Composition of the near-reef zooplankton at Heron Reef,Great Barrier Reef

Using a light trap, zooplankton was sampled at three stations at Heron Reef, Great Barrier Reef: (a) a typical patch reef in the Heron lagoon, (b) a site in 8 m water on the southern slope of Heron reef, and (c) a station approximately 300 m south of (b), in the open water of the channel between Heron and Wistari reefs. Samples were taken at the surface and on the substratum at the lagoon and reef-slope stations, and at 3 to 6 m depth at the open-water station. A total of 114 taxa, many recognized as species, were distinguished in the samples. Pronounced differences existed in abundance, diversity, and taxonomic composition of the samples obtained at different stations. Less pronounced differences existed between surface and substratum samples from the same station. Near-reef samples were more similar to one another than to open-water samples. Decapod larvae, amphipods, and cumaceans were all abundant in near-reef samples and very rare in open-water samples. Forams, isopods, mysids and polychaetes were common in near-reef samples, and rare or absent in open-water samples. Copepods were abundant in all samples but the near-reef samples contained predominantly different species than did samples from the open water. The near-reef fauna included 66 taxa which did not occur in open-water samples. Many of these were epibenthic rather than strictly planktonic in behaviour.     

Reef-basel herbivores and the distribution of two seagrasses (Syringodium filiforme and Thalassia testudinum) in the San Blas Islands (Western Caribbean)

Two seagrasses, manatee grass (Syringodium filiforme) and turtle grass (Thalassia testudinum), predominated in the areas bordering Ukkup Tupo, San Blas Islands, Western Caribbean. These seagrasses occupied the following three concentric zones extending outward from the reef: a near-reef turtle grass zone, an intermediate manatee grass zone and an off-reef turtle grass zone. Feeding experiments between January and March 1980 indicate that the absence of manatee grass close to the reef resulted from grazing by reef-associated herbivores, mainly day-active fishes and night-active sea urchins (Diadema antillarum). Grazing on manatee grass by fishes was approximately six times greater than grazing by sea urchins; thus, it appears that herbivorous fishes restrict the near-reef distribution of manatee grass at the study area. Where grazing was heaviest, the inner boundary of the manatee grass zone was farthest from the reef. The volume of manatee grass grazed during experiments was five times the volume of turtle grass consumed, strongly suggesting that the former species is a preferred food item. This is the first evidence for selective grazing on seagrasses.     

Use of pelagic prey subsidies by demersal predators in rocky reefs: insight from movement patterns of lingcod

Allochthonous subsidies of energy and nutrients can affect community structure in patchy marine habitats, including rocky reefs, and their ecological consequences may depend on the mechanism of energy transfer. Lingcod (Ophiodon elongatus) are demersal predators that trophically link nearshore rocky reefs with offshore pelagic habitats through consumption of pelagic fishes. We quantified lingcod habitat use and movement patterns to make inferences about the temporal and spatial conditions under which lingcod may acquire pelagic prey. Lingcod maintained small home ranges (21,272 ± 13,630 m²) within a rocky reef in the San Juan Archipelago, Washington; eight of nine individuals used rocky habitat exclusively. Depths occupied by lingcod (0–50 m) coincided with pelagic fish distribution on the rocky reef; however, diel patterns in lingcod activity varied inversely with occurrence of pelagic fishes on the reef. Our findings suggest that the pelagic subsidy to lingcod is not strongly mediated through directed off-reef foraging by lingcod.     

Increases in surgeonfish populations after mass mortality of the sea urchinDiadema antillarum in Panamá indicate food limitation

In 1983/1984,Diadema antillarum suffered mass mortalities throughout its West Atlantic range. Its populations were reduced by 95% and subsequently have failed to recover. These die-offs led to sustained increases in the abundance of soft algae, including types eaten by herbivorous reef fishes. I monitored adult populations of three herbivorous surgeonfishes (Acanthurus coeruleus, A. chirurugus andA. bahianus) between 1978 and 1990, and the recruitment of their pelagic juveniles between 1979 and 1989, on six patch reefs in Panamá. Adult populations ofA. coeruleus andA. chirurgus, which largely restrict their feeding to reef substrata, increased by averages of 250 and 160%, respectively, after the die-off ofD. antillarum in 1983. No increases occurred in the adult populations ofA. bahianus, which often feeds in off-reef habitats unaffected byD. antillarum. Average annual levels of juvenile recruitment of all three surgeonfishes did not differ before and after the die-off. These results support the hypothesis that adult populations of two herbivorous fishes that are strongly reliant on reef algae for food previously were limited by competition withD. antillarum.     

