Conceived and designed the experiments: WWT AOD PCH. Performed the experiments: WWT AOD PCH. Analyzed the data: WWT AOD MJAV. Contributed reagents/materials/analysis tools: MJAV. Wrote the paper: WWT AOD MJAV. Acquired funding for the study: PCH.
Current address: Accreditation Services International GmbH, Bonn, Germany
The authors have declared that no competing interests exist.
Saba Bank is a 2,200 km2 submerged carbonate platform in the northeastern Caribbean Sea off Saba Island, Netherlands Antilles. The presence of reef-like geomorphic features and significant shelf edge coral development on Saba Bank have led to the conclusion that it is an actively growing, though wholly submerged, coral reef atoll. However, little information exists on the composition of benthic communities or associated reef fish assemblages of Saba Bank. We selected a 40 km2 area of the bank for an exploratory study. Habitat and reef fish assemblages were investigated in five shallow-water benthic habitat types that form a gradient from Saba Bank shelf edge to lagoon. Significant coral cover was restricted to fore reef habitat (average cover 11.5%) and outer reef flat habitat (2.4%) and declined to near zero in habitats of the central lagoon zone. Macroalgae dominated benthic cover in all habitats (average cover: 32.5 – 48.1%) but dominant algal genera differed among habitats. A total of 97 fish species were recorded. The composition of Saba Bank fish assemblages differed among habitat types. Highest fish density and diversity occurred in the outer reef flat, fore reef and inner reef flat habitats. Biomass estimates for commercially valued species in the reef zone (fore reef and reef flat habitats) ranged between 52 and 83 g/m2. The composition of Saba Bank fish assemblages reflects the absence of important nursery habitats, as well as the effects of past fishing. The relatively high abundance of large predatory fish (i.e. groupers and sharks), which is generally considered an indicator of good ecosystem health for tropical reef systems, shows that an intact trophic network is still present on Saba Bank.
Saba Bank, located offshore from Saba Island, Netherlands Antilles, is a large (∼2,200 km2), isolated and completely submerged Caribbean atoll
Formation of coral reef atolls is relatively rare in the Atlantic
A depth profile of the study area (∼8 km in length) was generated in ArcGIS using bathymetric data from the Hydrographic Service of the Royal Netherlands Navy. Names of reef structures and reef zones are from Van der Land
In tropical marine ecosystems, habitat diversity partially underlies the diversity of organisms as local species richness increases with an increasing number of different critical habitat types
Despite the paucity of data on biogeographical patterns in marine species diversity across the Caribbean, Saba Bank provides a natural setting that is unique in the Caribbean region. The absence of mangroves and sea grasses that are found on many Caribbean islands will likely result in the absence of fish species that depend on such habitats as “nursery areas”
Despite the relatively isolated position of Saba Bank, it has not escaped the effects of anthropogenic disturbance such as fishing for benthic invertebrates (mainly spiny lobster,
Because Saba Bank is difficult to access due to its offshore location, not much is known about the species that are found there. This study aims to determine whether the aforementioned habitat types harbor different fish assemblages in terms of species richness and biomass. The presence of each habitat type was determined using bathymetric maps and remote sensing that led to delineation of five shallow-water benthic habitat types along a gradient from Saba Bank shelf edge to lagoon. Within each habitat type, we conducted underwater visual surveys to examine habitat characteristics and quantify the structure of reef fish assemblages.
All Saba Bank projects have collecting permits through CITES (where necessary) and the Saba Conservation Foundation (where CITES is not required).
Our study area was situated in a central part of eastern Saba Bank known as Overall Bank, located ∼16 km offshore from Saba Island (
The Saba Bank map was assembled in ArcGIS using available datasets for bathymetry of Saba Bank. Sampling was conducted on a region of Saba Bank named Overall Bank as shown (rectangle at right).
