Management implications of fish trap effectiveness in adjacent coral reef and gorgonian habitats

A combination of visual census and trap sampling in St. John, USVI indicated that traps performed better in gorgonian habitat than in adjacent coral reef habitat. Although most families were seen more commonly in coral habitat, they were caught more often in gorgonian areas. Traps probably fished more effectively in gorgonian habitats, especially for migrating species, because traps provided shelter in the relatively topographically uniform environment of gorgonian dominated habitats. Recently, trap fishermen on St. John have been moving effort away from traditionally fished nearshore coral reefs and into a variety of more homogeneous habitats such as gorgonian habitat. Consequently, exploitation rates of the already over-harvested reef fish resources may be increasing. Reef fish managers and marine reserve designers should consider limiting trap fishing in gorgonian habitats to slow the decline of reef fisheries.     

Variability in spatial and temporal occurrence of presettlement and settlement‐stage fishes associated with shallow reefs

Presettlement and settlement‐stage fishes were studied in a large, log‐spiral bay in temperate South Africa. The aim was to describe the assemblage composition, density and distribution associated with four types of habitats common to the bay: high profile reef, low profile reef, reef‐associated sand and open sand spatially separated from reef. Samples were collected with both a plankton ring net and a light trap at each habitat type as part of a mixed‐method approach. A total of 4084 presettlement and settlement‐stage fishes belonging to 31 teleost families and 84 species were captured. Reef‐associated sand and open sand habitats yielded higher species richness and diversity than the high and low‐profile reef habitats. Engraulidae, Gobiidae, Clupeidae and Cynoglossidae were the dominant fish families captured with the ring net, while Engraulidae, Clupeidae, Carangidae and Clinidae were captured with the light trap. A temporal difference in the abundance of presettlement fishes occurred between the sampling periods with highest values recorded during the summer settlement period. Habitat type together with associated physico‐chemical variables played a pivotal role in determining presettlement fish species composition, density and distribution across habitat types.     

The relationship between fish species richness, abundance and habitat complexity in a range of shallow tropical marine habitats

A simple habitat assessment score (HAS) was designed to assess habitat complexity across several different shallow tropical marine habitats including sandy patches, algal beds, seagrass beds and reefs. It measured rugosity, variety of growth forms, height, refuge size categories, percentage live cover and percentage hard substratum. Multiple regression models using HAS variables as predictors accounted for 71 and 22% of the variation in observed species richness and total fish abundance respectively. The two most important predictors of observed species richness were rugosity and variety of growth forms, while height was the most important predictor of total fish abundance. The HAS method worked consistently across a variety of habitat types and the complexity map closely mirrored the map of observed species richness, reflecting the patchy habitat mosaic of shallow tropical marine areas. Stations at the mouth of an enclosed lagoon, however, had a higher number of species than might have been expected judging from the habitat complexity scores. It is possible that this was linked to the preferential settling of pelagic fish larvae in this area as tidal water exchanges between the bay and the reef were funnelled through one small gap. This study highlights the need for fish biodiversity studies to take habitat complexity into account.     

Live coral repels a common reef fish ectoparasite

Coral reefs are undergoing rapid changes as living corals give way to dead coral on which other benthic organisms grow. This decline in live coral could influence habitat availability for fish parasites with benthic life stages. Gnathiid isopod larvae live in the substratum and are common blood-feeding parasites of reef fishes. We examined substrate associations and preferences of a common Caribbean gnathiid, Gnathia marleyi. Emergence traps set over predominantly live coral substrata captured significantly fewer gnathiids than traps set over dead coral substrata. In laboratory experiments, gnathiids preferred dead coral and sponge and tended to avoid contact with live coral. When live gnathiids were added to containers with dead or live coral, significantly fewer were recovered from the latter after 24 h. Our data therefore suggest that live coral is not suitable microhabitat for parasitic gnathiid isopods and that a decrease in live coral cover increases available habitat for gnathiids.     