Spatial and temporal variation in coral recruitment on the Great Barrier Reef: Implications for dispersal hypotheses

Over 15 000 coral recruits were counted on settlement plates from three mid-shelf reefs and six fringing reefs in the northern section of the Great Barrier Reef during two summers (1986 and 1987) and one winter (1987). The density of coral recruits on some settlement plates from a fringing reef was up to 4.88 cm^–2, the highest value ever reported. Mean density of recruits was greater on fringing reefs (81.1 recruits/settlement plate) than on mid-shelf reefs (15.6 recruits/settlement plate), but there was greater spatial variation in abundance of recruits between the fringing reef sites. Other differences between the mid-shelf reefs and the fringing reefs were that different taxa were dominant, and that settlement orientation differed, with mid-shelf recruits settling preferentially on horizontally oriented surfaces and fringingreef recruits preferring vertical surfaces. Of the three midshelf reefs, Green Island reef recorded the highest recruitment rate for each of the two summers, despite having a depauperate adult coral population following predation by the asteroidAcanthaster planci. This suggests that coral larvae frequently travel between reefs. In contrast with an earlier study, there was no consistent difference in abundance of recruits between forereef and backreef locations. Overall, the results indicated great spatial variation in the availability of coral larvae, both on the scale of whole reefs and within-reef habitats.     

Diel patterns of abundance of presettlement reef fishes and pelagic larvae on a coral reef

Most presettlement reef fish settled at night at One Tree Island, Great Barrier Reef. Fish were sampled day and night using channel nets located on the reef crest, and a plankton-mesh purse-seine net in the lagoon (1992–1994). Catches of fish at night were generally tens to hundreds of times greater than those taken during the day. Preflexion fish, as well as postflexion and pelagic juveniles, were taken in greater numbers at night. Preflexion forms were a combination of those that had hatched from demersal eggs and later stages that had been transported over the reef crest. Highest numbers of postflexion and pelagic juvenile forms of Apogonidae, Blenniidae, Gobiesocidae, Gobiidae, Labridae, Lutjanidae, Mugiloididae, Mullidae, Pomacentridae, Scaridae, Serranidae and Tripterygiidae were found at night. Observations, while SCUBA diving, and purse-seine samples in the lagoon indicated that the only resident larvae were of the genera Spratelloides and Hypoatherina; most of the fishes caught in nets, therefore, were immigrants. Patch reefs, sampled for new settlers early in the morning and late in the day, indicated that the majority of apogonids (Apogon doederleini, >95%) settled at night. Although greater numbers of pomacentrids were found in morning counts (e.g. Pomacentrus wardi), if data were converted to an hourly rate, many pomacentrids showed a similar hourly rate of settlement day and night. Depth-stratified sampling in waters near One Tree Island (to 20 m) indicated that some taxa rise to the surface at night. This behaviour, perhaps combined with avoidance of diurnal predators may explain on-reef movement of potential settlers soon after dark. Studies on settlement cues, therefore, need to focus on night-related phenomena. Received: 3 March 2000 / Accepted: 20 June 2000     

Settlement patterns and presettlement behavior of the naked goby,Gobiosoma bosci,a temperate oyster reef fish

Dispersion, distribution, development and feeding incidence of larvae of the naked goby,Gobiosoma bosci (Lacepéde), were examined for linkages between larval behavior while near the reef surface and later patterns of settlement and recruitment. Field sampling and experiments were conducted during the summers of 1988 and 1989 in the Flag Pond oyster reef along the western shore of the Chesapeake Bay near Camp Conoy, Maryland, USA. Results indicated that prior to settlement most demersal larvae aggregate in shoals and exhibit distinct microhabitat preferences on the reef. In a field experiment, larvae settled both during the day and at night. Dispersion at settlement was aggregated, suggesting that demersal shoaling influences settlement patterns in this species. The distribution of demersal larvae also indicated that larval swimming behavior is sufficiently strong to permit active control of position on reefs. Large demersal larvae settled rapidly when brought to the laboratory, but small larvae in demersal shoals appeared to require additional growth and morphological development prior to settlement. Development of the pelvic fins, used by juveniles and adults for perching on the substrate, may be a good indicator of competence to settle in this species. The adaptive significance of demersal shoaling by small larvae of the naked goby, and the fate of these larvae, remains perplexing because the low feeding rates found for larvae shoaling near the reef surface should slow or prevent the growth and development required prior to settlement. Observations made by other authors indicate that demersal shoaling and the use of water directly overlying reefs may be common behaviors of temperate and tropical reef fishes.     

