ARCHIPELAGIC COMPARISONS

433

Chapter 8 Archipelagic Comparisons

Each of the previous five chapters provided comprehensive and detailed descriptions of 

coral reef ecosystems in the Territory of American Samoa (Tutuila and Aunu`u Islands, 

Ofu and Olosega Islands, Ta`u Island, Rose Atoll, Swains Island). These discussions 

included the following topics: geopolitical context, survey efforts, benthic habitat mapping 

and characterization, oceanography and water, coral and coral disease, algae, benthic 

macroinvertebrates, reef fish, and island summary and ecosystem integration. In this chapter, 

the same topics will be discussed but the focus will shift from individual islands to interisland 

comparisons across the American Samoa Archipelago. The spatial patterns and temporal 

variability of the island/atoll ecosystems will be examined on a regional scale. This broaderscale ecosystem examination explicitly provides important information for local management 

and conservation at these islands/atolls. However, it also presents useful insights which can 

be applied more generally towards the management and conservation of coral reefs on a 

regional, national, and international level. 

8.1 Geopolitical Context

The Territory of American Samoa is composed of five volcanic high islands (Tutuila, Aunu`u, 

Ofu, Olosega, and Ta`u) and two low coral atolls (Swains and Rose). Tutuila (including 

Aunu`u) is the largest (145 km2) and most densely populated (55 885 residents) island in 

American Samoa (Figs. 8.1a and 8.1b). In addition, Tutuila harbors the most shallow-water 

coral reef habitat (315 km2) of any island in American Samoa (Fig. 8.1b).

Figure 8.1a. Human population numbers and densities of Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/

OLO), Ta`u (TAU), Rose (ROS), and Swains (SWA). The solid bars indicate population numbers per island, 

while the striped bars indicate population densities (U.S. Census Bureau, 2000).
ARCHIPELAGIC COMPARISONS

434

Because of higher population densities and large areas of coral reef habitat on Tutuila (Figs. 

8.1a and 8.1b), the potential for detrimental anthropogenic impacts (e.g., agricultural runoff, 

point and nonpoint source pollutants, and excessive resource use) is of concern. The Pago Pago 

Harbor/International Airport and densely populated coastline surrounding Pala Lagoon (Fig. 

3.1b) are areas potentially subject to elevated industrial and commercial pressures. Several 

marine protected areas have been established on Tutuila, including: Fagatele Bay National 

Marine Sanctuary near Step’s Point in the south point region (Fig. 3.1a); the National Park 

of American Samoa (NPAS), which spans a large stretch of northern coastline from Fagasa 

Bay to Cockscomb Point and Vatia Bay; and additional areas addressed by the communitybased Fisheries Management Program of American Samoa. Despite their protected status, 

these areas may still be subject to anthropogenic pressure. In particular, during a 7-week 

record from an ecological acoustic recorder deployed in the NPAS, the majority of vessel 

detections (23 out of 35 instances) occurred at night (Fig. 3.4.4c), which may indicate local 

fishing effort in that marine park.

Human population numbers and densities decrease as one travels east along the American 

Samoa Archipelago (Fig. 8.1a). The Manu`a Islands are located approximately 100 km east 

of Tutuila, and include Ofu, Olosega, and Ta`u. The population of the Manu`a Islands has 

been steadily decreasing since the 1950s as a result of a migration to Tutuila for employment 

opportunities and several damaging storms affecting the Manu`a Islands.

Ofu, with a land area of 7.2 km2, and Olosega, with a land area of 5.4 km2, have a combined 

population of 505 people, with 289 living on Ofu and 216 living on Olosega (Figs. 8.1a and 

8.1b). The populations are located around two primary villages: Ofu Village (northwestern 

coast of Ofu), which is the location of the small airport and protected harbor, and Olosega 

Figure 8.1b. Total banktop area and total terrestrial land area of Tutuila and Aunu`u (TUT), Ofu and Olosega 

(OFU/OLO), Ta`u (TAU), Rose (ROS) and Swains (SWA).High volcanic islands are denoted with the letter H, 

low carbonate islands/atolls with the letter L.
ARCHIPELAGIC COMPARISONS

435

Village (southwestern coast of Olosega). Sili Village, located on the northwestern coast of 

Olosega, remains sparsely inhabited. An estimated two-thirds of Ofu and Olosega remain 

undeveloped, with the majority of the southern coastline of Ofu under the administrative 

jurisdiction of the NPAS.

Ta`u, with a land area of 44 km2, is home to 380 people primarily located in the villages of 

Ta`u and Faleasao in the northwest and Fiti`uta in the northeast (Figs. 8.1a and 8.1b). The 

undeveloped southeastern and southern coastlines of Ta`u are under the jurisdiction of the 

National Park Service. Coastal subsistence agriculture/agrofarming and subsistence fishing 

likely constitute the majority of potential anthropogenic stressors to the reef environment, 

along with the activities associated with the boat harbor and airport in Ofu Village and the 

small airport near Fiti`uta.

Rose Atoll is the uninhabited, easternmost territory under the joint administration of the 

Territory of American Samoa and the United States Fish and Wildlife Service. The barrier reef 

encloses approximately 6.5 km2 (~ 1600 ac) of lagoonal habitat, with a 40-m wide channel 

located in the northern corner of the atoll (Figs. 8.1a and 8.1b). Two small islands (Rose and 

Sand) constitute only 0.06 km2 (~ 15 ac) of land. While Rose Island is vegetated, both islands 

do not have any streams, associated watershed areas or notable freshwater sources. Potential 

anthropogenic stressors to Rose Atoll are probably not a result of continued habitation by 

residents. However, the effects of the 1993 shipwreck, wildlife poaching (anecdotal evidence) 

and unmonitored fishing pressure likely constitute the main anthropogenic impacts at Rose 

Atoll.

Swains is a geographically small island and is geologically isolated from the rest of the 

American Samoa Archipelago (Chapter 1, Fig. 1.1a). The island is only 1.9 km2 and is 

consistently home to ≤ 30 residents who are concentrated in the village of Taulaga in the 

northwest (Figs. 8.1a and 8.1b). As a result of the small population and lack of industry on 

this island, the anthropogenic stresses on the coral reef ecosystems are limited to subsistence 

fishing pressures and localized subsistence agricultural runoff.

8.2 Survey Effort

A considerable amount of bathymetric, oceanographic, and biological data has been collected 

in the American Samoa Archipelago between 2002 and 2006. These data allow better 

understanding of the spatial and temporal variability of the coral reef ecosystems around 

each of the islands/atolls. Using spatially consistent Pacific Reef Assessment and Monitoring 

Program (RAMP) methodologies, a similar suite of observations and data have been collected 

for each island/atoll within the archipelago. Because of differences in the reef habitat areas, 

ship availability, and weather and sea conditions, the amount and spatial resolution of these 

data vary among islands. Details of survey efforts for each island/atoll are discussed in their 

respective chapters as follows: Tutuila and Aunu`u Islands (Chapter 3, Section 3.2: Survey 

Effort), Ofu and Olosega Islands (Chapter 4, Section 4.2: Survey Effort), Ta`u Island (Chapter 

5, Section 5.2: Survey Effort), Swains Island (Chapter 6, Section 6.2: Survey Effort), and 

Rose Atoll (Chapter 7, Section 7.2: Survey Effort). The respective island/atoll datasets have 

been coalesced to produce the archipelagic comparative analyses presented in this chapter.
ARCHIPELAGIC COMPARISONS

436

In addition to offering archipelagic comparisons of the mapping, oceanographic, and biological 

observations discussed in the five island/atoll chapters, these analyses explore information 

about the prevailing deep-water oceanographic environments surrounding each of the islands 

by using a series of the deep-water (to a ~ 500-m depth) conductivity, temperature, and 

depth (CTD) casts conducted from the National Oceanic and Atmospheric Administration 

(NOAA) Ships Townsend Cromwell, Oscar Elton Sette, and Hi`ialakai during American 

Samoa RAMP (ASRAMP) cruises in 2002, 2004, and 2006. The oceanographic data from 

these deep-water CTD casts are shown and discussed in Section 8.4.1: Vertical Structure. 

Regional oceanographic patterns on seasonal time scales were also examined.

8.3 Benthic Habitat Mapping and Characterization

The nearshore waters of the American Samoan Islands were surveyed using multibeam 

echosounders at depths between ~ 15 and at least 200 m, and in many cases, to depths as 

great as 3000 m. Optical imagery (video and still photographs) and diver observations of the 

seafloor at depths between ~ 3 and ~ 80 m were collected during Towed Optical Assessment 

Device (TOAD) surveys and towed-diver surveys. Multibeam bathymetry of the different 

islands revealed a trend of smaller insular shelves progressing away from Tutuila to the east 

(toward the Manu`a Islands and Rose) and north (toward Swains). The amount of reef habitat 

area (depths < 100 m) is substantially higher around Tutuila when compared with the rest 

of the archipelago, followed by Ofu and Olosega (combined), Ta`u, Rose, and Swains (Fig. 

8.1b). Possible reasons for this pattern will be discussed shortly.

As seen in Figure 8.1b, the relationship between total banktop area (defined as all submerged 

marine habitats at depths between the shoreline and 100 m) and total land area of the high 

islands (Tutuila, Ofu, Olosega, and Ta`u) is distinct from the low islands (Swains and Rose 

Atoll). Tutuila, the largest of the islands, has an extensive banktop surrounding the island. 

Depth intervals around Tutuila decline gradually from shoreline to ~ 100 m, an indication of 

Figure 8.3a. Percentage of the total banktop area included in each depth interval. Tutuila and Aunu`u (TUT), 

Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS) and Swains (SWA).
ARCHIPELAGIC COMPARISONS

437

the gently sloping and extensive banktop habitat surrounding the island (Figs. 8.1b and 8.3a). 

Ta`u, Ofu, and Olosega are all relatively small islands with significantly smaller surrounding 

banktop habitat areas. For these islands, bathymetric depth intervals surveyed from shoreline 

to ~ 100 m descend much more steeply than around Tutuila, with Ta`u descending the most 

steeply (Fig. 8.3a). Rose Island (within Rose Atoll) is so small that it cannot be displayed 

on Figure 8.1b; the total emergent land of the island is less than 8.2 ha. Both Rose Atoll and 

Swains have very limited shallow banktop (Fig. 8.1b). Figure 8.3a shows that depths rapidly 

descend from 20 to 100 m at both Rose Atoll and Swains. The percent that is shallower than 

20 m (backreefs, reef flats, reef crests, and shallow forereef slopes) is greatest at the smallest 

islands/atolls (Swains and Rose Atoll) and smallest at the largest island (Tutuila; Fig. 8.3.a).