Distributions and habitat associations of the bridled monocle bream Scolopsis bilineatus (Nemipteridae): a demographic approach

This study focussed on the demography and ecology of Scolopsis bilineatus at three locations on the Great Barrier Reef: the Lizard Island Group, Orpheus Island and One Tree Island. Scolopsis bilineatus lived for up to 16 years and had four distinct life‐history stages, which varied in their distribution patterns, habitat use and reproductive behaviour. Pre‐maturational sex change occurred whereby all males were derived from immature females, and males grew faster and larger than females. Small females and larger males generally formed pairs, which influenced their spatial distributions at small scales. Distributions of S. bilineatus were influenced by depth and exposure within reefs, particularly for juveniles, and most fish were found in shallow, sheltered habitats. Abundance was influenced by benthic cover, and was higher in areas of high coral cover and low where algae were abundant. Habitat associations were stronger at the microhabitat scale, and shelter sites were important for adults. Ontogenetic changes in microhabitat associations were found: juveniles occupied sand and rubble, and adults occupied shelters such as caves and overhangs. Adults showed site fidelity for shelter sites over a period of 4 days and returned to specific shelter sites repeatedly. These findings illustrate the importance of understanding the spatial ecology and habitat use of coral reef fishes, particularly with reference to size‐based changes within species.     

The structure of a nearshore fish community of Western Australia: diel patterns and the habitat role of limestone reefs

Synopsis Extensive limestone reefs are a characteristic feature of much of the coastline of Western Australia, and potentially represent a major habitat feature influencing the structure of the coastal fish community. The structure and temporal dynamics of the fish fauna and its relationships to nearshore patch reefs and surrounding habitat near Dongara, Western Australia, were examined using (1) diel gill-netting and (2) quantitative rotenone sampling of enclosed areas of substratum. Long-term and day-to-day variability of the fauna was low. Dominant species of gill-net collections were either associated with reefs or occurred in similar abundances at both reefs and surrounding sand/seagrass flats. The overall abundance, number of species and biomass of netted fishes was higher around reefs. Rotenone collections of the more sedentary species showed a similar pattern, but suggested, however, that a simple reef versus surrounding sand and seagrass habitat comparison is complicated by the canopy-forming seagrass Amphibolis that occurs on reef tops. Time of day had an important effect on overall fish abundance and number of species, with peaks occurring at crepuscular periods. This reflected dusk and dawn activity peaks of a dominant species rather than overlapping activities of many diurnal and nocturnal species. Diel switches between reef-edge habitat and surrounding sand/seagrass flats were uncommon despite expectations (based on literature examples) that patch reefs would function primarily as sheltering habitats and surrounding non-reef areas act as foraging habitat. High catches at reef-edge sites suggest that the majority of fishes forage on or near limestone patch reefs. Fish densities of around 0.8 individuals per m^-2 of bottom on these Western Australian reefs are relatively high in comparison to visual census estimates obtained for temperate reef systems in South Australia and New Zealand, but similar to those obtained using comparable netting methods in temperate Australian seagrass systems.     

Impact of hard-bottom substrata on the small-scale distribution of fish and decapods in shallow subtidal temperate waters

The micro-scale spatial distribution patterns of a demersal fish and decapod crustacean assemblage were assessed in a hard-bottom kelp environment in the southern North Sea. Using quadrats along line transects, we assessed the in situ fish and crustacean abundance in relation to substratum types (rock, cobbles and large pebbles) and the density of algae. Six fish and four crustacean species were abundant, with Ctenolabrus rupestris clearly dominating the fish community and Galathea squamifera dominating the crustacean community. Differences in the substratum types had an even stronger effect on the micro-scale distribution than the density of the dominating algae species. Kelp had a negative effect on the fish abundances, with significantly lower average densities in kelp beds compared with adjacent open areas. Averaged over all of the substrata, the most attractive substratum for the fish was large pebbles. In contrast, crustaceans did not show a specific substratum affinity. The results clearly indicate that, similar to other complex systems, significant micro-scale species–habitat associations occur in northern hard-bottom environments. However, because of the frequently harsh environmental conditions, these habitats are mainly sampled from ships with sampling gear, and the resulting data cannot be used to resolve small-scale species–habitat associations. A detailed substratum classification and community assessment, often only possible using SCUBA diving, is therefore important to reach a better understanding of the functional relationships between species and their environment in northern temperate waters, knowledge that is very important with respect to the increasing environmental pressure caused by global climate change.     