Differences in the structures of fish assemblages inThalassia testudinum beds in Guadeloupe,French West Indies,and their ecological significance

The structures of fish assemblages in twoThalassia testudinum beds in Guadeloupe, French West Indies, one adjacent to mangroves and the other adjacent to coral reefs, were compared between January 1983 and May 1984. The aim of the study was to compare the influences of mangroves and coral reefs on the utilization of seagrass beds by fishes through examination of species composition, catch rate, size of fishes and temporal changes. The two fish assemblages were similar in terms of the number of species they had in common (nearly 44% of the total number of species collected) and the great abundance of juveniles. They both comprised species that usually inhabit other habitats, i.e., estuaries, open waters or coral reefs. Estuary-associated species (e.g. Gerreidae) were the most abundant species in the seagrass bed near the mangroves, while small pelagic species (e.g. Clupeidae) were the most abundant species in the seagrass bed near the coral reefs. The seagrass bed near the mangroves was preferentially utilized as a nursery area by small juveniles of various species (e.g. Clupeidae, Sparidae, Gerreidae, and at least one coral reef species,Ocyurus chrysurus). The abundance of these species varied frequently, suggesting successive arrivals and departures of juveniles over time. The seagrass bed near the coral reefs was characteristically utilized by fishes that are more able to avoid predation, i.e., fishes that forage over seagrass beds at night and shelter in or near the coral reefs during the day (large juveniles of coral reef species and adults of schooling pelagic species, respectively). The constant migrations of these fishes between the coral reefs and seagrass beds explained the relative stability of the structure of the fish assemblage in the seagrass bed over time. Thus, the two seagrass beds were not equivalent habitats for fishes. The distinct ecological influences of the mangroves (as a nursery for small juveniles) and coral reefs (as a shelter for larger fishes) on the nearby seagrass beds was clearly reflected by the distinct utilizations of these seagrass beds by fishes.     

Copépodes planctoniques du lagon de Nouvelle-Calédonie: facteurs écologiques et associations d'espèces

In the southwestern part of the lagoon of New Caledonia (South Pacific Ocean), a plankton sampling programme was conducted from February 1978 to April 1979. During 11 cruises, 5 stations in the open sea and various bays were sampled at approximately monthly intervals. A transect of 3 stations (mid lagoon, near-reef and barrier-reef channel) visited each fortnight, completed the sampling programme. The copepods collected were identified to the specific level, counted and the 52 most abundant species analysed for seasonal and regional variations. Cruise and transect data, dealt with separately, were analysed as follows: (a) Correspondence analyses (reciprocal averaging) of qualitative and quantitative copepod counts were made; these revealed the relationships between species and between samples by means of factorial design. After computation of factorial axes, the barycentres of different sets of samples were projected as reference points (zero weight) in the factorial space; these reference points, characterizing different stations and sampling times, facilitate ecological interpretation. (b) Species partition was achieved by two successive methods: non-hierarchical, followed by hierarchical classification. Between-species distance was computed from their co-ordinates on the factorial axis. (c) The clusters of species obtained were plotted in the factorial planes to assess ecological preference. Then, clusters from cruise and transect data sets were compared to improve the copepod classification. The main ecological factors appear to be (i) spatial patterns, (ii) seasonal temperature cycle, (iii) changes in wind force and direction. Different populations inhabit the open sea, near-reef, mid-lagoon, shallow and deep-bay waters. Acartia australis outnumbers all other species in the reef vicinity. A. amboinensis is the most abundant in the deep, fjord-like bay. Canthocalanus pauper, Paracalanus parvus, Bestiola sp., Centropages orsinii, A. bispinosa are characteristic of shallow bays. In the factorial structure produced by correspondence analysis, certain seasonal barycentres appear close to some station barycentres: summer close to mid-lagoon, winter close to open sea, and spring close to near-reef barycentres, respectively. This may be explained by the seasonal dynamics of the lagoon water. After the heavy summer rainfalls, freshwater runoff carries midlagoon plankton towards the open sea; conversely, during the winter westerly gales, oceanic species enter the lagoon through the barrier-reef channel or above the reef with the swell breakers. In October, the surface-layer current induced by the strong trade wind carries large swarms of A. australis out to the open sea. Finally, the variations of plankton populations and biomass in the lagoon seem to be governed by the direction of water flow across the reef channels. Enrichment factors are terrestrial sediment wash-out after rainfall and, probably, trade wind-induced upwelling. Therefore, the more or less steady state of the lagoon's plankton biomass may result from the fact that enrichment factors are also exportation factors. Reciprocally, the entry of plankton-poor oceanic water cannot increase the plankton biomass in the lagoon.     