The American Samoan Islands rest on a portion of the earth’s crust known as the Pacific 

Plate, which, in the vicinity of these islands, is drifting to the west at a rate of ~ 7 cm per 

year (McDougall, 1985). Most of these islands, as well as the islands of Savai`i and Upolu 

in Samoa (west of American Samoa), were formed by volcanic eruptions when the portion 

of the plate on which they are resting passed over a thermal plume in the Earth’s mantle, 

the “Samoan hotspot” (Hart et al., 2000). Thus, islands farther to the southeast are generally 

younger, while the islands farther to the northwest are older. A new volcanic edifice named 

Vailulu`u that may one day become an island is currently being formed approximately 50 

km east of Ta`u. Tutuila is about 1.5 million years old, while Ofu and Olosega, and Ta`u are 

about 300 000 and 100 000 years old, respectively. Although politically part of American 

Samoa, Swains and Rose are built on much older volcanoes and are, geologically speaking, 

not part of the Samoan volcanic chain (Hart et al., 2004).

For at least the last 800 000 years, global sea level has fluctuated about 120 m every 100 000 

years or so in response to glacial-interglacial cycles. In the current interglacial period, sea 

level is near the top of its 120-m oscillation (Ruddiman et al., 1989). When sea level is static 

for a prolonged period, it tends to erode terraces into land areas. Tutuila has been exposed to 

erosion from glacial low-stands of sea level for far longer than the more easterly islands and 

consequently has an insular shelf that is ~ 4 km wide on average, with ~ 320 km2 of coral reef 

ecosystem area shallower than 100 m. Ofu and Olosega have a narrow shelf that is ~ 1 km 

wide in most areas, and banks eroded into several corners of the islands that extend ~ 2 km 

offshore. Ta`u has an insular shelf that is narrower still, being generally absent on the south 

and eastern sides of the island. Elsewhere it is generally a few hundred meters wide, except 

off corners of the island and along the western half of the north shore, where it may be ~ 1 km 

wide. The volume of sediment stored on the shelves appears to be generally correlated with 

shelf area. Sediment reservoirs are found in channels offshore of terrestrial drainages, at the 

base of the forereef slope, and in mid-shelf depressions around Tutuila.

The greater age of Tutuila has enabled the development of other features in addition to the 

insular shelf itself. Multibeam bathymetry of Tutuila provides a detailed picture of an elevated 

ridge around the seaward rim of the insular shelf of the island, which probably corresponds 

to a drowned barrier reef complex. Fishing boats have been observed congregating along 

this feature, and high coral cover areas have been seen in TOAD imagery in a number of 

locations (Chapter 3, Section 3.3: Benthic Habitat Mapping and Characterization). Given the 

potential importance of coral reef resources along the ridge, it will be an area of increased 

focus during the ASRAMP 2008 cruise.
ARCHIPELAGIC COMPARISONS

438

Multibeam bathymetry has also revealed details of other seafloor features of interest, 

including a series of small seamounts along an east-west tending ridge starting about 

~ 2 km west of Ta`u. TOAD imagery has revealed a luxuriant coral reef on the top of the 

seamount closest to Ta`u at 38-m depths and deeper (Chapter 5, Section 5.3: Benthic Habitat 

Mapping and Characterization). Other coral reefs have also been found between 30 and 50 

m or even deeper on several of the spurs extending seaward from corners of the Manu`a 

Islands. 

Towed-diver benthic surveys indicate that, on average, the American Samoan Islands had 

live scleractinian coral cover ranging from ~ 5% to ~ 40%. The Manu`a Islands tended to 

have greater cover on the southeastern half of the islands and Ta`u, in particular, had areas 

with > 50% live scleractinian coral cover. In contrast, high coral cover areas around Tutuila 

tended to occur in sheltered locations, such as Fagatele Bay or on ridges that extend out 

towards the open ocean. Swains had markedly higher live coral cover than the rest of the 

islands with values ranging from 20% to exceptionally high values of 75%, with a trend of 

increasing coral cover to the northeast.

Figure 8.3b. Island-wide mean benthic habitat composition for each of the islands/atolls of the American 

Samoa Archipelago from towed-diver benthic survey observations during ASRAMP 2002, 2004 and 2006 for 

Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS), and Swains (SWA).The 

shallow-water habitat was characterized by four measures: habitat complexity (L = low, ML = medium-low, 

M = medium, MH = medium-high, H = high, VH = very high), sand, hard substrate, and coral rubble. It is not 

clear at this time whether changes between years are indicative of actual changes in seafloor characteristics or 

are more reflective of variations in the location of towboard tracks and uncertainty in observer estimates. Note 

that lagoon data are not presented for Rose; only forereef data are presented for better comparison with the other 

islands that lack a lagoon. 
ARCHIPELAGIC COMPARISONS

439

Benthic habitat characterization data were collected at relatively shallow depths (majority 

< 20 m) during towed-diver surveys. Results varied within and between years because of 

changes in survey depths and locations, different divers, and changes in data collection 

protocols. However, even with these inconsistencies, trends were identified in benthic habitat 

characteristics from west (Tutuila) to east (Rose) to north (Swains).

Mean habitat complexity was fairly similar between islands and survey periods, generally 

falling in the range of medium to medium-high complexity from 2002 to 2006 (Fig. 8.3b). 

However, several notable trends were observed. Overall habitat complexity generally 

increased moving from west to east and north along the archipelago with Swains having 

the highest overall mean complexity in most years (2002 and 2006) followed closely by 

Rose. Habitat complexity was slightly lower around Tutuila versus the Manu`a Island group, 

possibly because of prolonged exposure to erosion, resulting in large sections of fairly flat 

shelf areas around Tutuila. Habitat complexity was highest at Swains and Rose partly because 

of the steepness of their slopes. In addition, both islands are subject to the full effects of largescale westerly currents and trade wind waves from the east, without the mitigating influence 

of an insular shelf. The resulting greater nearshore water motion at these two islands (Swains 

in particular) is hypothesized to at least partly explain why they have higher live coral cover 

than the islands with greater shelf area, and why coral cover is highest on their more easterly 

sides. 

In contrast, the percentage of benthic sand cover decreased from west to east and north 

across the archipelago, with Rose and Swains recording the least amount of sandy habitat. 

These results are supported by the bathymetric and backscatter data, which suggest that sand 

retention appears to be related to shelf area. Rose and Swains are older than Tutuila but lack 

the insular shelves found at other islands in American Samoa. The lack of an insular shelf is 

believed to be the reason for the almost complete lack of sandy habitat seaward of the reef 

crest. Note, however, that Rose lagoon is predominantly composed of sandy substrate.

Hard substrate/pavement composition varied between years and locations. However, an 

interesting, if perplexing, trend was noted between 2002 and 2004, with Swains recording 

the lowest overall hard substrate/pavement cover, followed by Ta`u and Tutuila. It is not 

clear at this time whether changes between years are indicative of actual changes in seafloor 

characteristics or are more reflective of variations in the location of towboard tracks and 

uncertainty in observer estimates. Ofu, Olosega, and Rose constituted the highest recorded 

hard substrate/pavement cover for 2002 and 2004. 

Rubble composition varied by location and by year, which was mostly attributable to changes 

in classification protocol, with no obvious or discernable archipelago-wide relationship.
ARCHIPELAGIC COMPARISONS

440

8.4 Oceanography and Water Quality

8.4.1 Vertical Structure

Shallow Nearshore Surveys 

Nearshore mean-temperature profiles from the shallow-water CTD surveys around each of the 

islands/atolls during ASRAMP 2002, 2004, and 2006 were relatively consistent throughout 

the five islands, with depth-associated variability in the upper 30 m of the water column 

typically < 0.5°C (Fig. 8.4.1a). Swains, the northern and most isolated island of the group, 

had the warmest, least vertically stratified, least variable temperature profile, with a mean 

temperature of ~ 29.5°C from the surface to the bottom. Rose, the easternmost island/atoll in 

American Samoa, had the coolest, most vertically stratified, and most variable temperature 

profile in the upper 30 m of the water column. The greatest mean temperature variation was 

observed around Ta`u, with temperature reduced 0.5°C from ~ 29.6°C at the surface (1 m) to 

~ 29.1°C at a 32-m depth. 

Mean island-wide salinity profiles show much more noteworthy differences among the islands 

(Fig. 8.1a). Rose had the highest mean salinity (35.3 psu), the largest variability with depth, 

and the highest variance in surface waters (above the sill depth [~ 6 m] in the channel pass).

This is reflected in the high salinity lagoonal waters below the sill depth. The high variance 

found in the surface waters could be caused by wave setup over the reef crest around the 

atoll. Mean salinity levels were lowest (~ 34.7 psu) around Swains, with little depth variation 

in the upper 30 m. Variability of salinity was also high in the surface layer (0–2 m) around 

Tutuila, probably reflecting a frequent freshwater lens caused by terrestrial runoff from high 

levels of precipitation. Mean transmission showed little depth variability around all islands. 

Rose and Ta`u had the highest transmission levels (97–98%) while Swains and Tutuila had 

the lowest transmission (~ 95%). The variance of transmission was generally low, except in 

the near-surface waters around Tutuila, probably reflecting terrigenous inputs from watershed 

runoff. Mean density profiles showed similar trends to that of salinity. Highest density water 

and most significant depth variation were found around Rose, while lowest density (21.75 

kg m-3) and least depth variability were observed around Swains. The highest variance of 

density was observed in the upper ~ 8 m around Tutuila, also probably reflecting watershed 

runoff.

Across the archipelago (Fig. 8.4.1b) mean profiles of water properties at each of the islands/

atolls showed that Swains, located over 350 km north of the other islands/atolls, was an 

outlier characterized by significantly different physical water properties. Data from the top 

1 m of the water column were potentially skewed because of surface heating and instrument 

temperature equilibration and should therefore be treated with caution. However, since the 

data are not unreasonable, they have been included for completeness.

Across the archipelago, Tutuila exhibited the highest concentrations of chlorophyll-a (Chla; 0.7 μg L-1), silicates (SiO2

 2.5 μM), and nitrates (NO2 0.025 μM), although these values 

were highly variable between sites, as indicated by the elevated standard deviations (Fig. 

8.4.1b). Phosphate (PO4) concentrations were similar amongst the five islands (0.10–0.15 

μM), with the exception of Ofu and Olosega, which had slightly lower concentrations. Mean 
ARCHIPELAGIC COMPARISONS

441

Figure 8.4.1a. Means and variances of temperature, salinity, beam transmission, and density were calculated 

from shallow-water (< 40 m) CTD casts around each island during ASRAMP 2002, 2004, and 2006 for Tutuila 

and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS) and Swains (SWA).Data obtained 

within the freshwater lagoon at Swains were removed before averaging as they were incomparable with the 

other CTD casts. Data from the lagoon at Rose were included for this analysis. 

