PACIFIC ISLANDS FISHERIES 

SCIENCE CENTER

 CORAL REEF

ECOSYSTEM DIVISION

a 2003-2007 Overview
II
CORAL REEF

ECOSYSTEMS

OF THE MARIANA ARCHIPELAGO:

a 2003-2007 Overview
The PIFSC mission is to conduct high-quality, timely research to support the stewardship of fisheries resources, protected species, and 

ecosystems in the central and western Pacific Ocean.

Front cover: Thysanostoma jellyfish commonly found throughout the Pacific. NOAA photo by Kevin Lino

To obtain copies of this document, contact PIFSC:

Pacific Islands Fisheries Science Center

2570 Dole Street

Honolulu, HI 96822-2396

(808) 983-5303

An online version is available at http://www.pifsc.noaa.gov/cred.

Unless otherwise indicated, photographs in this publication are in the public domain.

This document may be referenced in the following manner:

Pacific Islands Fisheries Science Center (2010) Coral reef ecosystems of the Mariana Archipelago: a 2003–2007 overview. NOAA Pacific Islands 

Fisheries Science Center, PIFSC Special Publication, SP-10-002, 38 p.

Back cover: The cone-like shapes and colorful, twin, spiral gills of tube-dwelling worms burrowed into this Porites coral colony have earned them the common name, “Christmas tree 

worms.” NOAA photo
This booklet provides the draft executive summary and key findings 

from the Coral Reef Ecosystem Monitoring Report of the Mariana Archipelago: 2003–2007 (in review). The full report is a comprehensive analysis of the Pacific Reef Assessment and Monitoring Program research 

surveys conducted in the Mariana Archipelago in 2003, 2005, and 2007 

by the Coral Reef Ecosystem Division (CRED) of the NOAA Pacific Islands Fisheries Science Center (PIFSC) with support from NOAA’s Coral 

Reef Conservation Program.

TABLE OF CONTENTS

32 REMOTE REEF AREAS

34 REFERENCES

29 CORAL REEFS OF THE 

SOUTHERN ISLANDS

03 HISTORY & MISSION OF THE CRED

07 THE MARIANA ARCHIPELAGO

11 METHODS OVERVIEW

13 SOUTHERN VS. NORTHERN ISLANDS 

NAME CODES USED IN TABLE 1 & FIGS. 7, 9, 11, & 12

FDP FARALLON DE PAJAROS

SUP SUPPLY REEF

MAU MAUG

ASC ASUNCION

AGR AGRIHAN

PAG PAGAN

ALA ALAMAGAN

GUG GUGUAN

ZEA ZEALANDIA BANK

SAR SARIGAN

ANA ANATAHAN

SAI SAIPAN

TIN TINIAN

TAT TATSUMI REEF

AGU AGUIJAN

ROT ROTA

GUA GUAM

SRR SANTA ROSA REEF

STI STINGRAY SHOAL

PAT PATHFINDER REEF

ARA ARAKANE REEF
2

HISTORY & MISSION OF THE CRED
3

2003 2004 2005 2006 2007 2008 2009

1st CRED Mariana Archipelago 

RAMP (MARAMP) cruise

2nd MARAMP cruise

Included mapping

Included mapping

3rd MARAMP cruise

4th MARAMP cruise

*2009 data are not included in this document

Added water-quality sampling

Added coral-disease surveys

Added sampling for carbonate 

chemistry 

Improved sh surveys: increased

number, added stratied random

sampling design and expanded 

depth range to 0−30 m

Executive order to protect reefs

U.S. Coral Reef Task Force (CRTF)

is formed

CRTF creates National Action 

Plan to Conserve Coral Reefs

Coral Reef Conservation Act 

of 2000 (CRCA)

NOAA creates Coral Reef 

Conservation Program (CRCP)

NOAA PIFSC forms Coral Reef 

Ecosystem Division (CRED)

NOAA & CRTF produce National 

Coral Reef Action Strategy

1998 1999 2000 2001 2002

1st Pacic RAMP cruises

The mission of the Coral Reef Ecosystem Division is to provide high- TIMELINE

quality, scientific information about the status of coral reef ecosystems 

of the U.S. Pacific islands to the public, resource managers, and policymakers on local, regional, national, and international levels.

National coral reef conservation efforts were formalized in 1998 with a presidential executive order to “preserve and protect the biodiversity, health, heritage, and social and economic value of U.S. coral reef ecosystems and the marine environment.” This executive order established the U.S. Coral Reef Task Force (CRTF) 

and emphasized the need to undertake a comprehensive approach to research, mapping, and monitoring 

of all U.S. coral reef ecosystems. In 2000, the CRTF developed The National Action Plan to Conserve Coral 

Reefs (U.S. CRTF 2000), and the Coral Reef Conservation Act laid out a national framework to address the 

degradation of U.S. coral reef ecosystems and other coral reef conservation issues (Coral... 2000). The Coral 

Reef Conservation Act also led to the creation of the national Coral Reef Conservation Program under the direction of the Secretary of Commerce. This legislation requires NOAA to conduct scientific research, mitigation, and outreach activities that directly contribute to the conservation of coral reef ecosystems. NOAA, in 

cooperation with the CRTF, produced A National Coral Reef Action Strategy in 2002 with goals and objectives 

that included mapping, information management, research, and monitoring (NOAA 2002). In response to 

these mandates and with the support of NOAA’s Coral Reef Conservation Program, the NOAA Pacific Islands 

Fisheries Science Center initiated the Pacific Reef Assessment and Monitoring Program (Pacific RAMP) in 

early 2000 and established the Coral Reef Ecosystem Division (CRED) in 2001.