The presence of each habitat type (see below) was determined using a combination of high-resolution multibeam bathymetry (Hydrographic Service of the Royal Netherlands Navy, 2006) and satellite imagery (LandSat). Spatial resolution of the bathymetric data was 2×2 m with a vertical resolution of <0.2 m. The former dataset was used to construct a bathymetric map of Saba Bank (
The sampling locations are shown on a composite image of depth and ocean color. Polygons labeled A to E show the sampling strata which correspond to five habitat types: (A) fore reef, FR; (B) outer reef flat, ORF; (C) inner reef flat, IRF; (D) softbottom lagoon, LSB; (E) hardbottom lagoon, LHB. Black dots show sample locations.
At Overall Bank, a broad reef zone (>2 km in width) occurs along the bank's eastern margin. The outermost reef structure in this zone is the fore reef, with a highly variable depth profile (
The lagoon zone (
Patch reefs (located inside the lagoon) and reef slope (separating inner reef flat from lagoon) were not sampled in our study due to the relatively small total area they occupy.
We used a random sampling design to investigate habitat types. A habitat map of the study area (see above) was used to randomly select survey positions using ArcGIS. A total of eight survey locations were selected within each of the five habitat types. An overview of all survey locations and habitat distribution is shown in
Owing to limitations of dive time and accessibility to sites on Saba Bank, our survey protocols were designed to efficiently record the habitat characteristics through a combination of quantitative and qualitative measures. Quantitative means were always used to estimate percent cover as described below. Where it was impractical to make quantitative measures, we recorded descriptive (qualitative) information that would enable us to examine for relative differences in habitat structure.
At each survey location, we examined physical characteristics and benthic community composition within a 4×25 m belt transect (100 m2). Percentage cover by each of the major benthic groups was estimated within each transect. The ability of each surveyor to accurately estimate benthic cover was tested beforehand using photoquadrats in which cover was measured exactly. Results from these preliminary studies indicated that estimation described 84.9% of the true variability present in reef communities. This value was deemed sufficiently high to use estimation during benthic surveys at the Saba Bank. Percentage cover of substrate (abiotic) was assigned into three categories: hardbottom (consolidated carbonate substrate), rubble (unconsolidated material of <0.5 m diameter), and sand (>0.5 cm thick layer). Percentage cover of corals, sponges, macroalgae and coralline algae was estimated to the nearest 1% and the dominant scleractinian and macroalgal genera were recorded at each sampling location. Note that abiotic and biotic descriptors were used independently to distinguish the physical composition of the substrate from that of the organisms that were observed growing on top of it.
A series of qualitative measures were also collected at each survey location in order to further characterize the relative differences in benthic composition among habitats. Vertical relief was defined as the largest vertical drop observed along the transect and it was assigned a qualitative score of low (1; <0.5 m), medium (3), or high (5; >1.0 m). Substrate rugosity was recorded into three categories of low (1), medium (3) and high (5) based on the surveyor's subjective assessment of the degree of substrate involution in relation to standardized line drawings on data forms. Depth and slope were also recorded for each location. Two qualitative descriptors were used for gorgonians: abundance (sparse, medium, dense) and height (small: 0–25 cm, medium: 25–100 cm and tall: >100 cm).
Fish surveys were conducted using a belt transect visual survey protocol
To increase the descriptive resolution of species richness of the fish assemblages at each sampling location, belt transect surveys were supplemented by roving diver (RD) surveys to provide a more complete estimate of local species richness. During RD surveys a diver swam for 10 min in a haphazardly chosen direction (i.e. approx. 100 m) and noted all fish species observed. Small-bodied demersal species (e.g. Gobiidae, Blenniidae), cryptic taxa and nocturnally active species were not included in our RD surveys because such taxa are more accurately surveyed using non-visual methods (see concurrent study by Williams et al.
Differences in benthic community structure were assessed using one-way ANOVA whereby the coverage of each benthic category was compared individually among the five habitat types. Differences in fish assemblage structure in terms of density, species richness and biomass were investigated using multifactor ANOVA after transformation of datasets (ln[x+1]) in order to meet assumptions of normality. Multidimensional scaling (MDS) was conducted using location-specific estimates of fish species richness and biomass to further visualize differences in fish assemblages among the five habitat types. Fish biomass estimates were transformed (ln[x+1]) and the data were standardized to equal mean (0) and standard deviation (1). The strength of the relationship between macroalgal cover and biomass of herbivorous fishes was tested by Poisson regression.