Habitat specialization in tropical continental shelf demersal fish assemblages

The implications of shallow water impacts such as fishing and climate change on fish assemblages are generally considered in isolation from the distribution and abundance of these fish assemblages in adjacent deeper waters. We investigate the abundance and length of demersal fish assemblages across a section of tropical continental shelf at Ningaloo Reef, Western Australia, to identify fish and fish habitat relationships across steep gradients in depth and in different benthic habitat types. The assemblage composition of demersal fish were assessed from baited remote underwater stereo-video samples (n = 304) collected from 16 depth and habitat combinations. Samples were collected across a depth range poorly represented in the literature from the fringing reef lagoon (1-10 m depth), down the fore reef slope to the reef base (10-30 m depth) then across the adjacent continental shelf (30-110 m depth). Multivariate analyses showed that there were distinctive fish assemblages and different sized fish were associated with each habitat/depth category. Species richness, MaxN and diversity declined with depth, while average length and trophic level increased. The assemblage structure, diversity, size and trophic structure of demersal fishes changes from shallow inshore habitats to deeper water habitats. More habitat specialists (unique species per habitat/depth category) were associated with the reef slope and reef base than other habitats, but offshore sponge-dominated habitats and inshore coral-dominated reef also supported unique species. This suggests that marine protected areas in shallow coral-dominated reef habitats may not adequately protect those species whose depth distribution extends beyond shallow habitats, or other significant elements of demersal fish biodiversity. The ontogenetic habitat partitioning which is characteristic of many species, suggests that to maintain entire species life histories it is necessary to protect corridors of connected habitats through which fish can migrate.     

Protozoal and epitheliocystis-like infections in the introduced bluestripe snapper Lutjanus kasmira in Hawaii

The bluestripe snapper, or taape, was introduced into Hawaii in the 1950s and has since become very abundant throughout the archipelago. As part of a health survey of reef fish in Hawaii, we necropsied 120 taape collected from various coastal areas south of Oahu and examined fish histology for extraintestinal organisms. Forty-seven percent of taape were infected with an apicomplexan protozoan compatible with a coccidian. Infection was evident mainly in the spleen and, less commonly, the kidney. Prevalence of this coccidian increased with size of fish, and we saw no significant pathology associated with the organism. Twenty-six percent of taape were also infected with an epitheliocystis-like organism that occurred mainly in the kidney and, less commonly, the spleen. In contrast to the coccidian, fish mounted a notable inflammatory response to the epitheliocystis-like organism, and this inflammation appeared to increase in severity with age. Prevalence of the epitheliocystis-like organism infection increased with age, but infection was not seen in fish greater than 26.5 cm fork length. The high prevalence of coccidial infection in introduced taape prompts the concern that these organisms, along with the epitheliocystis-like organism, have the potential to be transmitted to native reef fish. Given the impact of other introduced microbial organisms on native Hawaiian fauna, there is a clear need to assess whether protozoa and bacteria are endemic to Hawaii, and whether they negatively impact native reef fish that closely associate with taape.     

