Estimating total abundance of a large temperate-reef fish using visual strip-transects

Total abundance estimates for the large, common, reef fish Cheilodactylus spectabilis (Hutton) were obtained for a marine reserve and adjacent section of coast in north-eastern New Zealand during 1985. Visual strip-transects were used to estimate abundance and size structure in both areas. The accuracy, precision and cost efficiency of five transect sizes (500, 375, 250, 100, 75 m²) were examined over three times per day (dawn, midday and dusk), by simulating transects over mapped C. spectabilis populations. Two transect sizes showed similarly high efficiency. The smaller of the two (20x5 m) was chosen for the survey because of the general advantages attributable to small sampling units. Biases related to strip-transect size are discussed. Preliminary sampling indicated that C. spectabilis was distributed heterogeneously, and that density was habitat-related. An optimal stratified-random design was employed in both locations, to obtain total abundance and size-structure estimates. This reduced the between-habitat source of variability in density. The total number of sampling units used was governed by the time available. The resulting total abundance estimates obtained were 18 338±2 886 (95% confidence limit) for the 5 km marine reserve, compared to 3 987±1 117 for an adjacent, heavily fished 4 km section of coast. When corrected for total area and habitat area sampled, this represented a 2.3-fold difference in abundance. If sampling had been designed to detect an arbitrary 10% difference in abundance within each habitat, an infeasible 440 h of sampling would have been required. Size-frequency distributions of C. spectabilis at the reserve had a larger model size class than distributions from the adjacent area. The data suggest that reserve status is causal in these differing abundance and size structure estimates.     

Current and historic distribution and abundance of the inarticulated brachiopod, <Emphasis Type="Italic">Lingula reevii</Emphasis> Davidson (1880), in Kaneohe Bay,Oahu, Hawaii,USA

The inarticulated brachiopod, Lingula reevii Davidson (1880) is a filter-feeding invertebrate that burrows vertically in sandy or mixed sediments. Its only recorded occurrence is from Kaneohe Bay, Oahu, Hawaii, southern Japan, and Ambon, Indonesia. Past surveys of Kaneohe Bay populations suggested a distinct decrease in abundance following the diversion of sewage effluent from the bay in 1978/1979. In the summer of 2004 and 2007, visual surveys were conducted in areas of historical L. reevii abundance as well as in areas appearing to have suitable habitat. In 2004, approximately 2,950 m² at 20 sites within the bay were surveyed using quantitative belt transecting methods. A maximum density of 4 Lingula/m² was observed, a decrease from previous maximum estimates of 500 individuals/m² (Worcester, Dissertation, Zoology Department, University of Hawai′i, pp 49, 1969) and 100 individuals/m² (Emig, J Exp Mar Biol Ecol 52:47–61, 1981). When these 20 sites were revisited in 2007, many had fewer or no L. reevii; therefore, broader scale presence/absence surveys were conducted at 16 additional sites in the bay (also surveyed in 2004). The highest density of L. reevii found in 2007 was 0.94 individuals/m². The continued decline in abundance of L. reevii in Kaneohe Bay may be due, in addition to decreased organic enrichment from diversion of sewage discharge almost 30 years ago, to the more recent reduction of suitable habitat by the invasion of mat-forming alien algae species.     

Surface area in ecological analysis: Quantification of benthic coral-reef algae

The surface area of organisms and substrata is shown to be a significant ecological parameter because of its functional importance in the system. Quantification of surface area can be of particular value in morphologically complex environments such as coral reefs. The amount of surface in a reef habitat can be estimated by direct measurements and theoretical approximations, using a surface index (SI) for the amount of surface increase over that of a similarly bounded plane. SI values for a section of the British Honduran barrier reef ranged up to 15 in the reef-crest area at scales significant to macro-organisms. By combining substrate-area measurements with estimates of percent coverage of the major benthic algal components, a reef transect with a horizontal area of 300 m² was shown to have over 300 m² covered by benthic macroalgae. The potential for the further development of surface estimation techniques is discussed.     