Two models of algal abundance (δa) -constant, δa = α, and exponential with herbivorous biomass (H), δa = α exp(βH), where α and β are the biomass-independent and biomass-dependent terms, respectively, were compared for relative fit using a likelihood ratio test
General aspects of the five Saba Bank habitat types (
(FR) Fore reef habitat; (ORF) outer reef flat habitat; (IRF) inner reef flat habitat; (LSB) softbottom lagoon habitat; (LHB) hardbottom lagoon habitat.
The five habitat types varied significantly in terms of physical characteristics (ANOVA; p<0.05;
Category | Subcategory | Statistic | FR | ORF | IRF | LSB | LHB |
Depth (m) | Avg ± StDev | 23.6±3.0 | 13.8±0.4 | 13.8±0.6 | 19.4±0.4 | 20.1±0.7 | |
Range | 20.7–29.9 | 13.1–14.3 | 12.8–14.6 | 18.6–20.0 | 19.5–21.3 | ||
Substrate (%) | Hardbottom | Avg ± StDev | 87.4±14.6 | 56.0±42.0 | 68.0±29.5 | 2.5±7.1 | 44.4±37.6 |
Range | 55–99 | 0–100 | 5–95 | 0–20 | 0–95 | ||
Group | a | a,b | a,b | c | b,c | ||
Rubble | Avg ± StDev | 3.5±8.0 | 39.6±37.1 | 31.8±29.6 | 22.6±13.7 | 50.9±37.6 | |
Range | 0–23 | 0–85 | 5–95 | 1–45 | 4–100 | ||
Group | a | a,b | a,b | a,b | b | ||
Sand | Avg ± StDev | 9.1±8.2 | 4.4±6.8 | 0.3±0.5 | 74.9±17.5 | 4.8±7.1 | |
Range | 0–22 | 0–15 | 0–1 | 45–99 | 0–20 | ||
Group | a | a | a | b | a | ||
Vertical Relief | Avg ± StDev | 4.3±1.0 | 1.4±0.7 | 1.5±0.9 | 1.1±0.4 | 1.0±0.0 | |
Range | 3–5 | 1–3 | 1–3 | 1–2 | 1 | ||
Rugosity | Avg ± StDev | 4.5±0.9 | 2.6±0.7 | 1.9±0.8 | 1.0±0.0 | 1.3±0.5 | |
Range | 3–5 | 1–3 | 1–3 | 1 | 1–2 | ||
Slope (degrees) | Range | 5–10 | <1 | <1 | <1 | <1 |
Abbreviations of habitat types are: (FR) fore reef; (ORF) outer reef flat; (IRF) inner reef flat; (LSB) lagoon soft-bottom; (LHB) lagoon hard-bottom. Homogenous groups are indicated with letters (a, b, c). Differences between groups were significant (p<0.05) based on one way ANOVA. Vertical relief and rugosity were assigned qualitative scores (1 to 5).