Variation in emergence of parasitic and predatory isopods among habitats at Lizard Island, Great Barrier Reef

Gnathiid isopods are one of the most abundant groups of ectoparasites on coral reef fishes. They, and other isopods, have been shown to significantly affect the health and behaviour of many reef fish. Whether isopod emergence differs among habitats on coral reefs is not known. In this study, we measured emergence rates of parasitic isopods (Gnathiidea and Flabellifera) in six habitats at two sites at Lizard Island during new moon periods in March and December 2004. Isopods were collected from the periphery and centres of micro-reefs, patch reefs, continuous reefs, and from inter-reefal habitats (sand or rubble) with 1 m² emergence traps. Sites (Casuarina and Coconut Beach) were located on opposite sides of Lizard Island. Live gnathiids were collected with light traps in November 2005 to investigate species differences between sites. At both sites, the most abundant gnathiid species was exclusive to that site. More gnathiid larvae emerged at night, and emergence of fed gnathiids (pranizae) and flabelliferan isopods was almost exclusively nocturnal. Diurnal emergence was greater at Coconut Beach than Casuarina Beach. Although emergence counts were not consistently affected by parameters such as habitat, site, or sampling period, gnathiid size and feeding state were. Where significant differences existed, gnathiids were larger and more often fed over reef borders than centrally. We suggest first stage larvae (Z1) have the largest influence on total abundance and are patchily distributed in accordance with adults from which they have recently hatched. As later stage larvae depend on fish, more successful (fed) and older larvae are found on the edges of reefs where appropriate hosts may be more abundant, or predation is lower. Gnathiids were over-dispersed in all habitats investigated, including apparently homogeneous beds of coral rubble and sand. This indicates that their distributions may be better predicted by very fine scale differences in substrate or that aggregations are the result of gregariousness and may be difficult to predict on the basis of substrate. Emergence traps collected comparatively few parasitic flabelliferan isopods. This community differed greatly from the previously described community of scavenging isopods at Lizard Island. These differences are probably the result of differences in trapping methodology.     

Substratum preferences in planula larvae of two species of scleractinian corals, Goniastrea retiformis and Stylaraea punctata

To test whether coral planulae recruit randomly to different coral reef habitats or have specific substratum preferences, the settling behavior of planulae from two shallow water coral species from Pago Bay, Guam (13°25.02N, 144°47.30E) were examined in the laboratory in June and July of 1995. Goniastrea retiformis is generally restricted to the shallow reef front (<10 m depth) in areas dominated by crustose coralline algae (CCA), while Stylaraea punctata is abundant on inner reef flats were CCA coverage is low and sand and carbonate rubble covered by biofilms is common. When presented with four substrata (1) carbonate rock scrubbed free of biofilm and dried as a control, (2) the CCA Hydrolithon reinboldii, (3) the CCA Peyssonelia sp., and (4) naturally conditioned carbonate rubble covered by a biofilm, G. retiformis larvae showed a significant preference for H. reinboldii, and S. punctata larvae for the carbonate biofilm treatment. The preference shown by S. punctata larvae for biofilmed surfaces did not diminish with increasing larval age up to 11 days. These results suggest that the larvae of both species are capable of habitat selection, and that the preferred substrata among those tested bears a relationship to the habitats in which adult colonies were found. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

In situ swimming and settlement behaviour of larvae of an Indo-Pacific coral-reef fish, the coral trout Plectropomus leopardus (Pisces: Serranidae)

