Grey reef sharks (Carcharhinus amblyrhynchos) are apex predators found on many Indo-Pacific coral reefs, but little is known about their movement patterns and habitat requirements. We used acoustic ...telemetry to determine movements and habitat use of these sharks at the isolated Rowley Shoals atolls, 250 km off the coast of north-western Australia. We equipped 12 male and 14 female sharks ranging from 0.79 to 1.69 m in total length with transmitters that were detected by an array of 11 strategically placed receivers on two atoll reefs. Over 26,000 detections were recorded over the 325 days of receiver deployment. No sharks were observed to move between reefs. Receivers on the outer slopes of reefs provided nearly all (99%) of the detections. We found no differences in general attendance parameters due to size, sex or reef, except for maximum period of detection where larger sharks were detected over a longer period than smaller sharks. Male and female sharks were often detected at separate receivers at the outer slope habitat of one reef, suggesting sexual segregation, but this pattern did not occur at the second reef where males and females were detected at similar frequencies. We identified two patterns of daily behaviour: (1) sharks were present at the reef both day and night or (2) sharks spent more time in attendance during day than at night. Fast Fourier transforms identified 24-h cycles of attendance at the reef and a secondary peak of attendance at 12 h for most sharks, although no individuals shared the same attendance patterns. Our study provides baseline data that can be used to optimise the minimum area and habitat requirements for conservation of these apex predators.
Little information is available on the movements and behaviour of tropical rays despite their potential ecological roles and economic value as a fishery and a tourism resource. A description of the ...movement patterns and site fidelity of juvenile rays within a coral reef environment is provided in this study. Acoustic telemetry was used to focus on the use of potential nursery areas and describe movement patterns of 16 individuals of four species monitored for 1–21 months within an array of 51 listening stations deployed across a lagoon, reef crest, and reef slope at Mangrove Bay, Ningaloo Reef, Western Australia. Juveniles used a small (< 1 km²), shallow (1–2 m depth) embayment where three receivers recorded 60–80 % of total detections of tagged animals, although individuals of all species moved throughout the array and beyond the lagoon to the open reef slope. Detections at these primary sites were more frequent during winter and when water temperatures were highest during the day. Long-term use of coastal lagoons by juvenile rays suggests that they provide an important habitat for this life stage. Current marine park zoning appears to provide an effective protection for juveniles within this area.
Records of age and growth stored within otoliths were used to compare early life history traits with patterns of light trap catches for the damselfish Stegastes partitus (Poey). Otoliths provided ...strong evidence that fast growing cohorts of S. partitus larvae had higher survivorship than slow growing cohorts. Average growth rates during the larval phase accounted for 83% of the variability in the magnitude of catches in light traps on a monthly basis. This result suggests that fast growing cohorts of larvae contribute more to the replenishment of benthic populations than slow growing cohorts of this species. Multiple regression identified water temperature, rainfall and wind component as important determinants of larval growth, age at capture and monthly catches of this species. These variables accounted for 7 to 36% of the variance in growth rates, while water temperature was moderately correlated (r super(2) = 0.48) with catches. If such correlations between larval growth rates and replenishment are a general phenomenon, then this may provide a simple means of predicting year-class success in a range of reef fishes.
The late-stage larvae of the reef fishPomacentrus coelestiswere collected using light traps at stations on a cross-shelf transect near Northwest Cape, Western Australia, during the spring and summer ...months (October to February) of 1997–98 and 1998–99. Physical (water temperature, wind) and biological (chlorophylla, zooplankton abundance) variables were measured concurrently at each station. In 1997–98, environmental conditions were characterised by intrusive upwelling onto the shelf, relatively cooler water temperatures and higher chlorophyllaand zooplankton biomass. During the 1998–99 summer, water temperatures were warmer, and chlorophyllaand zooplankton biomass were relatively low. Catches ofP. coelestiswere much lower in the first summer (197 fish) than the second (1483 fish). Records of planktonic growth were obtained from otoliths of subsets of larvae from both summers. Growth varied among months within each summer; however, on average, larvae grew more slowly in the 1997–98 summer than the 1998–99 summer (0.48 mm d–1vs 0.53 mm d–1respectively), despite the presumed food (copepods) being more abundant in the plankton. Partial correlation analysis showed that water temperature explained approximately 30% of the variation in growth of larvalP. coelestis. In contrast, chlorophyllaand zooplankton abundance explained much less (4.1 and 3.5%, respectively).