Benthic community composition also varied significantly among the five habitat types (ANOVA; p<0.05;
Category | Subcategory | Statistic | FR | ORF | IRF | LSB | LHB |
Benthic Cover (%) | Live Coral | Avg ± StDev | 11.5±5.8 | 2.4±1.7 | 0.4±0.7 | 0.1±0.4 | 0.3±0.5 |
Range | 5–20 | 1–5 | 0–2 | 0–1 | 0–1 | ||
Group | a | b | b | b | b | ||
Sponge | Avg ± StDev | 4.2±2.2 | 2.0±0.5 | 2.1±1.7 | 0.4±0.5 | 1.8±1.5 | |
Range | 1–8 | 1–3 | 0–5 | 0–1 | 0–5 | ||
Group | a | b | a,b | b | b | ||
Macro Algae | Avg ± StDev | 37.5±22.8 | 46.6±14.6 | 48.1±21.2 | 32.5±16.7 | 43.8±22.8 | |
Range | 5–70 | 25–65 | 10–75 | 15–60 | 15–80 | ||
Group | a | a | a | a | a | ||
Coralline Algae | Avg ± StDev | 2.5±4.6 | - | - | - | - | |
Range | 0–10 | - | - | - | - | ||
Group | a | a | a | a | a | ||
Gorgonian Assemblage | Density | Avg ± StDev | 3.4±0.5 | 2.4±1.2 | 1.0±0 | 1.3±0.7 | 1.0±0 |
Range | 3–4 | 1–4 | 1 | 1–3 | 1 | ||
Height | Avg ± StDev | 3.1±0.4 | 1.9±1.0 | 1.3±0.7 | 2.9±1.7 | 1.0±0 | |
Range | 3–4 | 1–3 | 1–3 | 1–5 | 1 | ||
Dominant Coral Genera | none (6) | none (7) | none (7) | ||||
Dominant Algal Genera | |||||||
Habitat types are abbreviated as shown in
Thirty-four commercially valued fish species
Species | TG | FR | ORF | IRF | LSB | LHB |
HB | 0.25±0.71 (13) | 8.00±6.87 (75) | 6.13±4.26 (75) | 1.13±2.42 (38) | 1.50±1.60 (50) | |
HB | 2.00±2.33 (50) | 4.13±4.02 (75) | 2.38±2.88 (50) | - | - | |
PI | 1.50±1.41 (75) | 4.25±3.85 (100) | 1.38±1.51 (63) | 0.13±0.35 (13) | 0.13±0.35 (13) | |
HB | 2.25±2.31 (63) | 3.50±3.78 (50) | - | - | - | |
HB | 1.00±0.93 (63) | 1.75±1.75 (75) | 1.38±1.51 (50) | 0.25±0.71 (13) | 0.13±0.35 (13) | |
OM | 4.38±12.37 (25) | - | - | - | - | |
ZB | 0.88±0.99 (63) | 1.00±1.41 (50) | 2.00±3.66 (75) | - | 0.50±1.41 (13) | |
HB | - | 1.50±1.85 (50) | 2.50±3.34 (63) | - | 0.13±0.35 (13) | |
ZB | - | 1.00±0.76 (75) | 1.25±1.39 (63) | 0.25±0.71 (13) | 0.75±1.49 (25) | |
ZB | 0.25±0.46 (25) | 0.88±1.13 (50) | 0.25±0.46 (25) | - | 1.75±4.56 (25) | |
ZB | 0.13±0.35 (13) | 0.88±0.64 (75) | 1.00±0.93 (63) | 0.25±0.46 (25) | 0.13±0.35 (13) | |
HB | 1.88±1.64 (75) | 0.25±0.46 (25) | - | - | - | |
PL | 1.63±2.26 (50) | - | - | - | - | |
PI | 1.25±2.82 (25) | - | - | - | - | |
PI | 0.25±0.46 (25) | 0.50±0.53 (50) | 0.25±0.71 (13) | 0.13±0.35 (13) | - | |
ZB | 0.25±0.46 (25) | 0.88±1.13 (50) | - | - | - | |
ZB | - | - | 0.38±1.06 (13) | - | 0.50±0.93 (25) | |
PI | - | 0.38±0.74 (25) | - | 0.38±1.06 (13) | - | |
PI | 0.63±0.74 (50) | - | - | - | - | |
PI | - | - | 0.13±0.35 (13) | - | 0.50±1.07 (25) | |
ZB | - | 0.38±1.06 (13) | - | - | - | |
PI | 0.38±1.06 (13) | - | - | - | - | |
HB | - | 0.38±0.74 (25) | - | - | - | |
HB | 0.38±0.74 (25) | - | - | - | - | |
OM | 0.38±0.74 (25) | - | - | - | - | |
ZB | - | 0.25±0.71 (13) | - | - | - | |
ZB | 0.25±0.46 (25) | - | - | - | - | |
PI | 0.13±0.35 (13) | - | - | - | - | |
ZB | 0.13±0.35 (13) | - | - | - | - | |
ZB | - | - | 0.13±0.35 (13) | - | - | |
HB | 0.13±0.35 (13) | - | - | - | - | |
ZB | - | - | - | 0.13±0.35 (13) | - | |
ZB | 0.13±0.35 (13) | - | - | - | - | |
ZB | 0.13±0.35 (13) | - | - | - | - |
Fish density (No. individuals/100m2) is reported as the average ± standard deviation (frequency) from 8 belt transects per habitat. Trophic Guild (TG) is: (HB) herbivore; (PI) piscivore; (PL) planktivore; (OM) omnivore; (ZB) zoobenthivore. Habitat types are abbreviated as shown in
Fish assemblage structure was compared between habitat types based on estimates of fish density, species richness and biomass from belt transect surveys (