Figure 8.4.1b. Water sampling was conducted at American Samoa during ASRAMP 2006 for Tutuila and 

Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS), and Swains (SWA). Island-wide 

averages were derived to compare chemical variability throughout the whole archipelago. Data obtained within 

the freshwater lagoon at Swains were removed as they were incomparable with nearshore oceanographic 

samples and skewed the results. 
ARCHIPELAGIC COMPARISONS

442

nitrite (NO3 μM) concentrations were highest around Ofu, Olosega, and Ta`u (~ 0.4–0.5 μM), 

and lowest around Swains and Rose (~ 0.1 μM). Generally, variances of all water quality 

parameters were highest around Tutuila, probably reflecting the greater range of habitat types 

and exposures to ocean conditions, and terrigenous inputs from local watersheds.

Mean chemical concentrations from around the islands and atolls of American Samoa 

indicate that orographic relief, land mass, and population size strongly impact nearshore 

nutrient and water quality concentrations. The large landmass and higher elevation of Tutuila 

compared with the other islands produced more seasonal rainfall and increased sedimentation 

to surrounding waters. Because of these factors, combined with the large population and 

associated land use, it is not surprising that Tutuila had the highest nutrient and Chl-a

concentrations across the archipelago. The large standard deviation in the data obtained from 

Tutuila were probably caused by spikes in nutrient concentrations near developed areas, 

where nutrient and contaminant runoff is generally high compared with more rural areas 

where contaminants are relatively low. The variability of water quality parameters at all other 

islands is most likely linked to island size and population, as islands of similar land area and 

number of inhabitants appear to have markedly similar nearshore chemical concentrations.

Deep-water Offshore Surveys

For the deep-water (~ 500 m) CTD casts completed around each island/atoll during the 

ASRAMP 2002, 2004, and 2006 cruises, each of the four mean vertical profiles of water 

properties (temperature, salinity, density, dissolved oxygen, and fluorometry) showed 

that Swains was again an outlier compared with the other islands and atolls (Fig. 8.4.1c). 

Temperature was consistent throughout the five islands/atolls, each varying from ~ 30°C 

at the surface to ~ 8°C at 500 m. Although Swains exhibited higher temperatures in the 

surface waters and slightly lower temperatures below 200 m, the overall temperature 

variance was < 1°C and was significantly less than this in the surface waters (< 100 

m). The island’s mean salinity profiles showed similar patterns with depth, peaking at 

~ 270 m at 36 psu. The mean salinity profile around Swains followed the general pattern 

of the other islands but exhibited lower salinity in surface (< 100 m) and deeper (> 220 

m) waters. Variability was limited to the surface water and never exceeded 0.1 psu. 

Oxygen saturation in surface waters was similar around all islands and diverged and 

undulated with depths below 70 m. Swains had the lowest oxygen saturation with 

two main troughs: one at ~ 100 m deep (2.6 ml L-1) and another at ~ 350 m deep 

(at 2.4 ml L-1). Fluorometer levels were low (< 0.1 ml L-1) in surface waters, and peaked 

at depths around 100 m around each of the islands/atolls. Swains showed the greatest 

depth variability with a subsurface maximum of 0.35 ml L-1. Rose had the smallest peak in 

fluorometry at < 0.2 ml L-1. Fluorometry values tended to 0.0 ml L-1 below ~ 250-m depths, 

which corresponds to the deepest penetration of light into the water column. Variances of the 

fluorometry observations were greatest near the peak at 100-m depths.

Means from both deep- and shallow-water CTD casts show that the coral reef ecosystems 

around Swains consistently reside in a different oceanographic environment, with different 

temperatures, salinities, oxygen, and fluorometry readings than the other islands/atolls of the 

archipelago, at least during the austral summer months of the ASRAMP surveys.
ARCHIPELAGIC COMPARISONS

443

8.4.2 Seasonal Variation

In the previous section, the spatial variability of the vertical structure of water properties 

around the American Samoa Archipelago was described. Those observations characterized 

the oceanographic and water quality environment influencing the coral reef ecosystems 

around the islands and atolls at the times of the surveys, which were conducted only during 

the austral summer months. In this section, satellite remote sensing observations are used 

to explore the seasonal variability of key sea surface properties (temperature, Chl-a, and 

wind speed) across the region in an effort to better understand the range of environmental 

conditions influencing these reef ecosystems.

Mean sea surface temperature (SST) climatologies for the American Samoa region (10°–15° 

S; 167°–173° W) are presented for each season (summer [Jan–Mar], fall [Apr–Jun], winter 

[Jul–Sep], and spring [Oct–Dec]) to examine the seasonal temperature variability across the 

archipelago (Fig. 8.4.2a). This figure shows an annual temperature range across the region of 

~ 3°C; warm temperatures were observed during summer months (~ 29°–30°C) with cooler 

temperatures during the winter (~ 27°–28°C). During fall, winter, and spring, a pronounced 

SST gradient occurred between 12° and 13° S, with warmer waters observed to the north of 

this gradient and cooler waters observed to the south. The gradient weakened and became 

more diffuse during the summer months; however, north-south differences in SST are still 

Figure 8.4.1c. Means and variances of temperature, salinity, O2 saturation, and fluorometry were calculated 

from shipboard deep-water (~ 500 m) CTD casts taken around each island during ASRAMP 2002, 2004, and 

2006 for Tutuila and Aunu`u (TUT), Ofu, Olosega, and Ta`u (MAN), Rose (ROS) and Swains (SWA). 
ARCHIPELAGIC COMPARISONS

444

apparent. As shown in Figure 8.4.2a, Swains is sufficiently far north to lie within a different 

temperature regime, which is typically 0.5°–1.5°C warmer throughout the year than the 

waters surrounding the other islands of the archipelago.

Mean climatological Chl-a concentration showed seasonal variability in the American Samoa 

region (Fig. 8.4.2b), with the lowest concentrations occurring during the austral summer and 

the highest concentrations during the winter. Latitudinal gradients of Chl-a concentration 

were absent in the summer, weak in the fall, and relatively strong during the spring and 

winter, with a south to north increase in Chl-a occurring at ~ 11° S. Swains is located in the 

middle of this seasonal Chl-a gradient and was in a region of relatively high productivity 

during the winter and spring. The other islands in the archipelago are located well south of 

this Chl-a gradient and are surrounded by generally oligotrophic (low productivity) waters 

throughout the year. When examining specific island effects, it appears clearly that Chl-a

Figure 8.4.2a. Seasonal SST climatology for the American Samoa region (10°–15° S, 167°–173° W). Data is 

from NOAA Pathfinder 5.0 SST seasonal climatology from 1985 to 2001 produced by National Oceanographic 

Data Center (NODC; http://www.nodc.noaa.gov/sog/pathfinder4km). Seasons are defined as: Summer = Jan–

Mar, Fall = Apr–Jun, Winter = Jul–Sep, and Spring = Oct–Dec. Island abbreviations: TUT = Tutuila, MAN = 

Manu`a, ROS = Rose, and SWA = Swains.
ARCHIPELAGIC COMPARISONS

445

values increase to the southwest of Tutuila and Samoa, which may be caused by terrigenous 

input or topographically induced upwelling influencing local productivity. Rose was observed 

to have relatively high Chl-a throughout the year; however, this was perhaps an artifact of 

enhanced reflectance associated with the lagoon, aliasing ocean color in the area.

Mean climatological wind speed around the American Samoa region showed seasonal 

differences with greater speeds in the fall (7–8 m s-1) and winter (8–9 m s-1) compared to 

winds in the spring (6–7 m s-1) and summer (5–6 m s-1; Fig. 8.4.2c). Overall, winds in the 

American Samoa region decrease in strength towards the equator. Southeast trades are the 

dominant winds in this area; however, because of the northerly geographic location of Swains, 

the island is subject to both weaker annual mean wind speeds, as well as lower variability 

Figure 8.4.2b. Seasonal Chl-a climatology using National Aeronautics and Space Administration (NASA) 

SeaWiFS imagery for the American Samoa region (10°–15° S, 167°–173° W; http://oceancolor.gsfc.nasa.gov/

SeaWiFS). Climatology from 2000 to 2003 produced by Pacific Islands Fisheries Science Center Coral Reef 

Ecosystem Division (CRED) and NODC for OceanEye project. Seasons are defined as: Summer = Jan–Mar, 

Fall = Apr–Jun, Winter = Jul–Sep, and Spring = Oct–Dec. Island abbreviations: TUT = Tutuila, MAN = Manu`a, 

ROS = Rose, and SWA = Swains.
ARCHIPELAGIC COMPARISONS

446

throughout the year. Weaker mean wind speeds in the vicinity of Swains would be expected 

to locally increase upper ocean and nearshore water temperatures by decreasing vertical 

wind and wave-induced mixing.

Satellite-derived seasonal climatologies for SST, Chl-a, and wind speed were highly variable 

in the region surrounding American Samoa. Winter conditions were characterized by stronger 

winds, increased surface productivity, and decreased SST, all of which show strong latitudinal 

gradients. In contrast, summer conditions exhibit weak latitudinal gradients with generally 

weak winds, low surface productivity, and increased SST across most of the region.

Satellite-derived climatologies of oceanographic and meteorological data provide a regional 

and seasonal context for which in situ observations can be interpreted. Although in situ data 

Figure 8.4.2c. Seasonal wind climatology using NASA QuikScat Scatterometer Winds for the American 

Samoa region (10°–15° S, 167°–173° W; http://winds.jpl.nasa.gov/missions/quikscat/index.cfm).Climatology 

from 2000 to 2003 produced by CRED and NODC for OceanEye project. Seasons are defined as: Summer = 

Jan–Mar, Fall = Apr–Jun, Winter = Jul–Sep, and Spring = Oct–Dec. Island abbreviations: TUT = Tutuila, MAN 

= Manu`a, ROS = Rose, and SWA = Swains.
ARCHIPELAGIC COMPARISONS

447

from each island are discrete with respect to both space and time, incorporating remotely sensed 

climatological data allows, to a limited extent, for space-time discrepancies to be resolved 

and general conclusions to be made regarding interisland environmental variability.

Swains is geographically separate from the rest of American Samoa resulting in appreciably 

different physical oceanographic conditions, as seen via in situ observations and satellite 

climatologies. However, each of the three ASRAMP surveys occurred during late austral 

summer (February–March), when minimal gradients in climatological SST and wind were 

observed. Presumably, in situ oceanographic condition disparities at Swains compared with 

the rest of the islands in American Samoa would be accentuated during austral winter, when 

latitudinal gradients in SST and wind are more pronounced. 

Conversely, mean in situ Chl-a concentrations in American Samoa seem to be principally 

dependent on individual island characteristics rather than differences in geographical 

distribution. For example, Rose and Swains, nearly equal in size, have similar mean 

chlorophyll values, despite the substantial difference in latitude between them. Island means 

and interisland comparisons would undoubtedly change if ASRAMP surveys had occurred 

during the austral winter, when a pronounced north-south gradient and an overall increase 

in Chl-a are observed (Chapter 7, Fig. 7.3d). While satellite-derived data products are an 

excellent tool for illustrating temporal changes in Chl-a surface concentrations in the greater 

American Samoa region, they typically are not dependable for nearshore environments 

because of complex ocean color issues related to bottom reflectance. The measured in situChla values, for example, are orders of magnitude greater than satellite-derived climatological 

values. This demonstrates the significance of using high-resolution (spatial) in situ subsurface 

and nearshore data, in addition to remotely sensed data, to best describe and understand 

the oceanographic and water quality properties influencing the reef ecosystems around each 

island/atoll in the region.