Changes in Coral Reef Communities and an Associated Reef Fish Species, <Emphasis Type="Italic">Cephalopholis cruentata</Emphasis> (Lacépède), After 30 years on Curaçao (Netherlands Antilles)

Using the same methodology and identical sites, we repeat a study dating from 1973 and quantify cover of hard coral species, soft corals, sponges, hard substratum and soft substratum, and density of a commercially important reef fish species, the graysby Cephalopholis cruentata, along a depth-gradient of 3–36 m on the coral reefs of Curaçao. The objective was to determine the multi-decade change in benthic coral reef cover and structural complexity, and their effect on densities of an associated reef fish species. Total hard coral cover decreased on average from 52% in 1973 to 22% in 2003, representing a relative decline of 58%. During this time span, the cover of hard substratum increased considerably (from 11 to 58%), as did that of soft corals (from 0.1 to 2.2%), whereas the cover of sponges showed no significant change. Relative decline of hard coral cover and of reef complexity was greatest in shallow waters (near the coast), which is indicative of a combination of anthropogenic influences from shore and recent storm damage. Cover of main reef builder coral species (Agaricia spp., Siderastrea siderea, Montastrea annularis) decreased more than that of other species, and resulted in a significant decrease in reef complexity. Although density of C. cruentata was highly correlated to cover of Montastrea and Agaricia in 1973, the loss of coral cover did not show any effect on the total density of C. cruentata in 2003. However, C. cruentata showed a clear shift in density distribution from shallow water in 1973 to deep water in 2003. It can be concluded that the reefs of Curaçao have degraded considerably in the last three decades, but that this has had no major effect on the population size of one commercially important coral-associated fish species.     

Predictability of reef fish diversity and abundance using remote sensing data in Diego Garcia (Chagos Archipelago)

The diversity, abundance and distribution of reef fish are related to heterogeneity and physical complexity of benthic habitat. However, the field effort required to evaluate these aspects of the benthos in situ, at the scale of entire reefscapes, is greatly constrained by logistical and resource limitations. With moderate ground truthing, both substratum type and seabed topography are amenable to monitoring using satellite data. Here, remote sensing imagery was used to resolve the bathymetry and benthic character of a reef system in Diego Garcia (British Indian Ocean Territory). Replicate fish counts were made at seven measurement stations across the study area using visual census. Monte Carlo simulation revealed that species richness and abundance of several guilds and size groupings of reef fish appraised in situ were correlated with the satellite-derived seabed parameters over areas of seafloor as large as 5,030 m². The study suggests that satellite remote sensing is capable of predicting habitat complexity at a scale relevant to fish. Furthermore, as larger size classes of fish were better predicted with the satellite habitat complexity data, this technique could be used to predict fish stocks and identify potential sites for marine protected areas where intensive field surveys are not practical.     

Habitat selection by recruits establishes local patterns of adult distribution in two species of damselfishes: Stegastes dorsopunicans and S. planifrons

Local patterns of adult distribution in organisms that disperse young as pelagic larvae can be determined at the time of recruitment through habitat selection or, shortly thereafter, through post-recruitment processes such as differential juvenile survivorship and interspecific competition. This study addresses the importance of habitat selection by recruits in establishing the local pattern of adult distribution in two sympatric Caribbean damselfish species, Stegastes dorsopunicans and S. planifrons. Both species inhabit shallow reefs but show little overlap in their distribution; S. dorsopunicans predominates in the reef crest and S. planifrons occurs primarily on the reef slope. Furthermore, S. dorsopunicans is associated with rocky substrate, while S. planifrons occupies live coral. The substrate cover follows a similar pattern with coral being much less common on the reef crest than on the reef slope. Monitoring recruitment every other day in reciprocal removal experiments and artificial reefs indicates that the observed pattern of local adult distribution is a product of habitat selection for both species. The presence or absence of conspecifics did not influence recruitment patterns for either species. Stegastes dorsopunicans recruited primarily to shallow, rocky areas, appearing to cue on both substratum type and depth. Stegastes planifrons recruited exclusively to coral substratum independent of depth. These results indicate that local adult patterns of distribution can be explained by habitat selection at recruitment, and that substrate type and depth may be important cues. Received: 27 May 1997 / Accepted: 4 January 1998     

Herbivorous fish assemblages and herbivory pressure on Laamu Atoll, Republic of Maldives