Quantitative distribution of herbivorous reef fishes in the Gulf of Aqaba (Red Sea)

The distribution of the main herbivorous fishes (Acanthuridae, Scaridae, Siganidae) was studied across a coral reef of the Jordanian coast in the Gulf of Aqaba (Red Sea). Visual counts were realized by diving along transects (200 m long and 5 m wide), parallel to the shore, at 10 stations located from the lagoon to 40 m deep on the outer reef slope. Herbivorous reef fishes reach their highest abundance on the reef front, where 234 fishes were counted per 1,000 m². Their density decreases on the reef flat, with an average of 150 fish 1,000 m^-2, and is lowest on the outer reef slope (69 fish 1,000 m^-2). Surgeonfishes form 63% of the herbivorous ichthyofauna, parrotfishes 35%, and rabbitfishes 2%. Families and species display different distributions according to biota. The Acanthuridae dominate on the reef flat, whereas the Scaridae are more numerous on the outer reef slope. The evolution of the social structure of the main species was observed: the adults generally school in the lagoon and on the reef flat, but are mainly solitary on the reef slope. The distribution of juvenile individuals is more restricted: they are concentrated on the reef front and on the upper part of the reef slope.This study is part of a cooperation programme between the University of Nice (France) and the University of Jordan, to study the ecology of the coral reefs and the surrounding waters of the Jordanian coast (Gulf of Aqaba, Red Sea)     

Phytoplankton in the Weddell Sea,Antarctica: composition,abundance and distribution in water-column assemblages of the marginal ice-edge zone during austral autumn

At the compacted, north-south line of the ice edge, phytoplankton were sampled during early austral autumn of 1986 in the northwestern Weddell Sea. Cells from discrete water bottle samples from 12 stations on two east-west transects were counted to gain quantitative information on the composition, abundance, distribution, and condition of the phytoplankton in water-column assemblages. Over 70 species were found. The highest numbers of total cells (integrated through the top 150 m) were found in open water, well-separated from and to the east of the ice edge on the southern transect, with 6.01×10¹⁰ cells m^-2. The relative abundance of diatoms was low at ice-convered stations (< 35% of the total phytoplankton in preserved samples) and high at open-water stations (> 80%); however, the relative abundance of the prymnesiophyte Phaeocystis sp. was high at ice-covered stations (> 60%) and low at open-water stations (< 16%), with lower absolute abundances than during a previous austral-spring phytoplankton increase. In the open ocean, the dominants were the pennate diatoms Fragilariopsis cylindrus, Pseudonitzschia prolongatoides, F. curta, and a small form of the centric diatom Chaetoceros dichaeta in chains. Although the three pennate diatoms were frequently dominant in number, they represented less biomass than C. dichaeta in open waters. Mean phytoplankton abundance was low (0.2×10⁶ cells l^-1) but, overall, the diatom cell density (0.14×10⁶ cells l^-1) was similar to that found previously during a northward transect from ice-covered to ice-free water at the Weddell-Scotia Sea ice edge (spring 1983). The phytoplankton spatial patterns in the two autumn transects differed, with the more southerly transect exhibiting a higher abundance of diatoms and dinoflagellates. The ratio of full to empty diatoms was higher on the southern transect, indicating a healthy population, while lower ratios of full/empty frustules on the northern transect suggested a generally declining population. However, Phaeocystis sp. was more abundant on the northern transect.     