To fulfill its mission, the CRED conducts a comprehensive suite of integrated, interdisciplinary research 

activities, including habitat mapping, oceanographic studies, and long-term monitoring of multiple components of coral reef ecosystems in the U.S. Pacific islands (Fig. 1). The CRED conducts biennial Pacific RAMP 

Opposite page: Turbinaria coral polyps near the island of Asuncion. NOAA photo by Marie Ferguson
4

surveys in each study area to improve our understanding 

of ecosystem variability over climate time scales ranging 

from several years to decades and across broad spatial 

scales ranging from individual islands and atolls to entire archipelagoes and the Pacific Basin. Complementary 

methods are used to assess benthic community composition and abundance and diversity of invertebrates, algae, 

and fishes in the context of their benthic habitats and 

associated oceanographic and water-quality conditions. 

Using consistent methodologies across ~ 50 U.S. Pacific 

islands, atolls, and shallow banks enables unprecedented 

comparative analyses of coral reef ecosystems across diverse gradients of biogeography, environmental conditions, and human uses. Results from Pacific RAMP surveys 

significantly improve our understanding of ecosystem 

processes and cause-and-effect mechanisms that influence the status and resilience of coral reefs.

Pacific island communities are economically and culturally dependent on their marine resources. Accurate and up-to-date characterizations of coral reef ecosystems are necessary to develop and evaluate effective strategies for resource management. For a significant majority of the islands, atolls, and banks surveyed through the Pacific RAMP, 

virtually no prior ecological surveys, bathymetric or habitat maps, or in situ oceanographic observations had been completed. Little or no information was available about what to expect in terms of habitats, biogeographic structure, oceanographic conditions, or species composition, 

distribution, and abundance. In almost all regards, the initial surveys of the Pacific RAMP in 2000–2003 were exploratory baseline assessments 

with the purpose of shaping this monitoring program. The logistical and financial challenges presented by the remoteness of the U.S. Pacific 

islands and the vastness of this region as a whole had a major impact on the initial scope of this program and continue to shape its evolution. 

Figure 1. The CRED focuses on a study area that consists of ~ 50 islands, atolls, and shallow banks in 

American Samoa, the Hawaiian Archipelago, the Mariana Archipelago, and the Pacific Remote Island 

Areas. White areas represent U.S. Exclusive Economic Zones.

Opposite page: Two batfish (Platax tiera) near Farallon de Pajaros. NOAA photo by Chip Young

Main Hawaiian 

 Islands

Northwestern

 Hawaiian Islands Mariana

Archipelago

American 

 Samoa

Pacic Remote

 Island Areas

CRED STUDY AREA

Equator

U. S.

Australia

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5

“...preserve and protect the biodiversity, 

health, heritage, and social and economic 

value of U.S. coral reef ecosystems and 

the marine environment...”

—1998 executive order

With the exception of moored oceanographic and bioacoustic instruments that collect data nearly continuously, Pacific RAMP activities do not 

detect high-frequency (short-term) ecological fluctuations. Instead, biennial Pacific RAMP surveys are designed to examine ecological variability 

over a longer term of several years or decades, taking periodic “snapshots” of ecosystems when surveys are conducted. As such, many of these 

biennial ecosystem snapshots are needed before rigorous discussions about changes and trends become possible. 

Though changes in survey protocols inherently introduce some inconsistencies in data, the CRED has expanded or added some protocols in 

response to evolving management priorities and a continually improved understanding of factors that affect coral reef ecosystems, such as 

ocean acidification (see timeline on Page 3), and has continued to refine survey methods to better limit variability between survey staff members. The CRED conducted its last Pacific RAMP cruise in the Mariana Archipelago in 2009 on the NOAA Ship Hi`ialakai, and more information 

can be found in the cruise reports for this expedition at http://www.pifsc.noaa.gov/library/hiialakai.php. The next cruise in this archipelago is 

scheduled for 2011.
6

THE MARIANA ARCHIPELAGO
7

Opposite page: Since 2003, the CRED has mapped ~ 90% of the coral reef habitats across the Mariana Archipelago and has maintained an ongoing bathymetric data synthesis that 

combines CRED data with bathymetric data collected by other government and academic groups. Bathymetric and optical data are used for geologic studies, benthic habitat maps, 

tsunami inundation models, and nautical charts and are routinely provided to management partners in the Commonwealth of the Northern Mariana Islands and Guam to support 

improved implementation of ecosystem approaches to management of marine resources. 

The Mariana Archipelago is 890 km long and encompasses 15 islands, located on the 

Mariana Arc, and numerous offshore banks. The territory of Guam includes the island of 

Guam and adjacent offshore banks and reefs. All other islands and offshore banks compose the Commonwealth of the Northern Mariana Islands (CNMI). The islands and reefs 

of the Mariana Archipelago can be divided into 3 geologic groups (Figs. 2 and 3): (1) various offshore banks and submarine volcanoes located on the West Mariana Ridge; (2) the 

young, volcanic northern islands on the Mariana Arc, including, from south to north, Anatahan, Sarigan, Guguan, Alamagan, Pagan, Agrihan, Asuncion, Maug, and Farallon de 

Pajaros; and (3) the old, southern islands located on the Mariana Arc, including Guam, 

Rota, Aguijan, Tinian, Saipan, and Farallon de Medinilla. The 3 northernmost islands, the 

Mariana Trench, and several volcanic features between the Mariana Arc and Trough were 

designated as the Marianas Trench Marine National Monument by presidential proclamation in January 2009. 