Late larvae of the serranid coral trout Plectropomus leopardus (Lacepède), captured in light traps, were released during the day both in open water and adjacent to two reefs, and their behaviour was observed by divers at Lizard Island, northern Great Barrier Reef. Coral trout larvae (n = 110) were present in light-trap catches from 18 November to 3 December 1997, including new moon (30 November). The swimming speed of larvae in open water or when swimming away from reefs was significantly greater (mean 17.9 cm s⁻¹) than the speed of larvae swimming towards or over reefs (mean 7.2 cm s⁻¹). Near reefs, larvae swam at average depths of 2.7 to 4.2 m, avoiding 0 to 2 m. In open water, swimming depth varied with location: larvae >1 km east of Lizard Island swam steeply downward to >20 m in 2 to 4 min; larvae >1 km west oscillated between 2.6 and 13 m; larvae 100 to 200 m east of Lizard Island oscillated between 0.8 and 15 m. Nearly all larvae swam directionally in open water and near reefs. In open water, the average swimming direction of all larvae was towards the island, and 80% (4 of 5) swam directionally (p < 0.05, Rayleigh's test). Larvae swam directionally over the reef while looking for settlement sites. The frequency of behaviours by larvae differed between two reefs of different exposure and morphology. Depending on site, 26 to 32% of larvae released adjacent to reefs swam to open water: of these, some initially swam towards or over the reef before swimming offshore. In some cases, offshore-swimming seemed to be due to the presence of predators, but usually no obvious cause was observed. Depending on the reef, 49 to 64% of the larvae settled. Non-predatory reef residents aggressively approached 19% of settlers. Between 5 and 17% of the larvae were eaten while approaching the reef or attempting to settle, primarily by lizardfishes but also by wrasses, groupers and snappers. A higher percentage of larvae settled in the second week of our study than in the first. Average time to settlement was short (138 s ± 33 SE), but some larvae took up to 15 min to settle. Average settlement depth was 7.5 to 9.9 m, and differed between locations. No settlement took place on reef flats or at depths <4.2 m. Larvae did not appear to be selective about settlement substrate, but settled most frequently on live and dead hard coral. Late-stage larvae of coral trout are capable swimmers with considerable control over speed, depth and direction. Habitat selection, avoidance of predators and settlement seem to rely on vision. Received: 7 July 1998 / Accepted: 26 January 1999     

Influence of habitat on abundance and size structure of a large temperate-reef fish, Achoerodus viridis (Pisces: Labridae)

The relationship between densities of Achoerodus viridis (Pisces: Labridae) and reef habitats at various localities within New South Wales (NSW), Australia was examined. Types of habitats were quantified at inner, mid and outer estuarine locations in each of two estuaries (Botany Bay and Port Jackson) to determine whether purported patterns of movement from estuaries could be related to differences in habitat. Although the same types of habitat were generally found at all locations, differences in the proportion of habitat types were found between shallow and deep reefs and among inner, mid and outer estuarine locations for both estuaries. Shallow habitats were usually dominated by Ecklonia radiata, turf and/or fringe habitat in Botany Bay, whereas deep sites were generally dominated by urchin-grazed barrens habitat and, sometimes, sponge- and ascidian-dominated deep reef. Shallow sites in Port Jackson were dominated by a mixture of habitats, as were deep reefs at mid-estuarine locations. Other deep reefs in Port Jackson were dominated by E. radiata (inner estuarine) or barrens (outer estuarine) habitat. Thus, patterns of habitat cover were not consistent between estuaries and numbers of fish could not be related to proportional representation of habitat on reefs along estuarine gradients. Univariate and multivariate analyses showed that there was little evidence that any size class of fish was correlated with the proportional representation of a particular habitat or group of habitats. Counts of fish that focused on barrens and E. radiata forest habitats over a period of 10 yr showed that similar numbers and all sizes of fish were found in the two types of habitat. Greater numbers of small fish were, however, found in the E. radiata forest habitat than in the barrens habitat. Estimates of abundance along the coast of NSW (100s to 1000 km) in a range of habitats (e.g. ascidian-dominated reef, kelp forest, urchin-grazed barrens) showed that there was no indication that a particular habitat consistently had greater numbers of A. viridis than other habitats. Therefore, A. viridis of a range of sizes appears to be flexible in its use of habitats on reefs. Received: 24 December 1997 / Accepted: 23 June 1998     