Combined Effects of Levels of Protection and Environmental Variables at Different Spatial Resolutions on Fish Assemblages in a Marine Protected Area

Abstract: The links between species–environment relations and species’ responses to protection are unclear, but the objectives of marine protected areas (MPAs) are most likely to be achieved when those relations are known and inform MPA design. The components of a species’ habitat vary with the spatial resolution of the area considered. We characterized areas at two resolutions: 250 m² (transect) and approximately 30,000 m² (seascape). We considered three categories of environmental variables: substrate type, bottom complexity, and depth. We sought to determine at which resolution habitat characteristics were a better predictor of abundance and species composition of fishes and whether the relations with environmental variables at either resolution affected species’ responses to protection. Habitat features accounted for a larger proportion of spatial variation in species composition and abundances than differences in protection status. This spatial variation was explained best by habitat characteristics at the seascape level than at the transect level. Species’ responses to protected areas were specific to particular seascape characteristics, primarily depth, and bottom complexity. Our method may be useful for prioritizing marine areas for protection, designing MPAs, and monitoring their effectiveness. It identified areas that provided natural shelter, areas acting as buffer zones, and areas where fish species were most responsive to protection. The identification of such areas is necessary for cost‐effective establishment and monitoring of MPAs.     

Cross-shelf distribution of copepods and fish larvae across the central Great Barrier Reef

The distribution of total dry weight of zooplankton, copepod numbers and ichthyoplankton across the outer continental shelf in the central Great Barrier Reef was examined at bi-weekly intervals for three months over summer of 1983. Copepods were sampled (236 m net) within 10 m of the surface and within 10 m of the bottom. Mean densities in surface waters decreased markedly from the mid-shelf to outer shelf and the Coral Sea, but no cross-shelf gradient occurred in the bottom-water. Densities of copepods on the mid-shelf (surface and bottom waters) and in bottom-waters of the outer shelf were typically ca. 400 m^–3. Significantly lower densities (ca. 100 m^–3) occurred in surface waters of the outer shelf, except during outbursts of Acartia australis, when densities in these waters differed little from those elsewhere on the shelf. In oceanic waters, 10 km from the outer shelf station, copepod densities in surface waters were ca. 40 m^–3. Four of the five most abundant copepod taxa in surface waters, Paracalanus spp., Eucalanus crassus, Acrocalanus gracilis and Canthocalanus pauper, tended to be most abundant at the mid-shelf end of the transect. Acartia australis was sporadically very abundant in surface waters of the outer shelf, as was Paracalanus spp. in bottom-water of the outer shelf. An assemblage of Coral Sea species of copepod occurred in bottom-water of the outer shelf during two major intrusions, but not at other times. Densities of all common species varied considerably between cruises. Maximum densities of all common species except A. australis tended to be associated with diatom blooms linked to intrusions but a bloom did not necessarily mean all common species were abundant. Fish larvae included both reef and non-reef taxa, with reef taxa predominating on the outer shelf (approx 2:1 in density of individuals) and non-reef taxa dominating in nearshore samples (approx 2:1). Nine of the ten most abundant taxa analysed showed highly significant variation in numbers among stations and all but one of these also exhibited significant station x cruise interactions. Interactions generally reflected changes in the rank importance of adjacent stations from one cruise to the next or lack of any significant cross-shelf variation on some cruises where overall abundance of the taxa was low.     