GUAM

MAUG

MA

RIA

N

A

ARC

MARIA

N

A

T

ROUGH

MA

RIA

N

A

TRENCH

WEST

M

A

RIA

N

A

R

DI GE

MONUMENT BOUNDARY

0 100 km

Figure 2. Topography of the Mariana Archipelago and the location of the Marianas Trench Marine National Monument. 

This monument consists of the Mariana Trench, the 3 northernmost islands of the Mariana Arc, and several volcanic features (see red dots) between the Mariana Trough and Arc.

The islands of the CNMI and Guam differ in size, population, and isolation (Table 1 and 

Fig. 4). Guam is the largest island in the Mariana Archipelago with a land area of 544 km2. 

Saipan is the largest island in the CNMI with a land area of 119 km2, roughly similar to the 

combined area of 160 km2 for the 9 northern islands of the CNMI. Pagan and Agrihan (48 

and 44 km2) are the largest of the northern islands, the majority of which have land areas 

< 10 km2, very small relative to the southern islands. 

The combined population of Guam and the CNMI in 2009 was estimated at 245,319 by 

the Secretariat of the Pacific Community, with population centers largely focused on 4 of 

the 6 southern islands: Guam, with an estimated population of 182,207, as well as Rota, 

Tinian, and Saipan. The 9 northern islands are sparsely inhabited with total populations 

on Agrihan, Pagan, and Anatahan fluctuating from 0 to 100 persons since World War II. While 
8

GUAM

ROTA

AGUIJAN

TINIAN

SAIPAN

FARALLON DE

MEDINILLA

FARALLON DE 

 PAJAROS

MAUG

ASUNCION

AGRIHAN

PAGAN

ALAMAGAN

SARIGAN

GUGUAN

STINGRAY 

SHOAL

PATHFINDER REEF

ARAKANE REEF

ANATAHAN

SANTA ROSA REEF

MARIANA ARCHIPELAGO

3

1 2

Formed 15–20 million years ago

Seaoor and land characterized by low slopes except between 

uplifted layers

Though volcanic in origin, largely covered by layered, uplifted 

limestone from ancient coral reefs 

Consist of extinct volcanic edices 

3

Formed 0–5 million years ago by the subduction of the Pacic 

Plate under the Philippine Plate at the Mariana Trench, which 

lies ~ 130–210 km east of this island chain

Seaoor and land characterized by moderate to steep (10°–45°) 

slopes and periodic, explosive eruptions and landslides

All are active or dormant stratovolcanoes 

2

1

Formed after southern and before northern parts of the main arc

Near-optimal conditions for coral growth due to absence of 

terrestrially derived sediments, recent volcanic ash, and direct 

anthropogenic impacts

Some seamounts rise to within 9 m of the sea surface 

Remnant volcanic island arc that forms a series of seamounts ~ 

145–170 km west of and parallel to the main arc

WEST MARIANA RIDGE

NORTHERN ISLANDS OF THE MARIANA ARC

SOUTHERN ISLANDS OF THE MARIANA ARC

SUPPLY REEF
9

land cover on the southern islands is primarily influenced by population density, land cover 

on the northern islands is influenced by the shape and activity of volcanoes. Beyond population density and volcanic activity, a major determinant of land cover on all the islands is 

the presence of feral animals introduced by previous and present human inhabitants. On 

almost all islands of the Mariana Archipelago, introduced feral goats, pigs, deer, and cows 

destroy or alter native understory vegetation—typically causing an increase in erosion and, 

thus, sedimentation and runoff into nearshore waters that could have significant impacts 

on coral reef ecosystems. Only Guguan and Farallon de Pajaros are thought to have no introduced feral animals at this time (Table 1). Because of their isolated location and lack of 

inhabitants, the 3 islands within the Mariana Trench Marine National Monument have relatively few direct pressures from human activities.

Figure 3 (opposite page). The islands and reefs of the Mariana Archipelago can be grouped into 3 main geologic 

groups, shown here with information about their characteristics.

POPULATION DENSITY

(persons/km2)

PROPORTION OF POPULATION

IN ENTIRE ARCHIPELAGO

GUAM

SAIPAN

TINIAN

ROTA

OTHER

GUAM

SAIPAN

TINIAN

ROTA

OTHER

Figure 4 (right). Guam and Saipan have the largest human populations among the islands of the Mariana Archipelago, accounting for 69% and 28% of the population recorded for this archipelago in the U.S. Census 2000. The population of Saipan is only 40% of the population of Guam; however, because its land area is only 22% of the land area for 

Guam, Saipan has a higher overall population density. For more details and other island comparisons, see Table 1.