Ecology and characteristics of bdellovibrios from three tropical marine habitats

Marine bdellovibrios have not previously been reported from the southern hemisphere, and knowledge of their occurrence in marine ecosystems is rudimentary. This study examined quantitative and qualitative aspects of bdellovibrios parasitic to the bacterium Vibrio alginolyticus at each of three representative tropical marine habitats of the Great Barrier Reef. Bdellovibrios were found in the water column throughout a 12 mo period from May 1992 at a sandy beach, a mangrove and a fringing coral reef. Their abundance was correlated with water temperature (P<0.001) and was highest in summer, lowest in winter and intermediate in spring and autumn. Over the sampling period, bdellovibrios were most abundant at the mangrove habitat (36.6 ml^-1) and least abundant at the reef (9.5 ml^-1), but there was substantial variability in numbers at all habitats among seasons and months of the year. On some occasions no bdellovibrios were found in replicate samples from the beach and reef habitats, while on others the maximum detectable by the method used (180 ml^-1) was sometimes found at the beach and mangrove habitats. Bdellovibrios within each habitat were uniformly distributed among sampling sites (P>0.05). They were more abundant in sub-surface than bottom waters in summer, but the reverse occurred in winter. Midwater samples usually had least bdellovibrios. Bdellovibrio numbers were significantly correlated with those of potential host bacteria—colony-forming bacteria at all habitats and total bacteria at the beach and reef habitats. Strain characteristics, primarily based on host range, indicated qualitative differences in bdellovibrio populations among habitats. Pseudomonas atlantica, P. aeruginosa, P. marina, Cytophaga marinoflava, Vibrio gazogenes, V. mimicus and a Spirillum-like bacterium were not parasitised by bdellovibrios from any habitat. Of the other 25 Vibrio spp. tested, most were parasitised by the majority of bdellovibrio strains from each habitat. Strain differences were principally with respect to parasitism of non-Vibrio bacteria. All strains required Na⁺ and grew at 35°C, but some failed to grow at 15°C.     

Demographic plasticity in tropical reef fishes

We use age-based analyses to demonstrate consistent differences in growth, mortality, and longevity of coral reef fishes from similar habitats (exposed reef crests) 20 km apart. On outer-shelf reef crests of the northern Great Barrier Reef (GBR), size in four taxa of reef fishes (Chlorurus sordidus, Scarus frenatus, and S. niger and the acanthurid Acanthurus lineatus) was systematically and significantly smaller when compared with the same taxa on adjacent mid-shelf reef crests. Differences in size could be attributed to differences in growth between habitats (shelf positions). On outer reef crests the species examined had consistently lower size at age profiles and also reduced life spans compared with populations from mid-shelf reefs. To confirm this relationship, two of the most abundant species (C. sordidus and S. frenatus) were selected for more detailed spatial analysis of demographic patterns. Sampling adults of both taxa from reef crests on three mid- and three outer-shelf reefs revealed that most of the variation in growth was explained by shelf position, although C. sordidus also displayed differences in growth among mid-shelf reefs. We conclude that differences in body sizes across the continental shelf of the northern GBR are primarily determined by these trends in growth. Strong spatial patterns also existed in the mean ages of populations and longevity estimates for C. sordidus and S. frenatus between shelf positions. Both species on outer-shelf reefs displayed less variable cohort sizes, significantly reduced mean ages, and foreshortened longevity compared with populations on mid-shelf reefs. Furthermore, differences in these parameters were rare among replicate reefs within mid- and outer-continental-shelf positions. Age-based catch curves suggested that rates of S. frenatus natural mortality on the outer shelf were nearly twice as high as on the mid shelf. Visual surveys indicated that total scarid densities on outer-shelf reef crests are on average fourfold higher than for equivalent mid-shelf habitats. This fact, coupled with significantly reduced growth rates, reduced mean ages, and increased mortality rates, suggests that density-dependent processes may be responsible for observed differences among localities.     