Habitat and reef-fish assemblages of banks in the Northwestern Hawaiian Islands

The moderately deep terraces and banks of the Northwestern Hawaiian Islands (NWHI) were surveyed to describe their habitat and reef-fish assemblages. These tracts of bottom at 30–40 m comprise more than 4,500 km² of the regions reef area. The habitat was found to be dominated by algal meadows (65% cover of exposed bottom), with infrequent relief features. Annual monitoring of select stations for 4 years at Necker Bank indicated that the relative difference in algal abundance between stations persisted from year to year (at least in summer). Temperature records from year-long deployments of archival thermistors in high-cover (>70%) and low-cover (<30%) algal biotopes were indistinguishable, providing no explanation of the algal differences between stations. At all banks, Microdictyon was the primary alga, averaging 1.22 kg/m². In spite of the extensive standing primary production, and a historical lack of fishing, bank reef-fish populations were impoverished. Mean densities, sizes, and biomass of trophic groups were considerably less than values reported for NWHI reef shallows. An overall mean biomass was estimated at 22.5 g/m², which is a fifth of that reported for shallow reefs of the region. Fish biomass of all trophic groups was associated with the few sources of relief available on the banks. Apex predators (sharks, jacks, and snappers), common on all surveys (with a mean of five per station), were proposed to constrain fish populations to sparse sources of relief resulting in a skewed size structure of the two primary fish trophic components. Sizes of lower-level carnivores were tightly correlated with sources of relief whereas the size of herbivores were not, indicating that herbivores more often venture out and risk the exposed algal meadows. These bank summits are a rare example of a near pristine reef system with high benthic primary productivity and low fish biomass, and are a stark contrast to shallower coral-reef ecosystems of the NWHI.Communicated by P.W. Sammarco, Chauvin     

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.     

Zooplankton abundance and grazing at Davies Reef,Great Barrier Reef,Australia

Zooplankton abundance and grazing on autotrophic and heterotrophic particulate matter were measured along a transect across Davis Reef (18°5S; 147°39E) and in the back-reef lagoon over tidal and diel cycles during austral winter (August 1984). Zooplankton entering the reef from the surrounding shelf waters decreased in abundance over the reef flat, presumably because of predation. Within the reef lagoon, maximum daytime densities of pelagic copepods occurred during high water, suggesting an external input. At night, water-column zooplankton biomass increased by a factor of 2 to 3 due to the emergence of demersal reef zooplankton. Zooplankton grazing rates on heterotrophic particulate matter (bacteria + detritus and Protozoa) compared to phytoplankton were higher on the reef flat than on the fore-reef or lagoon. Within the lagoon, zooplankton grazing rates on heterotrophic material were maximum during high water, coincident with maximum tidal concentrations of particulate organic carbon. The combined demersal and pelagic zooplankton community were often able to crop 30% of the daily primary production by >2µm phytoplankton. However, >50% of phytoplankton biomass was in cells <2µm, presumably unavailable to these zooplankton. Our particulate production and ingestion measurements, together with zooplankton carbon demand extrapolated from respiration estimates, suggest that the zooplankton community of Davies Reef derives much of its nutrition from detritus.Joint contribution from the University of Maryland, Center for Environmental and Estuarine Studies (No. 2015), and the Microbial Ecology on a Coral Reef Workshop (MECOR No. 19)     

Latitudinal variation in abundance of herbivorous fishes: a comparison of temperate and tropical reefs

The aim of the study was to provide comparable estimates of abundance of herbivorous reef fishes at temperate and tropical localities using a standardized methodology. Faunas of herbivorous fish were sampled on the rocky reefs of temperate northern New Zealand and on the coral reefs of the northern Great Barrier Reef (GBR), Australia, and the San Blas Archipelago in the Caribbean. A pilot study established the most appropriate habitat setting and the scale and magnitude of replication for the sampling program in temperate waters. Herbivorous fishes, including members of families endemic to the southern hemisphere (Odacidae and Aplodactylidae), were most abundant in turbulent, shallow water (0 to 6 m) and had patchy distributions within this habitat. A hierarchical sampling program using 10-min transect counts within the 0 to 6 m depth stratum examined abundance patterns at a range of spatial scales including mainland and island coasts, localities separated by up to 100 km and sites separated by up to 10 km. This program identified a characteristic fauna of seven species of herbivorous fishes with mean total abundances ranging from 23 to 30 individuals per 10-min transect. Species composition of the fauna varied between islands and coasts. A similar methodology was used to sample the major families of herbivorous fish in a number of sites in each of the tropical regions. These sampling programs revealed a fauna dominated by acanthurids and scarids in both the GBR and Caribbean localities. Estimates of abundance from these regions were similar, with a mean of 108 individuals recorded on the GBR and 129 per 10-min transect in the Caribbean. Species richness varied between each region, with 44 taxa recorded from the GBR and 11 from the Caribbean. Abundances of temperate water herbivores in New Zealand were found to be 75 to 80% lower than those recorded from shallow water habitats sampled on coral reefs. This was not related to species richness, since both New Zealand and the Caribbean locality had patterns of low richness. We suggest that the differences in abundance found by our study between temperate and tropical regions are not restricted to herbivorous fishes, but are representative of general latitudinal trends in reef fish faunas. Received: 4 November 1996 / Accepted: 15 December 1996     