Table 1 (next page). Summary table of island parameters across the Mariana Archipelago: land area, proportion of 

archipelagic population, population density, year of last volcanic eruption, and animal threats. Land areas were calculated using geographic-information-system (GIS) techniques. See page 1 for island codes. Sources: U.S. Bureau of 

the Census, Smithsonian Institution, U.S. Geological Survey, CNMI Division of Fish and Wildlife, and Pacific Biodiversity 

Information Forum. 
10

ISLAND CODE

SHAPE &

RELATIVE

SIZE

AREA (km2)

TREE SNAKES

CATS

CATTLE

GOATS

PIGS

RATS

SHARE OF ARCH.

POPULATION

(% IN 2000)

POPULATION

DENSITY

(PERSONS/km2)

FDP

2

0

0

SOUTHERN NORTHERN

YEAR OF

LAST ERUPTION

GUG

4

0

0

1883 1967

GUA

544

69

284

—

TIN

101

2

35

—

119

28

524

—

ROT SAI

85

1

39

—

AGU

7

0

0

—

ANA

34

0

0

2007

SAR

4

0

0

—

ALA

13

<0.1

<1

1887

PAG

48

<0.1

<1

2006

AGR

44

<0.1

<1

1917

ASC

8

0

0

1924

MAU

2

0

0

—
11

MESOPHOTIC CORAL REEFS

Bathymetric and optical mapping have revealed extensive and dense coral 

reefs at depths > 30 m in numerous locations in the Pacific. Although few optical surveys have been carried out in such relatively deep areas in the Mariana 

Archipelago, those surveys that have been conducted have revealed light-dependent scleractinian (hard) corals. For example, a detailed survey carried out 

in 2004 at the Garapan Anchorage off Saipan revealed large expanses of corals 

in the mesophotic range (30–150 m) with reefs predominantly formed by Euphyllia paraancora, a hard coral species that was included in a recent petition to 

list 83 corals under the Endangered Species Act. The highest average live-coral 

cover from this study was observed in the depth range of 61–70 m with cover 

values > 33%. These results highlight the importance of monitoring at the full 

range of depths (0–150 m) in which coral communities may be present. More 

widespread surveys at these depths could define the extent of mesophotic 

reefs in this archipelago.

0 15

km

DEPTH (km)

2 1 0

0 1

km

SAND

CORAL COVER (%)

0 10 20 30 50 70 100

NOAA NOS Chart 81067_1 soundings in fathoms 

SAIPAN

TINIAN

Right: Bathymetric and optical data collected by the CRED in 2003 and 2004 were used to 

create a coral- and sand-cover map of the Garapan Anchorage and to aid management 

agencies in making decisions related to a proposed expansion of this anchorage into potentially coral-rich areas.
12

METHODS OVERVIEW
13

The CRED conducted its first Mariana Archipelago 

Reef Assessment and Monitoring Program 

(MARAMP) cruise in 2003, with subsequent cruises in 

2005, 2007, and 2009. Partners from Guam (Division 

of Aquatic and Wildlife Resources, Bureau of Statistics 

and Plans, Guam Environmental Protection Agency, 

and University of Guam) and the CNMI (Department 

of Environmental Quality, Department of Fish and 

Wildlife, and Coastal Resources Management) 

worked alongside CRED scientists to develop and 

implement a comprehensive monitoring program. 

Extensive biological, physical, and chemical surveys 

were conducted to document the conditions of and 

processes influencing the coral reef ecosystems around the islands of the Mariana Archipelago. Benthic habitat mapping data were collected 

using multibeam sonar surveys. Spatial and temporal observations of key oceanographic and water-quality parameters were collected using (1) 

a diverse suite of moored instruments designed for long-term observations; (2) closely spaced conductivity, temperature, and depth profiles of 

the vertical structure of water properties; and (3) discrete water samples for nutrient and chlorophyll-a analyses. Information on the condition, 

abundance, diversity, and distribution of biological communities around islands and on offshore reefs was collected using Rapid Ecological 

Assessment (REA), towed-diver, and towed optical assessment device (TOAD) surveys (Fig. 5). Towed-diver surveys, conducted on the forereef 

slopes of islands and banks loosely following the depth contour of ~ 15 m and encompassing various substrates, generate a broad overview of 

substrate types and associated large-fish and benthic communities. REA surveys, carried out at selected forereef locations at islands or banks at 

similar depths, produce more detailed, site-specific information on fishes of all sizes and community structure of hard substrate. TOAD surveys 

and a subset of towed-diver-survey data were used for optical validation and habitat characterization, documenting cover of live scleractinian 

(hard) corals, sand cover, and habitat complexity. The forthcoming Coral Reef Ecosystem Monitoring Report of the Mariana Archipelago: 2003–2007

provides detailed information on all methods used. 

SUMMARY OF METHODS

BIOLOGICAL

PHYSICAL/

CHEMICAL

MAPPING

TOAD - optical validation and habitat characterization

TOWED DIVER - broad surveys of benthic and sh communities 

REA - detailed view of benthic and sh community structure

MOORED INSTRUMENTS - collect continuous oceanographic data

CTD CASTS - measure conductivity, temperature, and depth

WATER SAMPLES - measure nutrients and Chl-a

MULTIBEAM SONAR - bathymetry, habitat characterization

Opposite page: CRED diver conducting a REA survey for coral disease. NOAA photo by Russell Moffitt
14

Figure 5. Schematic diagrams of (below) the main differences between towed-diver 

surveys and Rapid Ecological Assessment (REA) surveys, (top & middle right) 1 of 2 

divers conducting a towed-diver survey, and (bottom right) a diver conducting a REA 

survey along a 25-m transect line (the full report provides details about these methods).