While herbivory is recognized as a fundamental process structuring coral reef communities, herbivore assemblages and processes are poorly described for reefs in the Indian Ocean region. We quantified herbivorous fish assemblage structure (abundance and diversity) in Laamu Atoll, Republic of Maldives, in four reef habitat types: faro reef flats, faro reef slopes, inner and outer atoll reef slopes (20 sites in total). Herbivorous fish assemblages, representing a total of 30 species, grouped strongly by habitat type, with the highest absolute abundance observed on faro reef flats and lowest abundance on inside atoll rim reef slopes. Removal of Thalassia seagrass blades by ambient herbivore assemblages was used in a bioassay to assess relative herbivory pressure among four habitat types (eight sites). Also, at one site a choice herbivory assay was performed to assess herbivore preference among four benthic plants across three depth zones. Relative herbivory, as indicated by Thalassia assays, was highest on inside atoll rim reef slopes and lowest on outside atoll rim reef slopes. Thalassia consumption did not correspond to overall herbivorous fish abundance, but corresponded more closely with parrotfish abundance. In the choice assays, herbivores showed strong preferences among plant types and consumption of most plant types was higher at mid-depth than in the shallow reef flat or deep reef knoll zones.     

Occupancy Models for Monitoring Marine Fish: A Bayesian Hierarchical Approach to Model Imperfect Detection with a Novel Gear Combination

Occupancy models using incidence data collected repeatedly at sites across the range of a population are increasingly employed to infer patterns and processes influencing population distribution and dynamics. While such work is common in terrestrial systems, fewer examples exist in marine applications. This disparity likely exists because the replicate samples required by these models to account for imperfect detection are often impractical to obtain when surveying aquatic organisms, particularly fishes. We employ simultaneous sampling using fish traps and novel underwater camera observations to generate the requisite replicate samples for occupancy models of red snapper, a reef fish species. Since the replicate samples are collected simultaneously by multiple sampling devices, many typical problems encountered when obtaining replicate observations are avoided. Our results suggest that augmenting traditional fish trap sampling with camera observations not only doubled the probability of detecting red snapper in reef habitats off the Southeast coast of the United States, but supplied the necessary observations to infer factors influencing population distribution and abundance while accounting for imperfect detection. We found that detection probabilities tended to be higher for camera traps than traditional fish traps. Furthermore, camera trap detections were influenced by the current direction and turbidity of the water, indicating that collecting data on these variables is important for future monitoring. These models indicate that the distribution and abundance of this species is more heavily influenced by latitude and depth than by micro-scale reef characteristics lending credence to previous characterizations of red snapper as a reef habitat generalist. This study demonstrates the utility of simultaneous sampling devices, including camera traps, in aquatic environments to inform occupancy models and account for imperfect detection when describing factors influencing fish population distribution and dynamics.     

Mesophotic depths as refuge areas for fishery-targeted species on coral reefs

Coral reefs are subjected to unprecedented levels of disturbance with population growth and climate change combining to reduce standing coral cover and stocks of reef fishes. Most of the damage is concentrated in shallow waters (<30 m deep) where humans can comfortably operate and where physical disturbances are most disruptive to marine organisms. Yet coral reefs can extend to depths exceeding 100 m, potentially offering refuge from the threats facing shallower reefs. We deployed baited remote underwater stereo-video systems (stereo-BRUVs) at depths of 10–90 m around the southern Mariana Islands to investigate whether fish species targeted by fishing in the shallows may be accruing benefits from being at depth. We show that biomass, abundance and species richness of fishery-targeted species increased from shallow reef areas to a depth of 60 m, whereas at greater depths, a lack of live coral habitat corresponded to lower numbers of fish. The majority of targeted species were found to have distributions that ranged from shallow depths (10 m) to depths of at least 70 m, emphasising that habitat, not depth, is the limiting factor in their vertical distribution. While the gradient of abundance and biomass versus depth was steepest for predatory species, the first species usually targeted by fishing, we also found that fishery-targeted herbivores prevailed in similar biomass and species richness to 60 m. Compared to shallow marine protected areas, there was clearly greater biomass of fishery-targeted species accrued in mesophotic depths. Particularly some species typically harvested by depth-limited fishing methods (e.g., spearfishing), such as the endangered humphead wrasse Cheilinus undulatus, were found in greater abundance on deeper reefs. We conclude that mesophotic depths provide essential fish habitat and refuge for fishery-targeted species, representing crucial zones for fishery management and research into the resilience of disturbed coral reef ecosystems.     