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     

Home range, activity and distribution patterns of a temperate rocky-reef fish, Cheilodactylus fuscus

Red morwong (Cheilodactylus fuscus; Cheilodactylidae: Perciformes), are large temperate rocky reef fish that are patchily distributed in local aggregations of 3 to >100 fish. The home ranges and aggregating behaviour of red morwong were investigated using external tags (n = 114, over 35 mo) and ultrasonic transmitters (n = 9 over 20 d) at coastal and estuarine sites in south-east Australia. Adult red morwong had a mean home range during the day of 1865 m² (SE = 268). Night movement, determined by ultrasonic telemetry, indicated a significantly greater mean home range of 3639 m² (SE = 416). Home range did not differ significantly with size or sex. Fish movements were greatest and aggregation sizes varied most during the non-reproductive mid-summer period. Tagged fish (N = 20) displaced 200 to 900 m returned in 1 to 3 d to the point of capture, often traversing open sand habitat and other aggregations. During the day, fish were commonly found on bouldered habitat. During the night, these site-specific aggregations fragmented as fish dispersed over a variety of substrata, with crepuscular peaks in activity. Therefore, estimates of habitat-specificity and feeding patterns collected only by day may give misleading results. Received: 28 June 1996 / Accepted: 8 July 1998     

The structure of subtidal algal and invertebrate assemblages at the Chatham Islands,New Zealand

We examined the distribution and abundance of organisms on subtidal rocky reefs at nine sites around the Chatham Islands, a remote group 780 km east of southern New Zealand. We sampled five depth strata ranging from 1 to<16 m to identify spatial patterns in the abundance of algae and invertebrates and to assess their variation within and among sites. This information is used to discuss hypotheses concerning community structure at this remote locality. Several patterns were apparent. The immediate subtidal was occupied by the southern bull kelp Durvillaea spp. A suite of 11 fucalean species were dominant to a depth of 10 m with an average abundance of 28 m^-2, while one species, Carpophyllum flexuosum, occurred mostly in deeper water. Only two laminarian species of algae were present at the islands. The indigenous Lessonia tholiformis was abundant at 2.5 to 15 m and was not found in deeper water, while the giant kelp Macrocystis pyrifera was abundant at two sites in 12 to 18 m. The commercially valuable abalone Haliotis iris was extremely abundant in shallow water, with an overall mean of 6 m^-2 at 5 m. The sea urchin Evechinus chloroticus was common, but reached high densities only in small (<25 m²) patches. The characteristic urchin-dominated zones reported in kelp beds world-wide were not seen. There was considerable site-to-site variation in the occurrence and abundance of individual species. Some differences between sites were associated with shelter from swell (e.g. M. pyrifera was found only in sheltered sites) and physical habitat (e.g. juvenile H. iris were found only beneath boulders inshore), but much of the variation could not be explained by physical or depth-related factors alone. We hypothesize that the differences in these kelp bed assemblages compared to mainland New Zealand are partially due to the high degree of endemism at the Chatham Islands. Local variation cannot be explained by herbivory, and is most likely the result of the various life-history characteristics of the major habitat-forming species, the large brown algae.     