TRANSECT

1 M

60 M TO BOAT

MOUNTED CAMERA

TOWBOARD

(DIGITAL VIDEO AND STILLS)

side view

TOWED-DIVER SURVEY AREA & UNITS

5 M

5 M

60 M TO BOAT

SURVEYS OF 1.5−2.5 KM

5-MIN OR ~ 200-M SEGMENTS

DEPTHS OF 3−30 M

top view

TOWED-DIVER SURVEY AREA & UNITS

TOWED-DIVER & REA METHODS

DEPTHS OF 10–20 M

VARIOUS SAMPLING UNITS ALONG TRANSECTS

SURVEYS ALONG 2–3 TRANSECTS OF 25 M 

REA SURVEY AREA & UNITS

side view

TRANSECT

TOWED-DIVER TRACKS REA SITES

AGUIJAN

TOWED DIVER VS. REA

ONLY HARD-BOTTOM HABITATS

SPECIES-SPECIFIC INFORMATION

TOTAL FISH ABUNDANCE

BENTHIC COMMUNITY STRUCTURE

MACROINVERTEBRATE AND LARGE

FISH ABUNDANCE

WIDE RANGE OF HABITAT TYPES
15

To compare the coral reef ecosystems across the Mariana Archipelago, the CRED developed 3 composite metrics based on towed-diver visual 

observations from 2005 and 2007 (Fig. 6). The overall Coral Reef Condition Index has equally weighted benthic and fish components. The Benthic 

Condition Index was calculated using data for live-coral cover, stressed-coral cover, crustose-coralline-red-algal cover, macroalgal cover, and 

density of crown-of-thorns seastars (Acanthaster planci). The Fish Condition Index was calculated using data for density and biomass of large 

fishes (≥ 50 cm in total length). These indices were 

calculated at 2 scales for this report: (1) archipelagic level for comparisons among 13 islands of 

the CNMI and Guam and (2) island scale for comparisons among only the 4 populated, southern islands of Guam, Rota, Tinian, and Saipan. The ranks 

in these condition indices, for the purposes of discussion and visual representation, are grouped as 

high, medium, or low. A high rank means superior 

condition relative to (a), in the archipelagic indices, other geographic regions in the Mariana Archipelago or (b), in the island-level indices, other 

survey areas in the 4 populated, southern islands. 

The Archipelagic Coral Reef Condition Index is used 

in the next section as a basis for comparing the 

ecosystems around the southern islands to the 

ones around the northern islands. The Coral Reef 

Condition Index for Guam, Rota, Tinian, and Saipan

is used later in the discussion of the reef conditions 

observed around individual southern islands. 

Figure 6. Conceptual diagram of the calculation of the 3 condition indices, which are based on data from towed-diver surveys 

conducted on forereef habitats during MARAMP cruises in 2005 

and 2007. CCA stands for crustose coralline red algae and COTS 

for crown-of-thorns seastar.

MACROALGAL 

COVER

(10%)

CCA COVER

(10%)

COTS DENSITY

(10%)

STRESSED-CORAL

COVER

(10%)

CORAL COVER

(10%)

FISH DENSITY

(25%)

FISH BIOMASS

(25%)

CORAL REEF

CONDITION

INDEX

(100%)

BENTHIC

CONDITION 

INDEX

(50%)

FISH

CONDITION 

INDEX

(50%)
16

SOUTHERN VS. NORTHERN ISLANDS
17

Differences in the geologic histories of these islands, which result in large differences in land area and slope, have been identified as a major factor shaping the 

coral reef ecosystems of the Mariana Archipelago (Burdick et al. 2008; Houk and 

Van Woesik 2010; Riegl et al. 2008). The 6 southern islands are, in general, significantly larger than the northern islands and have uplifted calcium carbonate 

terraces with mostly low slopes (0°–10°) both onshore and offshore, resulting in a 

substantial amount of total potential reef area. In contrast, the 9 small, northern 

islands are active or dormant volcanoes with steep onshore and offshore slopes, 

narrow ridges, and few offshore terraces, resulting in total potential reef areas 

that are much smaller relative to the southern islands (Fig. 7). Other factors likely 

influencing coral reef conditions include the anthropogenic stressors of coastal 

sedimentation and pollution, fishing pressure, and land use that in the populated, southern islands are associated with increasing population, tourism, and development. In the northern, largely uninhabited islands, the frequency, duration, 

and magnitude of active volcanism can severely affect coral reef ecosystems. 