8.5 Coral and Coral Disease

8.5.1 Coral Surveys

In all survey years, visual estimates recorded by towed divers indicated the highest coral 

cover was observed around Swains (Fig. 8.5.1a; Table 8.5.1a). Rose and Ta`u consistently 

showed the next highest coral cover values in all survey years. The relative ranking of Tutuila 

and Ofu and Olosega with regard to coral cover values varied among survey years, though 

mean coral cover around Tutuila exceeded the coral cover around Ofu and Olosega during 2 

of the 3 survey years. Thus, the ranking of island-wide coral cover among American Samoan 

Islands and atolls, as determined through towed-diver surveys, is best described as Swains > 

Rose ≥ Ta`u > Tutuila > Ofu and Olosega.

Based on towed-diver surveys, mean island-wide live coral cover values around all islands 

declined from 2002 to 2004 (Fig. 8.5.1a; Table 8.5.1a). Between 2004 and 2006, mean live 

coral cover again declined around Tutuila, Ta`u, and Rose, but increased at Ofu and Olosega 

and Swains (although not to 2002 values). However, these data are based on visual estimates, 

which are subject to observer variability between survey years. Also, while every effort is 

made to replicate tracklines between survey years, slight differences in the lateral surface 
ARCHIPELAGIC COMPARISONS

448

position of the towboat can translate to the divers being in different depth strata between 

survey years. See depth histograms in towed-diver survey track figures in chapter texts (e.g., 

Chapter 4, Figs. 4.5.1a, 4.5.1d, and 4.5.1j). This effect is particularly pronounced at Rose and 

Swains, which have steeply sloping forereefs. Therefore, the apparent net decline in coral 

cover at all locations between 2002 and 2006 should be interpreted with caution.

Table 8.5.1a. Mean percent live coral cover values (as presented in Fig. 8.5.1a).N.A. = not available

Mean Live Coral Cover

Island Year REA Survey Towed-diver Survey

Unstressed Stressed

Tutuila 2002 N.A. 29.0 N.A.

Ofu/Olo 2002 N.A. 20.3 N.A.

Ta`u 2002 N.A. 30.8 N.A.

Rose* 2002 N.A. 29.1 N.A.

Swains 2002 N.A. 60.0 N.A.

Tutuila 2004 37.7 18.7 4.9

Ofu/Olo 2004 25.3 14.5 2.4

Ta`u 2004 24.3 22.7 5.8

Rose* 2004 23.6 23.5 5.6

Swains 2004 46.5 30.5 7.8

Tutuila 2006 24.1 13.7 3.2

Ofu/Olo 2006 29.4 17.9 3.3

Tau 2006 29.7 19.3 2.0

Rose* 2006 13.8 20.1 4.5

Swains 2006 42.8 43.9 1.9

*Forereef only

Figure 8.5.1a. Mean percent live coral cover measured from towed-diver and Rapid Ecological Assessment 

(REA) surveys around Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS), 

and Swains (SWA) during ASRAMP 2002, 2004, and 2006. Note that lagoon data are not presented for Rose; 

only forereef data are presented for better comparison with the other islands that lack a lagoon. 
ARCHIPELAGIC COMPARISONS

449

“Stressed” is an inclusive category including coral that may exhibit the effects of bleaching, 

disease, sedimentation or predation. It is not usually possible to clearly determine the cause 

of stress while being towed above the benthos. During 2004 and 2006, the survey years in 

which the stressed coral percent cover was estimated, island-wide mean values suggested 

that 1.9–7.8% of live coral exhibited some form of stress. Swains showed the extremes of 

this range, with the highest values of stressed coral in 2004 (7.8%) but the lowest values in 

2006 (1.9%; Fig. 8.5.1a; Table 8.5.1a). Between 2004 and 2006, stress values declined at 

all locations except Ofu and Olosega, which showed a slight increase. As with coral cover, 

these values are based on visual estimates, which are subject to observer variability between 

survey years.

During 2004 and 2006, the years in which quantitative surveys were conducted, the REA 

methodologies also indicated the highest island-wide mean live coral percent cover to be 

around Swains (Fig. 8.5.1a; Table 8.5.1a). The relative ranking of the other islands with 

regard to coral cover varied between survey years and also varied from that determined 

during towed-diver surveys. Thus, except for the highest coral cover being found at Swains, 

it is difficult to generalize archipelago-wide trends of coral cover from data collected during 

REA surveys. For all islands and survey years, except for Swains in 2006, mean estimates 

of island-wide live coral percent cover derived from REA surveys were higher than mean 

estimates derived from towed-diver surveys. Because towed-diver surveys include softbottom substrate (e.g., sand) while site-specific REA surveys target hard-bottom substrate, 

and because towed divers survey substantially more area than free-swimming divers during 

site-specific REA surveys, the estimates derived from towed-diver surveys probably provide 

a better measure of mean coral cover across large expanses of habitat than do site-specific 

surveys. Even though data from the two methods are not strictly comparable, they can be 

used to make broad generalizations that complement each other. The towed-diver surveys are 

designed to obtain estimates of benthic composition (including both hard- and soft-bottom 

substrates) over broad spatial scales, whereas the REA surveys are site specific and designed 

to better detect changes at target locations.

Between 2004 and 2006, REA surveys showed a decline in live coral percent cover around 

Tutuila, Rose, and Swains, and an increase around Ofu and Olosega, and Ta`u. The declines 

around Tutuila and Rose, and the increase at Ofu and Olosega, are congruent with the changes 

indicated by towed-diver surveys over the same time period and are, therefore, more likely 

to represent an accurate trend. While the declines in coral cover around Tutuila and Rose 

appear to be substantial (13.6% and 9.1%, respectively), these data must also be interpreted 

with caution, as different methods of estimating coral cover were used in 2004 and 2006 

(visual estimates and line intercept method, respectively), and estimates derived from the 

two methods can vary substantially. The line point intercept method will be retained on 

future surveys to facilitate comparability of data over time.

During both 2004 and 2006 surveys, the total number of coral genera recorded for all REA 

sites at each island showed the same pattern; the highest number of coral genera was found 

around Tutuila, followed by Ofu and Olosega, Ta`u, Rose, and Swains (Fig. 8.5.1b). Summing 

observations over both years, at least 37 scleractinian genera were observed around Tutuila, 

34 around Ofu and Olosega, 34 around Ta`u, 23 around Rose, and 18 around Swains (Table 
ARCHIPELAGIC COMPARISONS

450

Figure 8.5.1b. Number of coral genera recorded at all REA sites during ASRAMP 2004 and 2006 for Tutuila 

and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS) and Swains (SWA). 

Figure 8.5.1c. Mean coral generic richness per site for REA surveys conducted during ASRAMP 2004 and 

2006 around Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS) and Swains 

(SWA).
ARCHIPELAGIC COMPARISONS

451

8.5.1b). This trend is in agreement with the theory of Island Biogeography; the larger the 

area, the greater number of species occur. Although the relative proportions of the number 

of coral genera are not the same as the relative proportions of banktop area among islands 

(Fig. 8.1b), the same trend holds from “most” to “least.” However, the evenness with which 

these genera are distributed at REA sites around the islands varied slightly from this pattern 

(Fig. 8.5.1c). Although Ofu and Olosega had fewer genera than Tutuila (Fig. 8.5.1b), a higher 

number of genera per site were observed during both 2004 and 2006 around Ofu and Olosega 

than around Tutuila, indicating greater generic evenness around Ofu and Olosega than around 

Tutuila.

Using the number of colonies enumerated at REA sites as a metric, some taxa were ubiquitous and abundant (e.g., Montipora, Pocillopora, Porites), some were ubiquitous but uncommon (e.g., Acanthastrea, Leptoseris/Pachyseris, Psammocora), and others ranged from relatively common (e.g., Astreopora, Favia, Leptastrea) to rare (e.g., Gardineroseris, Mycedium, 

Plesiastrea, Stylocoeniella, Tubastrea) at the islands where they occurred (Table 8.5.1b). In 

Table 8.5.1b, the yellow boxes indicate the absence of observed colonies and provide a visual 

aide for island-wide variability in generic diversity and abundance. Figure 8.5.1d shows in 

graphic form the relative abundance of primary coral genera during REA surveys during both 

2004 and 2006. With the exception of Acropora, all the genera shown represent > 10% of 

colony abundance in at least one of the islands in American Samoa (Fig. 8.5.1d). 

TUT 04 TUT 06 OFU 04 OFU 06 TAU 04 TAU 06 ROS 04 ROS 06 SWA 04 SWA 06

10 4 14 10 9 12 16 1

414 191 67 98 187 207 179 157 3

172 65 169 162 364 489 196 120

53 23 7 3 11 3 15 20 11

15 5 10 16 6 22 45 38

8 2 0 4 5

24 14 10 5 4 2 3

95 29 13 14 6 5 1 1

144 64 271 108 344 76 499 287 19 8

44 29 61 190 105 304 7 1

141 107 31 63 6 22 40 30 61 6

194 115 285 215 79 112 1

1 8 3

70 19 509 581 183 320 3 3 1

27 26 6 2 1 2 2

2 3 5 8 2

68 36 13 16 5 292 5 6 2

452 133 145 188 138 1 43 25

92 31 6 7 2 8 30 15 38 12

186 27 6 10 2 8 7 2

23 2 8 5 2

29 8 23 7 1 2 1

584 337 153 289 102 218 711 514

2123 569 832 1026 627 547 631 367 1037 1642

2 2 2 5 8

23 3 16 26

90 7 7 4 11 26 1 1

804 412 76 190 40 116 148 121 19 40

65 30 257 186 91 108 2 1

1

461 305 306 451 164 203 1402 1375 1527 859

857 366 462 208 300 258 314 283 138 97

82 43 23 9 1 6 41 22 53 8

6 3 2 5 7 2

854 392 228 60 38 56 368 312

5 16

1 1 10

4 2 154 47

1

1 12 7 44 16 4 1

Taxa

Stylocoeniella

Stylophora

Tubastrea

Turbinaria

Sandalolitha/Halomitra/Herpolitha

Sinularia/Lobophytum/ 

Sarcophyton/Cladiella

Stylaster/Distichopora

Plesiastrea

Pocillopora

Porites

Psammocora 

Oulophyllia

Palythoa/Zoanthus

Pavona

Platygyra/Leptoria

Millepora

Montastrea

Montipora

Mycedium

Leptastrea

Leptoseris/Pachyseris

Lobophyllia/Symphyllia

Merulina/Scapophyllia

Goniastrea

GonioporaAlveopora

Heliopora

Hydnophora

Favites

Fungia/Cycloseris

Galaxea

Gardineroseris

Diploastrea

Echinophyllia

Favia

Echinopora

Acropora

Astreopora

Coscinaraea

Cyphastrea

Acanthastrea

Table 8.5.1b. Number of colonies of each coral genus (or group of similar genera) enumerated during 2004 and 

2006 REA surveys.Yellow boxes indicate no colonies of these taxa were observed.
ARCHIPELAGIC COMPARISONS

452

Montipora numerically dominated all locations with the exception of Rose (both years) and 

Swains (2004), where Pocillopora dominated. The rank order of generic abundance thereafter 

varies among islands, though generally Porites, Pocillopora, Astreopora, or Goniastrea are 

the next most abundant genera.