Annual abundance and biomass of aloricate ciliates in tropical neritic waters off Kingston,Jamaica

The daily abundance of aloricate ciliates at Lime Cay, Jamaica, a shallow neritic site, ranged from 29 to 118 × 10⁶ m^–2 (0.97 to 3.93 × 10⁶ m^–3) between November 1985 and November 1986. Biomass was converted to kilojoules (1 kcal=4.1855 kJ) assuming 42% carbon, 20.15 kJ (g dry wt)^–1, and 20% cell shrinkage. Biomass ranged from 0.40 to 3.00 kJ m^–2 (13.3 to 100 J m^–3; 0.28 to 2.08µg C l^–1) with an annual mean of 1.11 kJ m^–2 (36.8 J m^–3; 0.764µg C l^–1). Nanociliates (<20µm equivalent spherical diameter, ESD) dominated abundance, but microciliates (> 20µm ESD) dominated biomass.Strombidium, Strobilidium, Tontonia andLaboea species were conspicuous taxa. Annual production estimates of the aloricate assemblage, based on literature growth rates, ranged from 404 kJ m^–2 yr^–1 (37 J m^–3 d^–1) to 1614 kJ m^–2 yr^–1 (147 J m^–3 d^–1). A compromise estimate of 689 kJ m^–2 yr^–1 (i.e., 63 J m^–3 d^–1) is comparable to other estimates from tropical and subtropical regions. A model of annual energy flow through 11 planktonic compartments suggests the total ciliate assemblage (aloricates and tintinnines) to be as productive as metazoan herbivores and metazoan carnivores.     

Ecology of a Caribbean coral reef. The Porites reef-flat biotope: Part II. Plankton community with evidence for depletion

A quantitative assessment of drifting net plankton crossing a reef-flat biotope was obtained on a Caribbean coral reef. The spatial distribution and abundance of plankton were sampled to provide estimates of the removal of this potential food resource by suspension-feeding populations. Sampling was largely confined to the reef flat and adjacent waters of Laurel Cay, a flourishing coral reef present on the insular shelf off southwestern Puerto Rico. A prior study provides information on the meteorological and hydrographic characteristics of this area. Evidence for plankton accrual was found in the quantitative depletion of qualitatively similar populations sampled downstream of densely populated reef communities. Numerically, the diatom crop was reduced by 91% and zooplankton by 60% in water streaming off the reef. Significant diel and seasonal variations in plankton abundance were obcerved, as well as notable differences in volume flow, the latter closely related to the local wind regime. A time course of net plankton accrual was calculated, taking into account these various factors. During the summer season (July–August), when zooplankton was relatively abundant and water movement over the reef vigorous, the total gain from plankton reached 0.25 gC/m²/day; 75% of this occurred during a 4 h period at sunrise and sunset. Plankton retained on the reef flat in January of February and in September was around 0.1 g C/m²/day. Zooplankton biomass contributed the greatest share, exceeding that of diatoms by a factor of 10 during the day and 42 in the early evening. A mean annual accrual of 0.18 g C/m²/day is equivalent to 4 to 13% of net community metabolism.     

Population biology and antipredator defenses of the shallow-water Antarctic nudibranch Tritoniella belli