0 50 100 150 600

ARA

PAT

GUA

ROT

AGU

TIN

SAI

ANA

SAR

GUG

ALA

PAG

AGR

ASC

MAU

SUP

FDP

AREA (km2)

LAND SEAFLOOR AT DEPTHS OF 0–100 m

ANATAHAN

SAIPAN

TINIAN AGUIJAN

ROTA

GUAM

SANTA ROSA

FARALLON DE PAJAROS

SUPPLY MAUG

ASUNCION

AGRIHAN

PAGAN

ALAMAGAN

GUGUAN

ZEALANDIA

SARIGAN

STINGRAY

PATHFINDER

ARAKANE

12

14

16

18

20

22

LAT �N

0.02 0.04 0.06 0.08 0.1

CHL-a (μg/L)

CHL-a

MARAMP surveys found differences in several key oceanographic indicators between the southern and northern islands. The north–south orientation of the 

Mariana Archipelago results in latitudinal gradients in sea-surface temperatures 

(SST), wind conditions, currents, and chlorophyll-a (Chl-a) and nutrient concentrations across this archipelago. The southern islands exhibited warmer SSTs, 

stronger wind conditions and surface currents, lower salinities, and less productive waters (Fig. 8). Additionally, episodic volcanic eruptions, ash discharge, 

and landslides on the northern islands can significantly modify the quality and 

chemistry of surrounding coastal waters over short (days to weeks) and medium 

(months to years) time frames. As a result of such natural and human-induced 

coastal processes, the nearshore waters around the southern islands exhibited 

considerable spatial and temporal variability in water-quality parameters, including inorganic nitrogen and Chl-a, particularly around Guam.

Figure 7 (top). Land and potential reef area per island.

Figure 8 (bottom). Latitudinal gradients of seasonal Chl-a concentrations in the Mariana Archipelago for January–March using 

National Aeronautics and Space Administration SeaWiFS imagery 

Opposite page: A blacktip grouper (Epinephelus fasciatus (12°–22° N, 140°–150° E; http://oceancolor.gsfc.nasa.gov/SeaWiFS). ). NOAA photo by Chip Young
18

BLEACHING & STORM EVENTS AROUND GUAM

Corals bleach when the ocean temperature increases 1°C–2°C above the maximum monthly mean 

for ocean temperatures in their area. Satellite-derived sea-surface temperatures (SST) and in situ 

observations from subsurface temperature recorders (STRs) deployed around Guam show that 

water temperatures reached a maximum (29.5°C) in late August or September and a minimum 

(27.1°C) in February. These SST and STR data show that water temperatures around Guam in August and September 2006 surpassed the coral bleaching threshold, which is defined as 1°C above 

the monthly maximum climatological mean. David Burdick of the Guam Coastal Management 

Program reported field observations of bleaching occurring among numerous species at several 

sites around Guam to a depth of 7 m (Burdick et al. 2008). Follow-up observations in October 2006 

showed that some coral species had recovered whereas others exhibited partial to full mortality 

(Brown 2007). MARAMP data show that estimates of coral cover were lower in 2007 than in 2005 

from both towed-diver and REA surveys. 

Relief from temperature increases that can cause bleaching sometimes comes from storms, since 

they typically mix cool, deep waters with warmer, shallow waters. The waters around Guam are 

dominated by trade wind swells, characterized by frequent (6–10 s), relatively small waves (2–3 m), 

originating from the east. Typhoons are a common occurrence around Guam and generate large 

(> 4 m), mid-to-long-period (10–16 s) wave events, generally from the southeast (110o) and typically between August and December. Infrequent long-period swells with moderate wave heights 

(2.5–3.5 m) occur from the west-southwest and are associated with distant storms. While several 

tropical cyclones have passed close enough to Guam to influence its weather since 2003, the only 

large wave event (> 4 m) observed during the period of 2003–2007 occurred during the summer 

of 2004 with the passages of Typhoons Tingting and Chaba. Damage from these wave events did 

not appear to be extensive.

Left: Partially bleached Acropora coral near Guam. Photo by Dave Burdick (www.guamreeflife.com)
19

BENTHIC COMPOSITION AND DIVERSITY

Based on towed-diver surveys of forereef habitats for each of the 3 MARAMP cruises in 2003, 2005, and 2007, estimates of mean macroalgal 

cover generally were higher and estimates of mean coral cover were lower around the southern islands than around the northern islands (Fig. 

9). These differences in ecosystem conditions for the southern islands may result from differences in human impacts (for example, variation in 

levels of nutrient input and harvest of herbivorous fishes). It is important to note, however, that macroalgae are a normal and necessary part 

of reef ecosystems. The southern carbonate islands may naturally experience conditions that promote higher macroalgal growth than the 

growth seen in the northern, volcanic islands. Without a sufficient number of time-series observations, it is difficult to conclude that macroalgal 

populations are increasing or coral populations are decreasing around the southern islands, thus, underscoring the importance of the kind of 

long-term monitoring conducted by the CRED. 

Data from towed-diver surveys conducted on forereef habitats in 2005 and 2007 are integrated in the Archipelagic Benthic Condition Index by 

geographic region in the Mariana Archipelago. 

Based on these analyses, the benthic condition 

was generally better in the northern islands 

compared to the southern islands. Ranks in this 

index indicate that the benthic conditions for 

Maug, Alamagan, Guguan, and Sarigan were 

superior to the conditions for other islands 

in the Mariana Archipelago, a result of high 

or increasing coral cover, low stressed-coral 

cover, and low densities of crown-of-thorns 

seastars (COTS). Index ranks for the southern 

islands of Saipan, Tinian, Aguijan, Rota, and 

Guam were low in or decreasing between 

GUA

ROT

AGU

TIN

SAI

SAR

GUG

ALA

PAG

AGR

ASC

MAU

FDP

ALGAL COVER (%)

0 10 20 30 40 50

REA TOWED DIVER

GUA

ROT

AGU

TIN

SAI

SAR

GUG

ALA

PAG

AGR

ASC

MAU

FDP

0 10 20 30 40 50

CORAL COVER (%)