Figure 8.5.1e shows island-wide mean coral colony density per site at each island/atoll from 

Figure 8.5.1d. Relative abundance of coral genera from ASRAMP 2004 and 2006 REA surveys around Tutuila 

and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS), and Swains (SWA).
ARCHIPELAGIC COMPARISONS

453

REA surveys during ASRAMP 2004 and 2006. Mean colony densities were greatest around 

Ofu and Olesega (~ 9 colonies m-2) and Ta`u (~ 7.5 colonies m-2) and least around Tutuila 

(~ 5 colonies m-2). Mean colony densities around both Rose and Ta`u appeared to increase 

somewhat between survey years. Increased density can result from larval recruitment, 

fragmentation, and fissioning (partial mortality). However, it should be noted that site ROS23 was not surveyed in 2006 because of strong current, and that transect deployment at 

TAU-10 was shifted in 2006 relative to 2004; 2004 transects were occasionally deployed 

over habitats with partial composition of sand, whereas 2006 transects were more strictly 

deployed over hard substrates. These between-year variations in the assemblage of sites or 

substrate surveyed may contribute to the apparent difference in mean coral densities observed 

around Rose and Ta`u.

Figure 8.5.1e also shows colony size distributions around each island/atoll from REA surveys 

during ASRAMP 2004 and 2006. Colonies around Tutuila and Swains appear to have 

undergone a general shift towards smaller size classes. However, as discussed elsewhere 

in the individual chapter texts, field judgments regarding the boundaries of individual 

colonies are prone to variability among observers because of the highly clonal nature of 

most coral taxa. In this vein, it is noteworthy that Tutuila and Swains were surveyed by two 

different coral biologists in 2004 and 2006, whereas one of the coral biologists conducting 

2004 surveys also collected the majority of 2006 size class data at Ofu, Olosega, Ta`u, and 

Rose. Caution must therefore be exercised when examining datasets collected by different 

observers around Tutuila and Swains. At Ofu, Olosega, Ta`u, and Rose, where the same 

observer recorded most of the size data in both years, size class distributions did not change 

substantially between survey years.
ARCHIPELAGIC COMPARISONS

454

Figure 8.5.1e. Coral colony size class distributions and densities from ASRAMP 2004 and 2006 REA surveys 

around Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS), and Swains 

(SWA). 
ARCHIPELAGIC COMPARISONS

455

8.5.2 Coral Disease Surveys

Sixty-two coral disease surveys, totaling 25 375 m2 were conducted around Tutuila, Ofu, 

Olosega, Ta`u, Swains, and Rose. Within this area, an estimated total of 192 850 colonies 

were examined. Except for two lagoon surveys conducted at Rose, all belt transects were 

placed along forereef communities at depths ranging from 10 to 15 m. Within this context, 

a total of 352 individual cases of coral bleaching and disease, in addition to 217 cases of 

Acanthaster and Drupella predation, were enumerated. Disease conditions occurred at 

all islands, but not at all sites. Sixty-three percent of the sites surveyed exhibited disease, 

with Rose, Ta`u and Swains reporting cases at 81%, 88%, and 100% of the sites surveyed, 

respectively. Comparatively, only 55% of sites surveyed around Tutuila, Ofu, and Olosega 

exhibited the conditions mentioned above.

During the ASRAMP 2006 surveys, at least four disease states (i.e., bleaching, growth 

anomalies, band disease, and ‘other lesions’) with the potential for more were observed on 

corals in American Samoa. This is low compared to the nearly 30 diseases states reported 

for the Caribbean and western Atlantic (Green and Bruckner, 2000; Weil, 2004). Our surveys 

also revealed that the mean disease prevalence for all 62 sites combined was 0.34% (SE 

0.1). This number is at least one order of magnitude lower than those reported for either the 

Caribbean or the Great Barrier Reef (Weil, 2004; Willis, 2004). Distribution and overall mean 

prevalence of disease varied greatly. Figure 8.5.2a shows overall prevalence of disease and 

predation; predation was highest at Swains, but overall disease prevalence was the lowest, 

0.04% (SE 0.03), for all islands surveyed. The highest overall disease prevalence, 0.96% (SE 

0.6), was recorded at Rose. At Tutuila, Ofu and Olosega, and Ta`u, disease prevalence values 

were moderately low: 0.12% (SE 0.04), 0.05% (SE 0.03), and 0.1% (SE 0.03), respectively 

(Fig. 8.5.2a; Vargas-Ángel et al., in prep.).

Of the ~ 45 different scleractinian coral genera enumerated during the ASRAMP REA surveys, 15 exhibited signs of disease and predation. Figure 8.5.2b illustrates the percentage of 

coral genera affected by disease and predation derived from the 2006 disease survey data. 

The greatest number of afflicted genera (9) was detected around Ta`u, Ofu, and Olosega, 

contrasted by only 6 at Swains. Around Tutuila, 7 different coral genera exhibited disease 

and predation, compared to 8 at Rose. Figure 8.5.2b also shows that only a limited number of 

coral genera were disproportionately affected by disease and predation at the study locations. 

These patterns may be dictated by host/prey and predator abundance, as well as modes of disease transmission, pathogen life history and virulence. In the American Samoa Archipelago, 

the rank order of coral generic propensity to disease and predation appears to be Montipora, 

Pocillopora, and Porites. With respect to other taxa, the rank order varies among islands, 

although Acropora, Astreopora, and Montastrea appear to be the next more commonly afflicted coral genera. Trends like these suggest that the ecological impacts of disease and 

predation may be more severe in populations of infrequent or rare coral taxa.

Of the 352 cases of disease itemized in this study, the category ‘other lesions’ represented 

75.3%. Around Rose alone, 98% of lesions involved this condition affected, in decreasing 

order of importance, corals in the genera Favia, Montastrea, and Porites. Comparatively, ‘other 

lesions’ represented only 30% of cases for Tutuila and Ta`u combined. ‘Other lesions’ were not 

encountered at any of the sites surveyed around Ofu and Olosega. Lesions involving skeletal 
ARCHIPELAGIC COMPARISONS

456

growth anomalies represented 13.3% of all disease cases. Skeletal growth anomalies were 

also detected around all islands, but affected predominantly corals in the genera Acropora and 

Astreopora. Within colonies of Acropora, the greatest mean occurrence of growth anomalies 

was recorded around Ofu and Olosega, particularly on specimens of A. abrontanoides and A. 

cytherea. Coral bleaching was also detected around all islands; bleaching represented 9.4% 

of all disease cases and was most commonly observed around Ofu and Olosega. All lesions 

were small and focal, and severity was mild. Bleaching occurred most commonly on Porites, 

Montastrea, and Montipora. Lesions involving tissue loss represented 1.7% of all disease 

cases; a total of 6 instances were enumerated for all islands combined. Acroporid acute 

tissue loss (a.k.a. white syndrome) was registered at Tutuila and Rose only. Finally, black 

band disease was the least common type of lesion, with 1 case detected among the 62 sites 

surveyed; estimated prevalence was 0.02% (Vargas-Ángel et al., in prep.). Acanthaster and 

Drupella predation were also ubiquitous, with Rose exhibiting 58% of cases and the majority 

of these occurred on colonies of Pocillopora. Acanthaster and Drupella predation were also 

common around Swains, where 34% of cases were encountered mainly on plate-like colonies 

of Montipora and Pocillopora. The remaining cases were observed around Tutuila, Ofu, and 

Olosega. No Acanthaster and Drupella predation were observed around Ta`u (Vargas-Angel 

et al., in prep.).

Disease prevalence and predation, together with other coral reef ecosystem bio-indicators, 

such as changes in live coral percent cover, diversity, size class distribution, and changes 

in abundance and diversity of algae, fish density and biomass, and key macroinvertebrate 

assemblages, may provide proxies to ecosystem integrity and health. Although it appears 

Figure 8.5.2a. Mean overall prevalence of coral disease and predation determined from REA surveys during 

ASRAMP 2006 around Tutuila and Aunu`u (TUT), Ta`u (TAU), Ofu and Olosega (OFU), Rose (ROS), and 

Swains (SWA). PRE = Acanthaster and/or Drupella predation; OT = other lesions, including hyperpigmented 

irritations and discolorations other than bleaching; BBD = black band disease; TL = tissue loss; GA = skeletal 

growth anomalies; and BL = bleaching.
ARCHIPELAGIC COMPARISONS

457

that mean overall disease prevalence and predation were low in the American Samoa Archipelago, until extensive surveys are conducted and more is known about the etiology of 

Indo-Pacific coral diseases, it is difficult to compare prevalence levels in American Samoa 

with other parts of the Indo-Pacific. 

Disease and predator outbreaks are among several other recurrent natural disturbances 

of corals including episodic sea warming and storms. Coral disease may be exacerbated 

by human activities, which have been increasing in Indo-Pacific coral reef ecosystems, 

including those in the American Samoa Archipelago (Craig et al., 2005). At Tutuila, for 

example, deforestation and urban development along the south-facing shores, in concert with 

increased levels of rainfall, have the potential for adversely affecting coastal water quality 

and therefore the well being of nearshore coral communities. In addition to detrimental direct 

effects of siltation stress on coral reefs (see review by Rogers, 1990), equally important is the 

potential for increased occurrence and prevalence of coral disease, such as those observed at 

Rose. ‘Other lesions’, particularly hyperpigmented irritations with algal overgrowth, reached 

the greatest levels of prevalence for any disease (4.9%) at site ROS-23. It has been proposed 

that the iron leaching from the corroding metal debris scattered during the 1993 Jin Shiang 

Fa shipwreck is likely responsible for stimulating and maintaining elevated cover of benthic 

turf algae/cyanobacteria at the wreck and neighboring sites at Rose (Green et al., 1997; 

Schroeder et al., in prep.). Little is known about the secondary effects of this disturbance on 

adjacent and neighboring sites. However, it is plausible that the conditions mentioned above 

are connected to the prominent number of cases of coral irritations and algal overgrowth 

enumerated at Rose. Further research on this topic is needed.