Results of belt transect visual surveys are shown for each of the five different habitat types for: A) fish density; B) fish species richness; and C) estimates of biomass. The average values are presented for each habitat type (eight surveys per habitat). Error bars show standard deviation. Habitat types are abbreviated as in
MDS plot of: A) fish species diversity; and B) fish biomass at each sampling location for each of the five habitat types. Habitat types are abbreviated as in
Species richness, defined here as the average number of species observed per belt transect, was highest in FR and ORF (7.9 and 8.8 species/100 m2, respectively), intermediate in IRF (5.8 species/100 m2) and lowest in LSB and LHB (1.4 and 2.1 species/100 m2, respectively). Similar to fish density, species richness also differed between the five habitat types (F4,35 = 18.16, p<0.001;
Fish biomass (
The relative biomass of different trophic guilds also differed among habitat types (
The Roving Diver (RD) survey was included to strengthen local estimates of fish species richness at a locale. A total of 97 daily active, non-cryptic fish species were recorded from our 40 RD surveys (pooled data from the five habitat types;
The most commonly observed fish species were bicolor damselfish,
Species | FR | ORF | IRF | LSB | LHB | Signif.* |
7 | 7 | 7 | 2 | 1 | + | |
6 | 8 | 8 | 1 | 1 | + | |
7 | 8 | 8 | - | - | + | |
4 | 7 | 7 | - | 1 | + | |
7 | 7 | 2 | - | - | + | |
7 | 5 | 3 | - | 1 | + | |
6 | 5 | 4 | - | 1 | + | |
8 | 6 | 1 | - | - | + | |
6 | 5 | 2 | - | - | + | |
6 | 4 | - | - | - | + | |
- | - | 2 | 5 | 5 | + | |
- | 2 | 6 | 2 | 8 | + | |
7 | 7 | 8 | 5 | 7 | ns | |
4 | 7 | 8 | 6 | 7 | ns | |
5 | 8 | 7 | 4 | 4 | ns | |
7 | 7 | 8 | - | 5 | ns | |
6 | 8 | 8 | - | 4 | ns | |
6 | 7 | 4 | 3 | 6 | ns | |
1 | 7 | 7 | 6 | 5 | ns | |
1 | 7 | 6 | 8 | 3 | ns | |
2 | 8 | 8 | 1 | 5 | ns | |
1 | 5 | 5 | 7 | 5 | ns | |
6 | 5 | 4 | 2 | 4 | ns | |
4 | 6 | 3 | 1 | 4 | ns | |
- | 5 | 3 | 4 | 5 | ns | |
- | 7 | 5 | 2 | 2 | ns | |
1 | 7 | 5 | 2 | 1 | ns | |
1 | 5 | 2 | 2 | 6 | ns |
Habitat types are abbreviated as shown in
Twenty-eight fish species were sufficiently represented in RD surveys to investigate potential habitat associations. Of these, 12 species were unequally distributed among habitats (Chi-square test; p<0.01) and16 species showed no significant difference (
Habitat associations were also examined at the level of fish family after pooling RD survey data from all sites. The ten most common fish families were in declining order: Labridae, Scaridae, Serranidae, Acanthuridae, Pomacentridae, Holocentridae, Balistidae, Haemulidae, Chaetodontidae, and Carangidae. For eight families (
In this study, substantial coral cover was observed only in the fore reef (FR) zone of Overall Bank and rapidly diminished with increasing distance from the shelf edge. This suggests that significant reef accretion is restricted to a narrow zone along the Bank's periphery. Van der Land
Other reports already noted the unusual diversity and abundance of macroalgae on Saba Bank
Fish assemblages from the five Saba Bank habitat types differed in terms of species richness, density, biomass, and trophic structure (
Fish assemblages observed at the Saba Bank reef margin were exceptional in that they did not corroborate the hypothesized relation of habitat complexity to fish abundance and diversity. The fore reef showed the greatest habitat complexity (coral cover, vertical relief and rugosity) yet fish diversity and biomass were similar to or lower than in the adjacent reef flat habitat. The reason for this remains unclear. Commercial trap fishing may have acted selectively to reduce fish density and biomass in the bank's fore reef areas. Alternatively, natural productivity rates on the reef flat may be greater than on the fore reef.