REA TOWED DIVER

Figure 9. Comparisons of mean (left) algal cover (%) and 

(right) coral cover (%) from REA and towed-diver surveys of 

forereef habitats conducted in the Mariana Archipelago in 

2007. Error bars indicate standard error (±1 SE) of the mean.
20

2005 and 2007, except for a few geographic regions of Tinian and Saipan and the north coast of Aguijan. The low ranks for these 5 islands in 

this condition index resulted from relatively high levels of stressed-coral cover and macroalgal cover and low levels of coral cover. Particularly 

around Rota, some of the lowest coral-cover values in the Mariana Archipelago were observed (Fig. 9). Around the southern islands, compared 

to the northern islands, the occurrence of bleaching and coral disease was higher and COTS densities were found more often at levels of a 

potential outbreak. 

REA benthic surveys of hard-bottom habitats conducted in 2007 found that mean coral cover was moderate and similar around all southern 

islands. Conversely, around the young, sparsely inhabited or uninhabited, volcanic northern islands, coral cover was more variable, and both 

the highest and the lowest levels in the Mariana Archipelago were recorded there, with overall mean coral cover of 34.1% (SE 5.7) at Maug and 

3.6% (SE 0.7) at Farallon de Pajaros (SE stands for standard error of the mean). Macroalgal-cover estimates from REA surveys reveal a clear spatial 

pattern with high algal cover in the southern islands compared to the northern islands (Fig. 9).

The diversity of benthic communities, measured as mean richness of coral genera and of macroalgal genera per island, was greater in the southern islands than in the northern islands (richness was computed as the maximum number of coral genera or macroalgal genera recorded at a 

REA site). This difference is probably a result of the southern islands having greater potential habitat areas at shallow-water depths of < 100 m, 

compared to the northern islands (Fig. 7). These large areas in the southern islands support a wider range of habitat types and oceanographic 

conditions that result in a greater number of niche environments. The notion that the size of islands and disturbance frequency (predominantly 

volcanic activity) are likely predictors of taxonomic richness (Houk and Starmer 2010) is supported by data from MARAMP surveys (Tribollet 

and Vroom 2007). The greatest overall mean generic richness of corals for an island was recorded around Saipan (17.3 [SE 1.1]), and the highest 

overall mean generic richness of macoalgae was observed around Rota (13.1 [SE 0.74]) and Guam (13 [SE 0.68]). In this context, some coral and 

algal taxa were restricted to or particularly widespread around the southern islands, including the hard corals Diploastrea, Plesiastrea, Alveopora, 

and Coscinaraea and the red algae Amansia, Crouania, Gelidiella, Haloplegma, Halymenia, and Predaea. The lowest diversity was found around 

Farallon de Pajaros with a mean richness of 7.6 (SE 1.4) for coral genera and a mean richness of 6 (SE 0.47) for macroalgal genera. Farallon de 

Pajaros has a land area of a mere 2.3 km2 and experienced frequent volcanic activity during the 19th century and recurrent eruptions from the 

1940s to the 1960s.
21

CROWN-OF-THORNS SEASTAR DENSITIES

With the exception of Pagan, the northern islands had low islandwide mean densities of crown-of-thorns seastars (COTS). These corallivorous 

seastars oscillate in a boom-and-bust population cycle with natural fluctuations in food availability and successful recruitment events. Using 

a threshold of 0.15 organisms/100 m2, towed-diver surveys of forereef habitats in 2005 recorded potential outbreaks at Pagan with a mean islandwide COTS density of 0.24 organisms/100 m2, the greatest density value recorded for any island in the Mariana Archipelago. This high COTS 

density may have caused the observed decline in coral cover from 19% (SE 0.8) 

in 2003 to 10% (SE 0.8) in 2005. By 2007, COTS densities had decreased threefold 

at Pagan, and coral cover appeared to be recovering slightly with an islandwide 

mean of 12% (SE 0.7). At all of the southern islands, with the exception of Tinian, localized outbreaks were observed during towed-diver surveys in 2003 and 

2005. By 2007, however, COTS populations declined sharply around all of these 

islands except Guam. There, the islandwide mean density almost doubled from 

0.1 organisms/100 m2 (SE 0.03) in 2005 to 0.16 organisms/100 m2 (SE 0.03) in 

2007, with the highest densities recorded along the south and east coasts of 

Guam (Fig. 10). Across the Mariana Archipelago in 2007, signs of predation by 

COTS or snails were observed on a total of 14 genera of hard corals, belonging 

to 7 families, with more than 96% of all scars found on members of the families 

Faviidae, Acroporidae, Poritidae, and Pocilloporidae. Of these coral genera, Goniastrea, Porites, and Astreopora hosted 60% of all cases. Interestingly, around the 

northern islands of Asuncion and Maug, colonies of Goniastrea hosted 50% of 

all scars islandwide, whereas colonies of Astreopora, Porites, Pocillopora, and Stylophora hosted more than 60% of cases around the inhabited Guam and Saipan.

