early February (ANOVA, P = 0.06, Fig. 5) cruise analyzed for Auxis-Euthynnus.
In contrast to other tuna taxa, the abundance of K. pelamis larvae increased in the offshore direction. Despite a significant interaction among blocks and cruises (ANOVA, P = 0.004), K. pelamis larvae were more abundant in block E than in blocks A, B, and the lagoon on the early November cruise (Tukey's test, P < 0.02, Fig. 6), more abundant in blocks C and E than in the lagoon on the early February cruise (Tukey's test, P<0.002, Fig. 6), and more abundant in block E than the lagoon on the late February cruise (Tukey's test, P<0.05, Fig. 6). The statistical interaction among blocks and cruises was most likely caused by a relatively great abundance (mean 12.1 larvae/100 [m.sup.2]) of K. pelamis larvae in the lagoon on the late November cruise, compared with abundance in the lagoon on the early November, early February, and late February cruises (means 0.6, 0.0, and 3.5 larvae/100 [m.sup.2], respectively, Fig. 6). Apart from K. pelamis and T. alalunga larvae on the late November cruise, few larvae of any tuna taxon were found in the lagoon.
Tuna larvae ranged in size from 1.7 to 15 mm SL; however most taxa had a strong size mode at 2-4 mm SL. For all species, larvae were of similar size on each cruise, which were at least seven days apart.
[FIGURE 2 OMITTED]
Differences in size-frequency distributions of larvae between inshore and offshore zones of the Coral Sea were taxa-specific, however for those tuna taxa where significant differences were detected, there was a greater proportion of small larvae in the inshore zone. A greater proportion of small (2-3.5 mm SL) K. pelamis larvae was found in the inshore zone, compared with the offshore zone, on both the early February (K-S test, P < 0.01, Fig. 7A) and late February (K-S test, P < 0.02, Fig. 7B) cruises. A similar pattern was found for the late November cruise, with the result approaching significance (K-S test, 0.05 < P < 0.10, Fig. 7C). Despite a greater proportion of smaller larvae present in the inshore zone, small (~2 mm SL) K. pelamis larvae were present in the offshore zone on all cruises. There was also a greater proportion of small (3-4 mm SL) T. albacares larvae in the inshore zone, compared with the offshore zone, on the early February cruise (K-S test, P < 0.02, Fig. 8). Not enough T. albacares larvae were caught in each zone on other cruises for statistical analysis; however examination of size-frequency data indicated that there may also have been a greater proportion of smaller T. albacares larvae in the inshore zone on the late November cruise. No significant differences in size-frequency distributions were found for T. alalunga larvae between the inshore and offshore zones on either the early November (K-S test, P > 0.2, Fig. 9A) or late November (K-S test, P > 0.2, Fig. 9B) cruises. There was also no significant difference in the size of Auxis-Euthynnus larvae between the inshore and offshore zones on the early February cruise (K-S Test, P > 0.2, Fig. 9C).
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Mean near-reef (<4 km offshore) abundances of tuna larvae in the Coral Sea (range 20-120 larvae/100 [m.sup.2]) were similar to those found around the Hawaiian island of Oahu (~3-80 larvae/100 [m.sup.2], Boehlert and Mundy, 1994) and similar to estimates of near-reef abundance from French Polynesia (45-75 larvae/100 [m.sup.2], Leis et al., 1991). Although these values were not much greater than those determined for oceanic sites elsewhere (24-80 larvae/100 [m.sup.2], Strasburg, 1960; Nakamura and Matsumoto, 1966), the larvae of most tuna species were more abundant within 5.6 km of the Great Barrier Reef than further offshore in the Coral Sea. This pattern was consistent among cruises for particular taxa, indicating that near-reef larval distributions persist over seasonal time scales.