Distribution,abundance, and substrate preferences of demersal reef zooplankton at Lizard Island Lagoon,Great Barrier Reef

Demersal zooplankton, those plankton which hide within reef sediments during the day but emerge to swim freely over the reef at night, were sampled quantitatively using emergence traps planced over the substrate at Lizard Island Lagoon, Great Barrier Reef. Densities of zooplankton emerging at night from 6 substrate types (fine, medium, and coarse sand, rubble, living coral and reef rock) and from 5 reef zones (seaward face, reef flat, lagoon, back reef, and sand flat) were determined. A large population of nocturnal plankton including cumaceans, mysids, ostracods, shrimp, isopods, amphipods, crustacean larvae, polychaetes, foraminiferans and copepods are resident members of the reef community at Lizard Island. The mean density of plankton emerging throughout the reef was 2510±388 (standard error) zooplankton/m² of substrate. Biomass averaged 66.2±5.4 mg ash-free dry weight/m² of substrate. Demersal zooplankton exhibited significant preferences for substrate types and reef zones. The highest mean density of zooplankton emerged from coral (11,264±1952 zooplankton/m²) while the lowest emerged from reef rock (840±106 zooplankton/m²). The density of demersal plankton was six times greater on the face than in any other zone, averaging 7900±1501 zooplankton/m². Copepods dominated samples collected over living coral and rubble while foraminiferans, ostracods and decapod larvae were most abundant from sand. Plankton collected with nets at night correlated only qualitatively with plankton collected in emergence traps from the same location. Although abundant, demersal plankton were not numerous enough to meet the metabolic needs of all corals at Lizard Island Lagoon. Demersal plankton appear especially adapted to avoid fish predation. The predator-avoidance strategies of demersal plankton and maintenance of position on the reef are discussed. Our results indicate that much of the zooplankton over coral reefs actually lives on the reef itself and that previous studies using standard net sampling techniques have greatly underestimated plankton abundance over coral reefs.     

Hydrography and distribution dynamics of larval and juvenile fishes in the coastal waters of the Tanshui River estuary,Taiwan, with reference to estuarine larval transport

Distribution dynamics of fish larvae and juveniles in the coastal waters of the Tanshui River, Taiwan was studied fortnightly using surface horizontal tows with a larval net in daytime during the period from early April through early June 1991. Environmental factors, including water temperature, salinity, dissolved oxygen, pH, transparency and depth at sampling stations, were also monitored. A total of 10737 fish eggs and 1387 individuals, representing 43 families and 93 species, was collected during five cruises from 12 stations in the coastal waters. Most fish were estuarine-dependent marine species. Liza macrolepis, Ambassis gymnocephalus, Terapon jarbua, Mullidae and Gobiidae were the most dominant, making up 64.7% of the total catch. Early life stages, including egg, preflexion, flexion and postflexion larvae were abundant in surface samples. However, yolk-sac larvae were absent in the surface water, probably due to an ontogenetic behavioral shift as a consequence of a change in specific weight during early development. The species composition of fish larvae and juveniles was related to the microhabitats found in the coastal waters. The physico-chemical conditions, along with ontogenetic behavior, played an important role in larval fish distribution in the coastal waters.     

Dynamics of larval fish assemblages over a shallow coral reef in the Florida Keys

Few time series collections have been made of the larval ichthyofauna in waters directly above shallow coral reefs. As a result, relatively little is known regarding the composition and temporal dynamics of larval fish assemblages in shallow-reef waters, particularly those near a major western boundary current. We conducted a series of nightly net tows from a small boat over a shallow reef (Pickles Reef) along the upper Florida Keys during four new moon and three third-quarter moon periods in July (two new moons), August, and September 2000. Replicate tows were made after sunset at 0–1 m and at 4–5 m depth to measure the nightly progression in community composition, differences in depth of occurrence, and abundance and diversity with lunar phase. A total of 66 families was collected over the 3-month period, with a mean (±SE) nightly density of 23.7±2.1 larvae per 100 m ³ and diversity of 24.2±0.9 taxa per tow. A total of 28.8% of the catch was composed of small, schooling fishes in the families Atherinidae, Clupeidae, and Engraulidae. Of the remaining catch, the top ten most abundant families included reef fishes as well as mangrove and oceanic taxa (in descending order): Scaridae, Blennioidei (suborder), Gobiidae, Paralichthyidae, Lutjanidae, Haemulidae, Labridae, Gerreidae (mangrove), Balistidae, and Scombridae (oceanic). These near-reef larval fish assemblages differed substantially from those collected during previous offshore collections. Taxa such as the Haemulidae were collected at a range of sizes and may remain nearshore throughout their larval period. Overall, the abundance and diversity of taxa did not differ with depth (although within-night vertical migration was evident) or with lunar phase. Temporal patterns of abundance of larval fish families clustered into distinct groups that in several cases paralleled family life-history patterns. In late July, a sharp shift in larval assemblages signaled the replacement of oceanic water with inner shelf/bay water. In general, the suite and relative abundance of taxa collected each night differed from those collected on other nights, and assemblages reflected distinct nightly events as opposed to constant or cyclical patterns. Proximity to the Florida Current likely contributes to the dynamic nature of these near-reef larval assemblages. Our results emphasize the uniqueness of near-reef larval fish assemblages and point to the need for further examination of the biophysical relationships generating event-related temporal patterns in these assemblages.     