Figure 8.5.2b. Percent scleractinian coral taxa affected by disease and predation, based on the ASRAMP 2006 

REA surveys around Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u (TAU), Rose (ROS), and 

Swains (SWA).
ARCHIPELAGIC COMPARISONS

458

8.6 Algae

8.6.1 Algal Surveys

During the ASRAMP 2002 and 2004 survey periods, the towed-diver benthic survey protocol aggregated observations of turf and macroalgae into a single category. In contrast during 

ASRAMP 2006, macroalgae were recorded as a distinct category and turf algae were not 

recorded. Therefore, it is not surprising to see a two- to threefold decrease in algal cover 

around Tutuila, the Manu`a Islands, and Rose in 2006 over what was recorded in previous 

years (Fig. 8.6.1a). 

This does not indicate that macroalgal cover decreased across American Samoa from 2002 

to 2006. In fact, an increase in macroalgal cover is evident around Swains (Fig. 8.6.1a) despite the change in protocol to no longer include turf algae. The percent cover of macroalgae 

or macroalgae and turf algae was generally similar among all islands in all years (15–30% 

cover when turf algae and macroalgae were combined; typically < 10% cover when macroalgae were considered alone); the main exceptions being a higher turf and macroalgae 

cover around Rose in 2002 (~ 40%) and a higher macroalgae cover around Swains in 2006 

(~ 40%) than at other islands. The reason for the observed high mean macroalgae percent 

cover around Swains in 2006 is not clear, although it could reflect the perturbations to the 

ecosystem at Swains caused by both coral breakage associated with tropical cyclones Heta 

(observed) in 2004 and Percy and Olaf in 2005, and the ongoing increase of crown-of-thorns 

seastar (COTS) populations observed around Swains in both 2004 and 2006.

Figure 8.6.1a. Percent cover of algal functional groups from towed-diver surveys in the American Samoa Archipelago during ASRAMP 2002, 2004 and 2006 for Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), 

Ta`u (TAU), Rose (ROS forereef only) and Swains (SWA). Note that lagoon data are not presented for Rose; 

only forereef data are presented for better comparison with the other islands that lack a lagoon. 

During ASRAMP 2002, the Manu`a Islands (Fig. 8.6.1a) appeared to have a substantially 

higher crustose coralline algae percent cover (> 30%) than Tutuila, Rose, or Swains (all < 

20%). During ASRAMP 2004, crustose coralline algal cover appeared similar at all islands 

and ranged from ~ 25% to 45%, with Rose and Ofu and Olosega both having observed values 

of ~ 45%. During ASRAMP 2006, the crustose coralline algae percent cover around Rose 
ARCHIPELAGIC COMPARISONS

459

was almost 55%, while other islands ranged from 15% to 45%, with Ofu and Olosega again 

having ~ 45% cover. Differences among years may be attributed to other benthic organisms 

covering crustose coralline algae (e.g., macroalgal overgrowth), thereby influencing diver 

observations.

During ASRAMP 2004, 13 genera of green algae, 27 genera of red algae, 3 genera of brown 

algae, and 4 algal functional groups were reported from REA sites surveyed across American 

Samoa (Table 8.6.1a). The number of genera found at each site will increase as detailed 

taxonomic laboratory analyses at the species level are completed. During 2006, 17 genera 

of green algae, 32 genera of red algae, 2 genera of brown algae, and 4 algal functional 

groups were reported. The theory of Island Biogeography states that larger islands will 

contain greater biodiversity because they generally exhibit a greater number of habitat types. 

Table 8.6.1a reveals that field surveys around Tutuila, with the largest area of habitat and 

number of habitat types in American Samoa, documented substantially more algal genera 

or functional groups than other islands surveyed during both years of field sampling. The 

three geographically close, high, volcanically derived Manu`a Islands that are situated east 

of Tutuila exhibited the second highest number of recorded algal genera. The two smallest 

islands in American Samoa, Rose and Swains (both low, oceanic atoll systems), contained 

the least amount of algal diversity. Swains had less than half the number of algal genera as 

Tutuila, which is consistent with the small habitat area and number of habitat types.
ARCHIPELAGIC COMPARISONS

460

Table 8.6.1a. Number of green, red, and brown macroalgal genera and functional groups (non-geniculate 

branched red coralline algae, crustose coralline red algae, turf algae, cyanophytes) reported during in situ 

surveys around each island or island group in American Samoa during ASRAMP 2004 and 2006 cruises. The 

number of genera found at each site will increase as detailed taxonomic laboratory analyses are completed. Xs 

represent genera or functional groups found during 2004, Ys represent genera or functional groups found during 2006.

Manu`a 

Islands Rose Swains Tutuila

Total # Green Macroalgal Genera

6 – 2004

12 – 2006

12 – total

6 – 2004

12 – 2006

12 – total

5 – 2004

8 – 2006

8 – total

11 – 2004

12 – 2006

14 – total

Boergesenia X

Boodlea XY Y XY

Bornetella Y

Bryopsis Y XY XY

Caulerpa Y XY XY XY

Chaetomorpha Y Y

Chlorodesmis XY Y XY

Cladophora Y Y Y

Codium Y Y

Dictyosphaeria XY XY XY XY

Halimeda XY XY XY XY

Microdictyon XY XY

Neomeris XY Y XY

Rhipilia XY

Tydemania XY X

Udotea Y Y Y

Valonia Y XY XY

Ventricaria Y XY

Total # Red Macroalgal Genera

10 – 2004

24 – 2006

25 – total

4 – 2004

13 – 2006

14 – total

4 – 2004

6 – 2006

7 – total

24 – 2004

28 – 2006

32 – total

Actinotrichia XY Y XY

Acrosymphyton X

Amansia Y

Amphiroa XY XY XY

Botryocladia Y

Callophycus X

Carpopeltis XY

Champia Y Y Y

Cheilosporum Y Y XY

Chrysymenia Y XY

Corynocystis XY

Dasya Y Y Y Y

Dudresnaya X

Galaxaura XY XY XY XY

gelid XY Y XY

Gelidiopsis Y Y Y XY

Gibsmithia Y XY

Gracilaria Y

Grateloupia Y Y Y

Griffithsia Y Y

Haloplegma XY XY
ARCHIPELAGIC COMPARISONS

461

Halichyrsis Y Y

Halymenia XY

Hypoglossum Y Y

Jania Y Y XY XY

Laurencia/Chondrophycus XY X Y XY

Liagora X XY

Lithanophora Y

Martensia Y Y XY

Peyssonnelia XY XY XY XY

Portieria XY XY

Predaea XY

Titanophora Y XY

Trichogloea X

Tricleocarpa XY

Wrangelia Y X

Total # Brown Macroalgal Genera

1 – 2004

2 – 2006

2 – total

3 – 2004

2 – 2006

3 – total

2 – 2004

2 – 2006

2 – total

2 – 2004

2 – 2006

2 – total 

Dictyota Y XY XY XY

Lobophora XY XY XY XY

Stypopodium X

Total # Algal Functional Groups

3 – 2004

4 – 2006

4 – total

3 – 2004

4 – 2006

4 – total

4 – 2004

4 – 2006

4 – total

4 – 2004

4 – 2006

4 – total

non-geniculate branched coralline red algae Y X XY XY

crustose coralline red algae XY XY XY XY

turf algae XY XY XY XY

cyanophytes XY XY XY XY

Total # of macroalgal genera or functional groups 

documented 43 33 21 52

To gain a sense of which algae were the most common at each island, the percentage of quadrats in which a genus or functional group occurred at a given island within each sampling 

period was calculated (Table 8.6.1b). These were then added together, and the contribution 

of each single genus or functional group divided by the total to determine the relative proportion of occurrence for each genus or functional group at each island (Fig. 8.61d). If algal 

species composition at a single island begins to change temporally, the relative proportion of 

occurrence of algal genera or functional groups will change.
ARCHIPELAGIC COMPARISONS

462

Table 8.6.1b. Percentage of quadrats in which select macroalgal genera or functional groups occurred at each 

island.Data without parentheses represent 2004, data within parentheses represent 2006. *The person recording 

data had difficulty identifying algal genera at Rose in 2006, so these numbers may be incorrect.

Ofu Olosega Ta`u Tutuila Rose Swains

Green Macroalgal Genera

Dictyosphaeria 11.1 

(6.7)

4.2 

(1.7)

4.6 

(28.1)

5.3 

(10.8)

16.7 

(40.8*)

13.5 

(19.8)

Halimeda 22.2 

(33.3)

20.8 

(10.0)

11.1 

(19.8)

29.6 

(38.8)

37.1 

(36.7*)

1.0 

(0)

Microdictyon 0 0 0 0 31.9 

(91.7*)

58.3 

(51.6)

Rhipilia 0 0 0 0 0 74.0 

(74.0)

Red Macroalgal Genera

Peyssonnelia 47.2 

(83.3)

10.4 

(58.3)

3.7 

(79.2)

45.8 

(43.0)

2.1 

(74.2*)

13.5 

(20.8)

Brown Macroalgal Genera

Lobophora 30.6 

(0)

33.3 

(0)

10.2 

(12.5)

10.2 

(6.3)

38.2 

(74.2*)

6.3 

(16.7)

Algal Functional Groups

turf algae 97.2 

(96.7)

83.3 

(98.3)

91.7 

(95.8)

93.9 

(92.5)

83.0 

(94.2)

85.4 

(55.2)

crustose coralline red algae 97.2 

(95.0)

100.00 

(93.3)

73.2 

(88.5)

76.1 

(82.1)

86.1 

(95.0)

93.8 

(76.0)

non-geniculate branched 

coralline red algae

0 

(3.3) 0 0 1.5 (1.3)

45.8 

(0*)

32.3 

(0)

cyanophytes 30.6 

(86.7)

54.2 

(55.0)

14.8 

(58.3)

24.2 

(12.5)

62.5 

(93.3*)

46.9 

(17.7)
ARCHIPELAGIC COMPARISONS

463

Figure 8.6.1b. Percent occurrence of select algal genera and functional groups at REA sites around American 

Samoa during ASRAMP 2004 and 2006 for Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), Ta`u 

(TAU), Rose (ROS) and Swains (SWA). Note that lagoon data are not presented for Rose; only forereef data 

are presented for better comparison with the other islands that lack a lagoon. 

Figure 8.6.1b clearly shows how algal composition at the various islands compare within a 

given year. Table 8.6.1b lists the percentage of quadrats in which select macroalgal genera or 

functional groups occurred around each island. It is clear that the algal functional groups of 

turf algae and crustose coralline red algae were the most commonly encountered algae in almost every quadrat across all islands during both years sampled. This is not surprising since 

dense turf algal communities rapidly colonize available substrate. Crustose coralline red algae have also been found to be an early colonizer with rapid rates of growth. Crustose red 

algae play a critically important role in reef ecosystems by cementing together loose rubble 

and serving as a suitable settling surface for invertebrate (especially coral) larvae.

Some macroalgal genera reveal distinct differences among islands and may be useful in 

terms of spatially describing differences in reefs across American Samoa. Swains was the 

only ecosystem reported to contain the green alga, Rhipilia, where it was very common. 