Newman et al.
Many typical coral reef fish species are highly dependent on nursery habitats such as mangroves and sea grass beds
Large piscivores and apex predators were abundant compared to elsewhere in the region
Our observations from Saba Bank indicate a simultaneous abundance of predatory fishes and macroalgae in the same area, which is remarkable. Common trajectories of Caribbean reef decline require the removal of the highest trophic levels (e.g. predatory fishes) so biomass production at lower trophic levels (e.g. macroalgae) is no longer transformed through various trophic linkages to large and long-lived organisms such as corals and large predatory fish
Results of this study provide an initial description of typical fish assemblages from one area of Saba Bank, and give some indication of the role that habitat plays in structuring the composition and abundance of assemblages. Keeping in mind that the Overall Bank study area (40 km2) represents only 1.8% of Saba Bank's total area, generalizations must be made with caution. Further exploration is required to quantify the distribution of natural resources on Saba Bank and to elucidate the ecological processes which are at play in the biological communities of this unique ecosystem. Detailed benthic habitat maps have already been developed for some Caribbean reef ecosystems
Findings presented here raise questions about the importance of nursery habitats for some fish species and the role of disturbance in structuring benthic communities. From the perspective of managing fisheries resources, it is essential to understand which habitat types serve as alternative nursery areas for commercially valued species, and whether such areas require special protective measures. Factors that have contributed to the good ecosystem health of Saba Bank likely include the Bank's inaccessibility, its distance from major coastal sources of pollution, the small size of the fishing fleet operating on the bank, and the large size of the bank itself. However, formal protection and strategic management of Saba Bank will ensure that anthropogenic stressors do not lead to degradation of this unique ecosystem in the future.
Fish species number across Saba Bank habitat types* as observed in roving diver surveys. Values are the number of location within each habitat type where a species was observed (8 surveys per habitat type). (*) Abbreviations for habitat types are: FR = fore reef; ORF = outer reef flat; IRF = inner reef flat; LSB = lagoon soft-bottom; LHB = lagoon hard-bottom. (**) Chi square test to determine the statistical significance of observed species distribution among habitat types. NS = not significant; (+) = significant at p>0.05; (++) = significant at p>0.01. The dagger symbol indicates a significant difference in family-level distribution among habitat types. For Acanthuridae and Balistidae, differences observed in among-habitat distribution were not significant (ns).
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The studies presented here formed one component of The Saba Bank Project. Shelley Lundvall, Project Manager, provided essential logistical support and diving assistance. Peter Etnoyer compiled bathymetric datasets, provided LandSat images, and assisted with GIS analyses and sampling design. Jan den Dulk, Manager of Saba National Marine Park, provided office space and logistical support to the survey team. The following people assisted with field surveys: Harry Eyre, Sue Hurrell, Nicholas Johnson, Ryan Johnson, Leendert van Driel, and Greg Laake. We are grateful to two anonymous referees for critical review of the manuscript.