The circumpolar nudibranch Tritoniella belli Eliot occurs in abundance in shallow-water benthic communities of McMurdo Sound, Antarctica. Density estimates based on belt transects averaged collectively 0.46 individuals m⁻² at three study sites between depths of 6 and 30 m in November 1996. At two of the sites, population densities increased linearly between 18 and 30 m depth (up to 0.7 and 1.15 individuals m⁻² at 30 m depth). Individuals at all sites were rare or absent at depths shallower than 12 m. Size frequencies of individuals at the sites were similar, and a pooled analysis revealed a unimodal distribution skewed highly towards juvenile size classes. This suggests both recent recruitment and constant rates of mortality across size classes. The relationship between foot length and wet weight best fits an exponential growth equation, indicative of an allometric growth pattern. Distribution of T. belli in the field suggests that it is a habitat and diet generalist. Potential invertebrate predators include sea anemones and seastars, both of which co-occur in abundance in McMurdo Sound. Laboratory experiments indicate that the sea anemone Isotealia antarctica can capture and ingest T. belli. However, 70% of T. belli that are captured escape from the tentacles or, following ingestion, are rejected from the gastrovascular cavity. The seastars Odontaster validus, Perknaster fuscus, and Acodontaster conspicuus, avoid contact with T. belli, but if forced into contact with mantle tissues, retract their tube-feet. Mucus secreted from the mantle tissues, coated on to the tips of glass rods, and presented to seastar tube-feet, causes significantly longer tube-foot retraction times than control rods. Moreover, pieces of freeze-dried krill coated with mantle mucus are consumed significantly less often than untreated control pieces of krill by a benthic scavenging fish (Pseudotrematomas bernacchi). Employing seastar tube-foot retractions as a bioassay, we found the bioactive compound(s) are soluble in ethyl acetate, indicating they are lipophilic or moderately hydrophilic in nature. Chemical defenses in the mucus of T. belli probably contribute to its high abundance in Antarctic benthic communities. Received: 6 October 1997 / Accepted: 24 March 1998     

Diversity,abundance, and distribution of reef sharks on outer-shelf reefs of the Great Barrier Reef,Australia

Quantifying the distribution and habitat use of sharks is critical for understanding their ecological role and for establishing appropriate conservation and management regimes. On coral reefs, particularly the Great Barrier Reef (GBR), little is known regarding the distribution of sharks across major reef habitat types. In this study, we surveyed shark populations across outer-shelf reefs of the GBR in order to determine the diversity, abundance, and distribution of reef sharks across three major coral reef habitats: (1) the reef slope, (2) the back reef and (3) the reef flat. Model selection revealed that habitat was the principal factor influencing shark distribution and abundance. Specifically, overall shark abundance and diversity were significantly higher on the reef slope (and to a lesser degree, the back reef) than the reef flat. This confirms that shark populations are not homogeneously distributed across coral reefs. Thus, the results presented herein have important implications for shark population assessments. In addition, our results highlight the potential importance of the reef slope, with high levels of live coral cover and structural complexity, for sustaining reef shark populations. As this habitat is highly susceptible to disturbance events, this study provides a useful context for predicting and understanding how environmental degradation may influence reef shark populations in the future.     

Abundances,growth rates,and production of tropical neritic copepods off Kingston,Jamaica

Weekly samples were collected near Kingston, Jamaica in 27 m vertical hauls, using 200 and 64µm mesh plankton nets, from July 1985 to January 1987. Thirtytwo copepod species were identified; nauplii and all copepodite stages were enumerated. Total copepod abundance ranged from 2.56 to 87.3 × 10⁴ m^–2. The annual abundance cycle was bimodal with peaks in October–November and May–June corresponding to the rainy seasons. Mean annual copepodite biomass was 0.15 g AFDW m^–2 ranging from 0.03 to 0.41 g AFDW m^–2. Mean generation time (from egg to adult) at 28°C was 19.5 d for the common speciesCentropages velificatus, Paracalanus aculeatus, andTemora turbinata. Isochronal development was demonstrated for copepodites ofP. aculeatus andT. turbinata, but not forC. velificatus. Mean daily specific growth rates (G) were 0.63, 0.63, and 0.48 d^–1 forC. velificatus, P. aculeatus, andT. turbinata, respectively. In general, daily specific growth rates decreased in the later copepodite stages. Thus, it is postulated that growth of later stages and egg production may be food limited. Annual copepodite production was estimated as 419 kJ m^–2 yr^–1, while annual exuvial production and naupliar production were 35 and 50 kJ m^–2 yr^–1, respectively. Egg production was estimated as 44% (184 kJ m^–2 yr^–1) of the total copepodite production. Thus, mean total annual copepod production was 688 kJ m^–2 yr^–1. This estimate is within the range of copepod production estimates in coastal temperate regions.