Similarly, both Rose and Swains, two island systems remote from the rest of American Samoa, supported communities of the green alga, Microdictyon, and non-geniculate branched 

coralline red algae. Because these algae were either not found or else scarce on larger, high 
ARCHIPELAGIC COMPARISONS

464

islands, these algal distributional patterns may suggest oceanographic or geomorphic differences between islands. Other macroalgal genera, such as the green alga, Dictyosphaeria, 

appear ubiquitous across islands, appearing with similar frequency everywhere. Such genera 

might be useful indicators of reef health, signaling a decline in health if their presence is no 

longer recorded or if blooms occur.

Temporal comparison of 10 common algal genera or functional groups found at islands in 

American Samoa (Fig. 8.6.1b) reveal genera such as the green alga, Dictyosphaeria, to be 

a common component of reef systems across the American Samoa Archipelago, although 

in relatively low density (Fig. 8.6.1b). Other genera such as the green alga Halimeda (Fig. 

8.6.1b) are fairly prevalent on some islands (Tutuila, Manu`a, and Rose), but less common on 

others (Swains). Finally, other genera are only found in specific locations (e.g., Rhipilia orientalis only occurs at Swains, Microdictyon [Fig. 8.6.1b] only occurs on Rose and Swains). 

Such island differences may be useful in helping to define attributes of a healthy reef system. For instance, if the ubiquitous alga, Dictyosphaeria, suddenly disappears from certain 

reef settings, this may indicate a decrease in reef health. Alternatively, if an alga that is only 

known from one island suddenly appears on geographically distant islands, it may indicate a 

human introduction that will necessitate monitoring to ensure than it does not become invasive in its nonnative environment.
ARCHIPELAGIC COMPARISONS

465

8.6.2 Coralline Algal Disease Surveys

Surveys during ASRAMP 2006 enumerated a total of 504 cases of coralline algal disease in 

the American Samoa Archipelago (Vargas-Ángel et al., in prep.); this is nearly 45% higher 

than the incidence of coral diseases. Of this number, nearly 94% of cases occurred around 

Tutuila, Ofu, and Olosega and 6% around Ta`u and Rose; no cases were reported for Swains. 

Of the 504 cases, 92% corresponded to coralline lethal orange disease (CLOD). Cases of 

CLOD were highly abundant around Tutuila and around the more remote Ofu and Olosega 

(Fig. 8.6.2a). Around Tutuila, CLOD occurred preponderantly along the south-facing shores, 

in both relatively pristine as well as impacted locales, such as Fagatele Bay (TUT-21) and 

the area south of the airport (TUT-09), respectively. Comparatively, around Ofu and Olesega, 

CLOD was ubiquitous, occurring at 11 out of 12 survey sites. For both islands, CLOD was 

distributed in an aggregated pattern, suggesting that this disease pathogen(s) may be readily 

transmitted by organisms such as grazers. CLOD is caused by a bacterium that moves in a 

band, similar to the black band disease of corals (Littler and Littler, 2003) and leaving behind 

the dead algal carbonate structure.

Coralline fungal disease and other coralline discolorations were also present in the American 

Samoa Archipelago, but in low abundance (Fig. 8.6.2a). Because these pathogens can result 

in elimination/reduction in coralline algal cover with subsequent replacement by mats of

cyanobacteria or turf algae, CLOD, coralline fungal disease, and other coralline diseases can 

adversely affect successful coral larval recruitment (Vargas-Ángel et al., in prep.). Further 

research is indicated in this matter.

Figure 8.6.2a. Relative abundance (# cases m-2) of coralline algal disease from ASRAMP 2006 REA surveys 

for Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU), Ta`u (TAU), Rose (ROS) and Swains (SWA).CLOD 

= coralline lethal orange disease; CFD = coralline fungal disease; Other = undetermined coralline discolorations.
ARCHIPELAGIC COMPARISONS

466

8.7 Benthic Macroinvertebrates

Figure 8.7a. Island-wide mean densities of key macroinvertebrates from towed-diver surveys around American 

Samoa during ASRAMP 2002, 2004 and 2006 for Tutuila and Aunu`u (TUT), Ofu and Olosega (OFU/OLO), 

Ta`u (TAU), Rose (ROS) and Swains (SWA). *Only forereef communities at Rose were included in this analysis for continuity across islands.See Chapter 6, Section 6.6: Benthic Macroinvertebrates, for a discussion on the 

unique lagoonal communities at Rose. Note that lagoon data are not presented for Rose; only forereef data are 

presented for better comparison with the other islands that lack a lagoon. 

Island-wide mean densities (organisms ha-1) of key macroinvertebrates, including COTS, 

giant clams, sea cucumbers, and sea urchins around the islands and atolls of American Samoa 

during ASRAMP 2002, 2004, and 2006 cruises are shown in Figure 8.7a. Archipelago-wide 

densities of COTS were extremely low, with the exception of large and increasing numbers of 

COTS observed around Swains in successive years where a 300% increase in mean density 

of COTS was observed between 2002 and 2004, and a nearly 500% increase between 2002 

and 2006 (50% between 2004 and 2006). The spatial distribution of COTS was variable 

between years. In 2004, COTS were more abundant along the southern tip of Swains, while 

in 2006 they were more abundant on the southeastern, western, and eastern shores and 

relatively uncommon in the north. Cyclone Heta, nearly a category 5 typhoon, passed to the 

west of Swains (traveling south to north) in January 2004, shortly before ASRAMP 2004, 

generating high winds and damaging swells. REA sites along the northern and northeastern 

shore displayed the greatest degree of coral breakage and dispersal, and towed-diver surveys 

around Swains recorded high values of stressed coral (7.8%). This may have corresponded 

with the overall island-wide increase in COTS between 2002 and 2004. An additional 

category 3 tropical cyclone (Percy), which produced 6-m wave heights, passed within 50 

km of Swains in 2005; however, lower coral stress values were recorded in 2006 (1.9%) in 

comparison with 2004. The effects of such disturbances on COTS population dynamics are 

poorly understood and the topic needs further study.
ARCHIPELAGIC COMPARISONS

467

Only 3 COTS were observed around Tutuila during all 3 survey years; around Ofu and 

Olosega, 5 were recorded in 2002 with no subsequent sightings in 2004 and 2006, and 7 

were recorded in a localized area inside the lagoon at Rose in 2004. There were no sightings 

of COTS around Ta`u during any of the 3 survey years. 

Island-wide mean densities of sea urchins for each of the islands and atolls of American Samoa 

displayed a significant degree of spatial and temporal variation, which could be attributed 

to differences in survey attributes, sea urchin population dynamics, and other factors (Fig. 

8.7a). The highest numbers and densities of sea urchins were consistently reported around 

Tutuila, Ofu, Olosega, and Ta`u. Mean densities of sea urchins were consistently very low 

around Rose and Swains. 

Around Tutuila, higher densities of sea urchins were consistently reported in the northwest 

and north regions in 2002 and 2004. Densities were much lower in 2006 (Figs. 3.6a, 3.6b, 

and 3.6d). High densities were also reported along the southern coastline in 2004 and 2006, 

although surveys immediately to the east and west of Pago Pago Harbor entrance were 

surprisingly devoid of sea urchins over large areas of habitat. Mean density of sea urchins 

around Ofu and Olosega was relatively similar between years. Sea urchins were observed 

in high densities off the west coast of Nu`utele in 2004 and 2006. Other areas that recorded 

higher densities included the northernmost point of Olosega (2004/2006) and the southern 

points of both Ofu and Olosega (2004). Around Ta`u, a similar trend was noted with the 

highest densities being observed around the southwest and northwest points in 2004 and 

2006, with consistent, but lower, densities noted in the southeast and northeast corners in 

both years. Coastline surveys between those points generally encountered low densities of 

sea urchins. The numbers and densities of sea urchins around Rose and Swains were very 

low, with specimens observed only along the western, northern, and eastern points of the 

island at Rose and the west forereef of Swains. 

Island-wide mean densities of sea cucumbers around the islands and atolls of American Samoa were fairly consistent, with the highest numbers and densities reported around Ofu and 

Olosega in all survey years. Ta`u recorded the next highest numbers for all years surveyed, 

followed by Tutuila in 2004 and 2006. Few sea cucumbers were reported around Rose and 

Swains in all survey years.

Around Tutuila, Ofu, and Olosega, sea cucumbers were widely distributed, albeit at low 

densities. At Ta`u, sea cucumbers were also evenly distributed, although slightly higher 

densities were reported along the western and northern coasts.

Island-wide mean densities of giant clams were relatively consistent between islands, with 

Ta`u recording the highest mean densities of all islands in the American Samoa Archipelago 

in 2002, 2004, and 2006, followed by Ofu and Olosega and Tutuila in 2002 and 2006. In 2004, 

densities were slightly higher around Tutuila than Ofu and Olosega. At Rose and Swains, 

giant clams were recorded in low numbers along the forereef and fringing reef habitats. In the 

lagoon at Rose, on the other hand, giant clams were extremely dense in localized areas. 

Towed-diver and REA surveys around Tutuila found giant clams to be widely and evenly 

distributed in low densities. Giant clams were relatively common around Ofu and Olosega, 
ARCHIPELAGIC COMPARISONS

468

with no apparent skew towards any particular coastline or region. Around Ta`u, giant clams 

were generally present at all but one REA site and were evenly distributed around the island. 

High densities were reported within the inner lagoon of Rose Atoll, with over 1000 individuals 

noted during each survey year; the highest concentration was observed along the southern end 

of the lagoon (see Chapter 6, Section 6.6: Benthic Macroinvertebrates). The forereef surveys 

recorded few giant clams and at low mean densities. Giant clams were mostly distributed in 

the northwest on either side of the channel. Around Swains, REA survey records indicate that 

giant clams were evenly distributed in low numbers .

8.8 Reef Fish

Figure 8.8a shows island-wide mean fish biomass recorded during ASRAMP 2002, 2004, 

and 2006. Total fish biomass around the populated high islands of American Samoa (Tutuila 

and Aunu`u, Ofu and Olosega, and Ta`u) was stable in 2002 and 2004 at around 1.0 ton ha-1 

(SE 0.3), but fell to 0.5 ton ha-1 (SE 0.1) in 2006. Conversely, fish biomass around Rose went 

from 1.5 ton ha-1 (SE 0.8) in 2002 to 2.3 ton ha-1 (SE 1.2) in both 2004 and 2006. Future 

surveys will confirm whether these changes form clear trends or are simply within the range 

of natural population variability. The fish biomass around Swains was dominated by large 

transient schools and is thus harder to quantify with site-scale surveys. This is apparent in the 

highly variable interyear averages for total fish biomass around Swains (0.6 ton ha-1 (SE 0.3) 

in 2002, 2.9 ton ha-1 (SE 2.0) in 2004, and 0.7 ton ha-1 (SE 0.3) in 2006). It is clear, however, 

that Swains harbors a high fish biomass ( average 1.4 ton ha-1 (SE 0.8) over all survey years) 

which is comparable to Rose. This is confirmed by towed-diver surveys which show that 

Swains consistently has the highest biomass of large fish (> 50 cm) in the archipelago. When 

all years are averaged, the total fish biomasses around the two low islands, Swains (1.4 ton 

ha-1) and Rose (2.1 ton ha-1), was substantially higher than the mean for the high island 

group (~ 0.8 ton ha-1).