Behavioural response of Mullus surmuletus to habitat modification by the invasive macroalga Caulerpa taxifolia

Densities of red mullet Mullus surmuletus have declined since 1996 at Cap Martin, Alpes‐Maritimes, France, following colonization by Caulerpa taxifolia . Foraging M. surmuletus were rarely observed over C. taxifolia or the seagrass Posidonia oceanica , but were observed over bare sand and endemic macroalgae. Within colonized sites (where the mean cover of C. taxifolia was 30–100%), fish were concentrated on areas with low cover of C. taxifolia (<21%). A difference in the frequency of observations of foraging groups (17%) was observed between sites with and without C. taxifolia . The observed patterns were probably due to a physical barrier to foraging that is presented by meadows of C. taxifolia . Changes to habitat structure that occur when the substratum is colonized by C. taxifolia influence the accessibility to benthic food resources, and consequently the foraging activities of fish that feed on benthic invertebrates. The decline in density of M. surmuletus at Cap Martin could be the result of fish emigration from colonized sites (unfavourable to the species) to uncolonized sites.     

Community structure and biogeography of shore fishes in the Gulf of Aqaba,Red Sea

Shore fish community structure off the Jordanian Red Sea coast was determined on fringing coral reefs and in a seagrass-dominated bay at 6 m and 12 m depths. A total of 198 fish species belonging to 121 genera and 43 families was recorded. Labridae and Pomacentridae dominated the ichthyofauna in terms of species richness and Pomacentridae were most abundant. Neither diversity nor species richness was correlated to depth. The abundance of fishes was higher at the deep reef slope, due to schooling planktivorous fishes. At 12 m depth abundance of fishes at the seagrass-dominated site was higher than on the coral reefs. Multivariate analysis demonstrated a strong influence on the fish assemblages by depth and benthic habitat. Fish species richness was positively correlated with hard substrate cover and habitat diversity. Abundance of corallivores was positively linked with live hard coral cover. The assemblages of fishes were different on the shallow reef slope, deep reef slope and seagrass meadows. An analysis of the fish fauna showed that the Gulf of Aqaba harbours a higher species richness than previously reported. The comparison with fish communities on other reefs around the Arabian Peninsula and Indian Ocean supported the recognition of an Arabian subprovince within the Indian Ocean. The affinity of the Arabian Gulf ichthyofauna to the Red Sea is not clear. Received in revised form: 2 November 2001 Electronic Publication     

Reef fish assemblages in north-western Sri Lanka: distribution patterns and influences of fishing practises

The distribution and abundance of reef fishes in relation to habitat structure were studied within Bar Reef Marine Sanctuary (BRMS) and on an adjacent reef, disturbed by destructive fishing techniques, in north-western Sri Lanka, by visually censusing 135 species groups using fifty metre belt-transects. Two types of continental shelf patch-reefs are found in the study area: coral reefs and sandstone reefs, which are divided into distinct habitats, four for the coral reef (shallow reef flat, shallow patch reef, deep reef flat and Porites domes) and two for the sandstone reef (structured sandstone-reef and flat sandstone-reef). Fish assemblages varied in structure between reef types and among habitats within reef types. Functional aspects of habitat structure and composition, such as available food and shelter, seemed to be important factors influencing distribution patterns. The strongest separation in the organisation of fish assemblages in BRMS was between reef types: 19% of all species were confined to the coral-reef patches while 22% were restricted to the sandstone reef patches and 59% were represented on both reef types. In terms of distribution among habitats, 21% of all species were restricted to one habitat while only 1.5% were present in all. The highest density of fish was in the coral reef habitats while highest species diversity was found in the most structurally complex habitat: the structured sandstone-reef. This habitat also had the highest proportion of species with restricted distribution. Planktivores were the most abundant trophic group in BRMS, and the species composition of the group varied among habitats. The comparison of the disturbed reef with BRMS suggested that habitat alteration caused by destructive fishing methods has strongly influenced the fish community. Within the fished area the structure of the fish assemblages was more heterogeneous, fish abundance was lower by an order of magnitude and species numbers were lower than in BRMS.     