Assessment of the nutritional condition of larval and early juvenile tuna and Spanish mackerel (Pisces: Scombridae) in the Panamá Bight

The nutritional condition of first-feeding and late larval/early juvenile scombrids was investigated in waters of the northwestern Panamá Bight from May through early November 1988. Wild-caught larvae and juveniles of three taxa, black skipjack tuna (Euthynnus lineatus), bullet and/or frigate tuna (Auxis spp.) and sierra (Scomberomorus sierra), were examined histologically to determine nutritional condition. The incidence of malnourishment in wild-caught preflexion (first feeding—prior to notochord flexion) larvae of all taxa was high. Starvation rates for E. lineatus and Auxis spp. preflexion larvae ranged from 62 to 63% d^-1, while the percentage of larvae actually dying of starvation was estimated at 41 to 43% d^-1. The nutritional point-of-no-return for preflexion larvae was estimated at 1 to 2 d maximum. The cellular condition of liver hepatocytes, particularly the relative amount of vacuolation related to storage of glycogen and lipid, proved to be a sensitive indicator of nutritional condition. In laboratory trials, late larval (postflexion) and early juvenile black skipjack exhibited a nutritional point-of-no-return of 2 to 3 d. Although postflexion larvae were moderately vulnerable to malnourishment in laboratory trials, <13% of wild-caught postflexion larvae exhibited even mild nutritional stress, and no postflexion larvae or juveniles showed signs of severe malnourishment. This pattern of starvation incidence suggests that tropical scombrids undergo stagespecific starvation mortality. Preflexion larvae can suffer significant daily losses due to starvation, while postflexion larvae and early juveniles seem to experience a rapid improvement in feeding ability and/or food availability.     

Among-habitat variation in herbivory on Sargassum spp. on a mid-shelf reef in the northern Great Barrier Reef

Herbivory is widely acknowledged as a key process determining the benthic community structure and resilience of coral reefs. Despite numerous studies that have examined herbivory across reef gradients in the Caribbean, few studies have directly quantified this process on Pacific reefs. Bioassays of two species of erect macroalgae (Sargassum swartzii and S. cristaefolium) were used to quantify variation in grazing intensity across seven habitats of varying depth and wave exposure on a mid-shelf reef in the northern Great Barrier Reef. Removal rates of Sargassum varied significantly among habitats, with both species displaying broadly similar patterns. The shallow habitats on the exposed aspect of the reef (i.e. reef crest, flat and back reef) experienced the highest reductions in mass (81.4–91.6% day⁻¹) for both S. swartzii and S. cristaefolium, while the deeper exposed habitats (reef slope and base) displayed the lowest reductions (3.8–13.4% day⁻¹) over a 24 h period. In contrast, the grazing intensity varied between the two species in the three habitats on the leeward aspect of the reef. Reductions in mass remained relatively high for S. swartzii on the patch reef and sheltered reef base and flat (62.7–76.5% day⁻¹) but were considerably lower for S. cristaefolium (37.9–63.5% day⁻¹) across the same habitats. Surprisingly, the rates of removal of Sargassum displayed no relationship with the density or biomass of roving herbivorous fishes or those species known to consume erect macroalgae, either collectively or independently. These results suggest that the relationship between browsing rates and herbivorous fish biomass is complex and may be driven by species that are underestimated in visual surveys. Direct quantification of browsing intensity using assays revealed a different pattern to inferences based on herbivore densities and highlights the potential difficulties of evaluating ecosystem processes based on visual census data alone.