[FIGURE 5 OMITTED]
Larvae of the genus Thunnus may generally be more abundant in near-reef waters than farther offshore, because in all studies of tuna larvae in near-reef waters consistently high concentrations or abundances of Thunnus larvae have been found there. Greatest abundances of Thunnus larvae were often found within 2 km of the outer Great Barrier Reef in the present study, and, as with our findings, small ([less than or equal to] 3.0 mm SL) Thunnus spp. larvae were ~10 times more abundant 1.8 km offshore, than 9.3 km offshore, of the leeward side of Oahu Island (Boehlert and Mundy, 1994). In an earlier study at Oahu Island high concentrations (up to 220 larvae/500 [m.sup.3]) of T albacares larvae were found within 2 km of shore (Miller, 1979), and similar concentrations (up to 224 larvae/500 [m.sup.3]) of Thunnus spp. larvae (either T. albacares or T. alalunga) were found in samples taken within 200 m of reefs in French Polynesia (Leis et al., 1991). The present study is the first to investigate near-reef distributions of tuna larvae outside of central Pacific Ocean island environments and thus extends the near-reef distributional pattern to include continental slope environments of the western Pacific Ocean, but further research on on-offshore patterns of abundance in other regions is required to confirm the generality of this phenomenon.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Considerable differences in on-offshore larval distributions may exist among genera of tuna, perhaps even among species, and these differences may not necessarily reflect similarities in adult distributions. We have confirmed the opposing on-offshore distributions of Thunnus spp. and K. pelamis larvae previously discovered in Hawaii, where Thunnus spp. larvae were more abundant near the reef on the leeward side of Oahu Island, while K. pelamis larvae increased in abundance in the offshore direction (Boehlert and Mundy, 1994). Interestingly, the on-offshore distributions of Thunnus spp. larvae in the Coral Sea were more similar to the larval distributions of E. affinis (and possibly Auxis spp.) than to the distributions of K. pelamis larvae. This finding is remarkable, considering T. albacares and K. pelamis are considered to be truly oceanic species with similar adult distributions, whereas adult E. affinis 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).
[FIGURE 8 OMITTED]
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. larvae). 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. affinis (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 spawning and capture and therefore would have reduced the potential effect of subsequent modification on their observed distributions.
The greater abundance and size of K. pelamis larvae offshore indicates that observed distributions of this species most likely arose from considerable modification 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.sup.2]) and T alalunga (10.3 larvae/100 [m.sup.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 during the study period in the Coral Sea most likely resulted 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 near-reef (within 2 km offshore) distributions of Makaira indica (black marlin), M. mazara (blue marlin), and Istiophorus platypterus (Indo-Pacific sailfish) larvae in the Coral Sea ("the anstau hypothesis," Leis et al., 1987). The istiophorid larvae examined by Leis et al. (1987) for horizontal distributions were taken from the same samples used in this study. Like billfish, larvae of Thunnus spp. also have relatively shallow distributions; greatest abundances were found in the upper 20 m of the water column around Oahu Island (Boehlert and Mundy, 1994) and higher concentrations were found at 5 m depth than at 10 m depth in French Polynesia (Leis et al., 1991). Larvae of T. atlanticus (blackfin tuna) were caught in greatest numbers in the upper 20 m of the water column in the northern Caribbean Sea, and few larvae were caught below 40 m depth (Hare et al., 2001). We cannot confirm this hypothesis, however, because we did not take direct measurements of either currents or the vertical distributions of tuna larvae during the present study period.
[FIGURE 9 OMITTED]
Downwelling on the seaward side of the outer reefs in the Coral Sea could account for the simultaneous occurrence of opposing on-offshore distributions of K. pelamis and Thunnus larvae because of known differences in the vertical distributions of larvae between these two genera. Larvae of K. pelamis have deeper distributions 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 al., 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 downwelling 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 larvae 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 downwelling 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 vertical 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. albacares 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 present 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 pelagic 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.
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 statistical analyses.
Manuscript submitted 8 January 2008. Manuscript accepted 23 June 2008.
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Ashley M. Fowler (contact author) (1, 2)
Jeffrey M. Leis (2)
Iain M. Suthers (1)
Email address for A. M. Fowler: email@example.com
(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
Table 1 Species composition of tuna larvae (family Scombridae) caught during four cruises in the Coral Sea, near the Great Barrier Reef, between November 1984 and February 1985. Values are numbers of larvae caught. Taxa are ordered to allow comparison of larval numbers among groups comprising the same genera. Early Late Early November November February Taxon cruise cruise cruise Katsuwonus pelamis 55 89 109 Thunnu sspp. 3 24 116 Thunnus albacares 11 19 49 Thunnus alalunga 69 62 26 Auxis-Euthynnus 13 2 72 Euthynnus affinis 49 4 54 Auxis spp. 5 0 10 Total 205 200 436 Late Proportion February Species of total Taxon cruise total catch (%) Katsuwonus pelamis 94 347 34 Thunnus spp. 29 172 17 Thunnus albacares 16 95 9 Thunnus alalunga 14 171 17 Auxis-Euthynnus 9 96 10 Euthynnus affinis 1 108 11 Auxis spp. 7 22 2 Total 170 1011
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