Coastal populations of maritime countries in eastern Africa rely on fish as a primary source of protein, but baseline information on the abundance of fish communities on these coastlines is often ...lacking. We used baited remote underwater video stations to compare the abundance and diversity of reef fishes targeted by fishing at two sites in southern Mozambique, one at Lighthouse Reef within the Bazaruto Archipelago National Park and the other to the south at San Sebastian Reef on the San Sebastian Peninsula. Fish that are known targets of fisheries (mostly small-scale and artisanal) had an abundance that was almost three-times greater at San Sebastian Reef (80.22 ind. h
-1
SE 18.00) than at Lighthouse Reef (29.70 ind. h
-1
SE 8.91). Similarly, there was greater mean species richness at San Sebastian Reef (38.74 species h
-1
SE 2.79) than at Lighthouse Reef (25.37 species h
-1
SE 3.66). The main drivers of targeted fish abundance were habitat and depth, with shallow (<15 m) and mixed reef areas having the greatest abundance and richness. More sampling was done over sand habitat at Lighthouse Reef, which likely led to the lower abundance and species richness observed at this site; however, that finding could also be attributable to the fact that protection is provided to only a section of available coral reef habitat in a small area. Nevertheless, fish community structure was comparable between the sites, with similar proportions of carnivores (78-81%), herbivores (12-14%) and omnivores (7-8%). Our findings highlight the variation in species abundance and assemblages of coral-reef fish targeted by fishing in Mozambique and emphasise the importance of localised environmental variables as a driver of these patterns. To ensure maximum protection of Lighthouse Reef fish communities, we recommend an extension of the no-take zone to include the entire reef complex.
A total of 23 whale sharks were identified over a 5 d period in the Arta Bay region of the Gulf of Tadjora, Djibouti. Most of the sharks aggregating in this area were small (<4 m TL) males. ...Individuals were identified using photographs of distinctive scars and spot and stripe patterns on the sides of the animals. Of these, 65% had scarring that was attributable to boat or propeller strikes. Most of the whale sharks we encountered were feeding on dense accumulations of plankton in shallow water just off (10-200 m) the shoreline. This food source may account for the aggregation of sharks in this area. One 3 m male shark was tagged with an ARGOS (Splash) satellite tag for 9 d. During this time the shark traversed to the shoreline on the opposite side of the Gulf (a distance of 14 km) and then returned to the Arta Bay area before retracing his path to the other shore. The shark spent most of the daylight hours at the surface, while at night dives were more frequent, deeper and for longer durations.
Coral reef fishes, like many marine organisms, have a complex life history that consists of a planktonic larvae stage and a benthic juvenile or adult stage. We used the growth records in the otoliths ...of a common damselfish to investigate the extent to which processes in the plankton determined the outcome of events after benthic settlement. Sequential samples of the same cohort showed that individuals that survived intense selective mortality 1-3 months after settlement were those fish that were the larger members of the cohort at hatching and grew faster during planktonic life. Such links between life history phases are likely to occur in reef fishes whenever there is selection for a trait that is cumulative, such as size. They may not only operate between life history stages in the same individuals, but even between those of different generations via maternal effects on size at hatching.
While growth rates of pelagic larvae have been argued to be one of the principal determinants of the recruitment success of temperate marine fishes, it is not known if this is the case in the ...tropics. Here, we use larval growth histories derived from otoliths of a Caribbean reef fish to show that monthly variation in the intensity of settlement and recruitment of pelagic juveniles onto reefs is positively correlated with variation in growth rates 1-2 weeks after larvae begin feeding. Our results suggest that the processes thought to underlie recruitment of marine fishes in temperate regions may also operate in the tropics and contrasts with current research on the causes of recruitment variability in coral reef fishes, which emphasises the role of larval transport.
Temporal variance in species abundance, a potential driver of extinction, is linked to mean abundance through Taylor's power law, the empirical observation of a linear log-log relationship with a ...slope between 1 and 2 for most species. Here we test the idea that the slope of Taylor's power law can vary both among species and spatially as a function of habitat area and isolation. We used the world's most extensive database of coral reef fish communities comprising a 15-year series of fish abundances on 43 reefs of Australia's Great Barrier Reef. Greater temporal variances were observed at small and isolated reefs, and lower variances at large and connected ones. The combination of reef area and isolation was associated with an even greater effect on temporal variances, indicating strong empirical support for the idea that populations on small and isolated reefs will succumb more frequently to local extinction via higher temporal variability, resulting in lower resilience at the community level. Based on these relationships, we constructed a regional predictive map of the dynamic fragility of coral reef fish assemblages on the Great Barrier Reef.
We tested the hypothesis that fast growing Atlantic cod Gadus morhua survived better than their population of origin in the winters of 1991–1992 and 1992–1993 on the Scotian Shelf (northwest ...Atlantic). Survivors were defined as fish >90 d of age at capture which comprised epibenthic juveniles >20 mm sampled mainly near the bottom. The majority of larvae and juveniles aged ≤90 d at capture were sampled with a midwater trawl and were assumed to be representative of the pelagic population (survivors + non-survivors) from which epibenthic survivors originated. Standard length corrected for shrinkage was linearly correlated to lapillar radius (r2 = 0.97). Individual growth histories were reconstructed from the width of lapillar increments. Selection for fast growth was weak in the winter of 1991–1992 and back-calculated growth and length at age of survivors were not significantly larger than that of the population (repeated-measures MANOVA). In winter 1992–1993, a strong selection for fast growth was evident in late larvae 41 to 80 d old. The divergence in length at age between survivors and the population reached 4 mm at an age of 70 d, corresponding to a 13 d reduction in the duration of the larval phase. Survivors in the winter of 1992–1993 had larger hatch marks than the population, suggesting that the potential for fast growth may be reflected in traits present at hatching. Our results support the hypothesis that fast growth increases the survivorship of Atlantic cod during larval life in the plankton and indicate that the intensity of size-selective mortality may vary considerably from year to year.
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