Habitat correlates of the distribution and biomass of Seychelles' reef fishes

Synopsis Relationships between quantitative measures of habitat type and the biomass of Chaetodon, Scarus and Parupeneus species were investigated across 35 reef sites in the Inner Seychelles Group. Multiple regression was used to determine the proportion of variance in biomass between sites which could be explained by depth, exposure, vertical relief, topographic complexity, live coral cover, coral rubble cover, rock cover, sand cover, underlying carbonate substrate, underlying sand substrate, underlying rock substrate and an index of fishing intensity. A significant proportion of the variance in biomass was explained by habitat variables and the index of fishing intensity for 7 of 12 Chaetodon species (23–52% of variance explained), 3 of 6 Parupeneus species (33–40%), and 10 of 13 Scarus species (14–46%). Within genera, different groups of habitat variables explained the variance in biomass for different species and, of the variables studied, only the proportion of underlying sand substrate failed to explain a significant proportion of the variance in biomass for any species. Quantitative relationships between the biomass of Chaetodon and habitat were often in accordance with those suggested by previous studies of their ecology, life-history and distribution at other Indo-Pacific locations. However, the habitat associations of the Parupeneus and some Scarus species have not been studied at other locations and clearly warrant further investigation. It was concluded that habitat was an important determinant of the distribution of many Seychelles reef fishes, but that the habitat variables examined were rarely the most important determinant of biomass. However, the inclusion of a procedure to collect habitat data provided a useful means by which to reduce the unexplained variance associated with visual census biomass estimates and therefore improves the possibility of elucidating the effects of other factors on the biomass of Seychelles reef fishes.     

Thresholds in seascape connectivity: the spatial arrangement of nursery habitats structure fish communities on nearby reefs

Ecosystems are linked by the movement of organisms across habitat boundaries and the arrangement of habitat patches can affect species abundance and composition. In tropical seascapes many coral reef fishes settle in adjacent habitats and undergo ontogenetic habitat shifts to coral reefs as they grow. Few studies have attempted to measure at what distances from nursery habitats these fish migrations (connectivity) cease to exist and how the abundance, biomass and proportion of nursery species change on coral reefs along distance gradients away from nursery areas. The present study examines seascape spatial arrangement, including distances between habitats, and its consequences on connectivity within a tropical seascape in Mozambique using a seascape ecology approach. Fish and habitat surveys were undertaken in 2016/2017 and a thematic habitat map was created in ArcGIS, where cover and distances between habitat patches were calculated. Distance to mangroves and seagrasses were significant predictors for abundance and biomass of most nursery species. The proportions of nursery species were highest in the south of the archipelago, where mangroves were present and decreased with distance to nurseries (mangroves and seagrasses). Some nursery species were absent on reef sites farthest from nursery habitats, at 80 km from mangroves and at 12 km from seagrass habitats. The proportion of nursery/non-nursery snapper and parrotfish species, as well as abundance and biomass of seagrass nursery species abruptly declined at 8 km from seagrass habitats, indicating a threshold distance at which migrations may cease. Additionally, reefs isolated by large stretches of sand and deep water had very low abundances of several nursery species despite being within moderate distances from nursery habitats. This highlights the importance of considering the matrix (sand and deep water) as barriers for fish migration.