The fish communities around the entire archipelago were mainly composed of herbivores (> 

50% of total biomass), with the exception of Swains where herbivores represented only 10% 

of biomass. Nearly 60% of the biomass around Swains was composed of large schools of 

predators (barracudas, snappers, and jacks).

Similar to total fish biomass, large fish biomass (> 50 cm total length) was consistently 

low in the high island group for all years surveyed (0.03–0.04 ton ha-1 (SE 0.01)). The 

two remote islands, Rose and Swains, had clearly higher large fish biomasses, but observed 

values declined from 0.20 ton ha-1 (SE 0.09) in 2002 to 0.13 ton ha-1 (SE 0.08) in 2004, and 

0.10 ton ha-1 (SE 0.05) in 2006 (Fig. 8.8a). When all years were averaged, Rose and Swains 

had a fish biomass almost 4 times greater than that of the high islands (0.14 ton ha-1 vs. 0.04 

ton ha-1).

The total fish biomass values around the populated high islands (~ 0.8 ton ha-1; 50% 

herbivores) were comparable to values reported for the populated Main Hawaiian Islands 

(MHI; ~ 0.7 ton ha-1; 55% herbivores; Friedlander and DeMartini, 2002). Conversely, 

the biomass values for the two low islands (~ 1.7 ton ha-1) compare more closely to those 

reported in the uninhabited Northwestern Hawaiian Islands (NWHI; ~ 2.4 ton ha-1), with the 
ARCHIPELAGIC COMPARISONS

469

higher top predator biomass proportion found on Swains (60%) also matching the average 

value reported in the NWHI (55%; Friedlander and DeMartini, 2002). Similarly, the large 

fish biomass around the populated high island group in American Samoa (~ 0.037 ton ha1) was only slightly higher than the value for the MHI (~ 0.028 ton ha-1; Zgliczynski and 

Richards, unpubl. data). The large fish biomass around Rose and Swains (~ 0.143 ton ha1) was also closely comparable to the NWHI values (~ 0.126 ton ha-1; Zgliczynski and 

Richards, unpubl. data). However, the relatively high large fish biomass recorded on the two 

remote atolls is still low compared to the very high values encountered in the U.S. Pacific 

Figure 8.8a. Total (all families and sizes) and large fish (> 50 cm length) mean biomasses observed during ASRAMP 2002, 2004, and 2006. Total fish biomasses are divided into trophic groups, while large fish biomasses 

are divided into the 10 main families.
ARCHIPELAGIC COMPARISONS

470

Remote Island Areas, which are on the order of 0.700 ton ha-1 (Zgliczynski and Richards, 

unpubl. data).

Finally, reef fish diversity, measured as the number of species encountered per 100 m2, is 

a more variable measure than fish biomass, making comparisons between years less clear. 

When all years are combined, a pattern of diversity similar to that for biomass emerges 

where the two isolated atolls, Rose and Swains, have slightly higher fish diversity (36 and 32 

species 100 m-2, respectively) compared to the populated high islands of Ofu and Olosega 

(31 species 100 m-2), Ta`u (29 species 100 m-2), and Tutuila (28 species 100 m-2).

8.9 Archipelagic Summary

The archipelagic comparison of biotic and abiotic components of the coral reef ecosystems 

of American Samoa reveals diverse and complex spatial patterns coupling together human 

populations, biogeography, oceanographic, and environmental conditions. Not surprisingly, 

human population was an order of magnitude greater at Tutuila than at the other islands, 

presumably due to substantially greater land area, complex island topography which affords 

numerous separate watersheds and protected embayments, and relatively level coastal terrain 

to support more nearshore villages and higher population densities. Tutuila also has extensive 

shallow water coral reef habitats, including significant submerged relic reef structures located 

3 to 5 km offshore around most of the island. These areas likely provide more marine 

food sources than the other islands, which have comparatively limited shallow water benthic 

habitats. 

Human populations generally deceased eastward from Tutuila to Ofu, Olesega, Ta`u, and 

uninhabited Rose Atoll. Though Ta`u is substantially larger in total land area than Ofu and 

Olesega combined, the steep topography of the island probably limits the areas suitable for 

human habitation. North of these geologically-linked islands, Swains had a small caretaker 

population of 37 residents during the 2000 census, though there were less than 10 residents 

on the island during each of the three ASRAMP surveys in 2002, 2004, and 2006. Both 

Rose and Swains have small land and shallow habitat areas that are probably not capable of 

supporting large human populations. 

The high islands (Tutuila, Ofu, Olosega, and Ta`u) generally had slightly lower benthic habitat 

(topographic) complexity and significantly higher percent cover of sand habitats than the low 

islands (Rose and Swains), likely reflecting greater terrigenous inputs from the watershed 

drainages into the nearshore ecosystems. A suite of oceanographic observations indicated 

that Swains, which is located at 11° S, is situated in a different oceanographic environment 

that the other four islands located along 14°S, with warmer water, lighter winds, and higher 

chlorophyll a. Nutrient levels, on the other hand, were generally highest and most variable 

around Tutuila, most likely due to a combination of larger landmass of higher elevation 

producing more seasonal runoff and sedimentation in local watersheds, nutrient loading from 

agriculture, and contaminant runoff from urban areas

As a likely function of island biogeography, greater shallow water habitat area, and a more 

diverse range of habitat types and oceanographic conditions, diversity of coral and algae, 

as measured by mean coral richness, coral genera, and algal genera/functional groups, was 
ARCHIPELAGIC COMPARISONS

471

greater around the high islands than around the low islands. Swains was similar to Rose with 

smaller benthic habitat and land areas, lower numbers of coral genera and mean coral generic 

richness, and fewer algal genera and functional groups. While lower in coral diversity, the 

overall mean coral cover, as measured by both towed-diver surveys covering all habitats 

and REA surveys, were consistently highest around Swains, presumably due to the different 

oceanographic environment and lack of significant terrigenous inputs and sedimentation. 

Despite localized impacts around portions of the high islands, the mean overall prevalence of 

coral disease for all of the islands was an order of magnitude less than equivalent contemporary 

surveys have indicated for well surveyed sites in either the Caribbean or Great Barrier Reef 

ecosystems. Coral disease prevalence levels were generally very low (less than 0.5%), with 

the highest disease prevalence values observed around Rose. That said, a large proportion of 

these elevated prevalence values were the result of predation by Drupella snails. The second 

highest disease prevalence values were observed around Swains, of which about 85% was 

recorded as predation by elevated numbers of COTS. 

The presence and distribution of coralline algae disease across an archipelagic continuum 

(94% of all recorded cases for Tutuila, Ofu and Olosega, 6% for Ta`u and Rose, and 0% for 

Swains) indicates that there may be a direct or indirect link with higher human population 

numbers and densities, or that these diseases may be propagating naturally through localized 

grazer movements. 

Invertebrate surveys across the archipelago found extremely low densities of COTS around 

each of the islands except Swains, where high densities and associated coral predation were 

observed in 2004 and 2006. Sea urchins were observed in moderate densities around the high 

islands and in extremely low densities around the low islands. Mean densities of giant clams 

were consistently highest around Ta`u, although some extremely high giant clam densities 

were found at localized sites within the lagoon at Rose. 

Finally, total reef fish (all sizes) and large fish biomass (TL<50 cm), were lowest around the 

most populated islands (Tutuila, Ofu, Olosega, Ta`u) and highest around the least populated 

(Swains and Rose). This suggests that a combination of anthropogenic stressors, including 

but not limited to fishing, coastal development, sedimentation, and pollution are likely 

impacting reef fish populations. 
ARCHIPELAGIC COMPARISONS

472

References

Craig P, DiDonato G, Fenner D, Hawkins C (2005) The state of coral reef ecosystems of 

American Samoa. In: Waddell, J (ed), The state of the coral reef ecosystems of the United 

States and Pacific Freely Associated States: 2005. NOAA Tech. Memo. NOS NCCOS 11, 

NOAA NCCOS Center for Coastal Monitoring and Assessment’s Biogeography Team, 

Silver Spring, MD. p. 312–337.

Friedlander AM, DeMartini EE (2002) Contrasts in density, size, and biomass of reef fishes 

between the northwestern and the main Hawaiian Islands: the effects of fishing down 

apex predators. Mar Ecol Prog Ser 230:253–264.

Green E, Bruckner A (2000) The significance of coral disease epizootiology for coral reef 

conservation. Biol Conserv 96:347–361.

Green E, Burgett J, Molina M, Palawski D, Gabrielson P (1997) The impact of a ship grounding and associated fuel spill at Rose Atoll National Wildlife Refuge, American Samoa. 

U.S. Fish and Wildlife Service Report, Honolulu, Hawaii, 60 pp.

Hart SR, Coetzee M, Workman RK, Blustajn J, Johnson KTM, Sinton JM, Steinberger B, 

Hawkins JW (2004) Genesis of the Western Samoa seamount province: age, geochemical 

fingerprint and tectonics. Earth Planet Sc Lett 227:37–56.

Hart SR, Staudigel H, Koppers AAP, Blustajn J, Baker ET, Workman R, Jackson M, Hauri E, 

Kurz M, Sims K, Fornari D, Saal A, Lyons S (2000) Vailulu`u undersea volcano: the new 

Samoa. Geochem Geophys Geosyst GC0000108:1–13.

Littler DS, Littler MM (2003) South Pacific reef plants: a diver’s guide to the plant life of the 

South Pacific coral reefs. Off Shore Graphics Inc, Washington, p. 331.

McDougall I (1985) Age and evolution of the volcanoes of Tutuila, American Samoa. Pac 

Sci 39:311–320.

Rogers C (1990) Responses of coral reefs and reef organisms to sedimentation. Mar Ecol 

Prog Ser 62:185–202.

Ruddiman WF, Raymo ME, Martinson DG, Clement BM, Backman J (1989) Pleistocene 

evolution: northern hemisphere ice sheets and North Atlantic Ocean. Paleoceanography 

4:353–412.

Schroeder RE, Green AL, DeMartini EE, Kenyon JC (2008) Long-term effects of a shipgrounding on coral reef fish assemblages at Rose Atoll, American Samoa. BULL MAR 

SCI 82(3):345–364.

Vargas-Angel B, Kenyon JK, Fenner D, Brainard RE (In prep.) Rapid ecological assessment 

of coral disease on shallow coral reef habitats of the American Samoa archipelago: distribution and abundance. Available from Pacific Islands Fisheries Science Center Coral