.17

.03

.03

.01

.02

.00

.00

.55

.77

.24

.49

.08

.42

.02

.05

.03

.05 .01

.11

N O R T H W E S T

N O R T H

E A S T

S O U T H

W E S T

2007 COTS DENSITY

SURVEY TRACKS

AVERAGE COTS DENSITY/100 m2 .##

.## POTENTIAL COTS OUTBREAK

GUAM

Figure 10. Densities (organisms/100 m2) of crown-of-thorns seastars (COTS) from towed-diver surveys 

of forereef habitats conducted around Guam in 2007.
22

OCEAN ACIDIFICATION AROUND MAUG

During the CRED’s MARAMP cruise in 2003, a hydrothermal vent system (depth of ~ 10 m and 

seafloor area of ~ 400 m2) was observed within the caldera of Maug. In 2007, area-wide and 

near-vent water samples were collected to gain insight into the biogeochemistry of hydrothermal vents. Divers collected (1) gas bubbles for analysis of carbon dioxide (CO2

) and hydrogen sulfide concentrations; (2) water samples for dissolved inorganic carbon, total alkalinity, 

pH, iron, manganese, trace metals, and nutrient concentrations; and (3) water temperatures 

from multiple locations within and around the area of this vent system. The seafloor within 

this vent system was composed of boulders and sand, layered with accumulations of iron-rich, 

orange-tinted flocculent material. Spatoglossum sp. was the dominant alga within this vent 

system, covering every hard surface. No calcium-carbonate-producing organisms were observed within this vent system. However, a REA survey conducted only 15 m south of this vent 

revealed live-coral cover of 67% in 2007, reaching close to 100% cover of Porites rus within 

one part of this REA site. Water temperatures were 45°C–63°C at the vent openings, 16°C–34°C 

above the surrounding subsurface water temperatures. Gas bubbles collected consisted of 

CO2 and probably contributed to pH values of 6.07 in this vent system, much more acidic than 

the pH of 8.13 of surrounding waters. Calculated states of calcite and aragonite saturation of 

undiluted vent waters were < 1, demonstrating that these waters were highly corrosive to 

calcifying organisms (calcium carbonate generally dissolves in waters with saturation states of 

< 1). The unique temperature and pH values for this vent system clearly regulated the benthic 

community structure there, but they had only a very local impact on coral growth as nearby 

water temperatures (29°C) and pH (8.13) reflected levels typical for this region. These preliminary results provide the only known example of present day ocean acidification within a coral 

reef ecosystem.

Left: Vents release gas bubbles and warm water west of East Island, one of the 3 islands that make up the Maug 

island group. The seafloor is covered by “fluffy” orange silt. NOAA photo by Ellen Smith
23

INFORMING RESOURCE MANAGEMENT

Climate change and the anticipated expansion of the military presence on Guam are primary factors affecting coral 

reefs around Guam and the Commonwealth of the Northern Mariana Islands (CNMI), as outlined in the NOAA Coral Reef 

Conservation Program (CRCP) Management Priority Setting documents for Guam and the CNMI, along with impacts of 

fisheries and land-based sources of pollution, especially around populated areas. 

The population of military personnel and their dependents on Guam is expected to increase 160% from 15,000 persons 

in 2009 to more than 39,000 by 2020—a change expected to directly affect this territory’s coral reef ecosystems. Resource managers on Guam have explicitly expressed a need for baseline data, which they view as crucial to measuring 

the effects of the military buildup and potential mitigation activities. Continuing ecosystem monitoring, along with 

examination of existing data from previous CRED research activities, will provide valuable guidance for setting future 

priorities for coral reef management and research tied to the CRCP’s National Goals and Objectives. 

Interdisciplinary data from the CRED are presented in detail in the Coral Reef Ecosystem Monitoring Report of the Mariana 

Archipelago: 2003–2007. Since 2003, the CRED has been conducting a broad range of surveys of the coral reef ecosystems around Guam, including nearshore waters adjacent to the Piti-Asan and Geus-Manell watersheds. These watersheds, which discharge into ocean areas that include the Piti Bomb Holes Marine Preserve and Achang Reef Flat Marine 

Preserve, along with other Pacific RAMP survey areas, such as the Tumon Bay and Pati Point Marine Preserves, have been 

identified as priority management sites by the CRCP and local Guam resource management agencies. 

Similarly, Pacific RAMP surveys have been conducted in priority management sites and marine protected areas around 

the more populated areas of the CNMI, including around Saipan, the Bahia Laolao Sea Cucumber Reserve, Managaha 

Marine Conservation Area, Lighthouse Reef Trochus Reserve, Bird Island Marine Sanctuary, and Forbidden Island Marine 

Sanctuary; at Rota, the Sasanhaya Fish Reserve and Talakhaya Watershed Revegetation Area; and a marine reserve designated on the southwest coast of Tinian.

The CRED will continue to monitor these and other areas through the Pacific RAMP and other collaborative opportunities. Improved techniques implemented in 2009 will yield enhanced data about nearshore fish communities, data that 

are important in developing stock assessments and annual catch limits for commercially important reef-fish species. In 

2009, the CRED performed some limited studies and established a permanent benthic Rapid Ecological Assessment site 

within Apra Harbor. In 2010, a PIFSC-led fisheries cruise in this archipelago focused on filling gaps in mapping data and 

improving studies of coral reef fishes. These projects were designed to respond to questions, interests, and information 

gaps presented by local scientists, managers, and fisheries groups in the CNMI and Guam.

To find out more about the information available from the CRED or to make suggestions for future research and outreach activities, please contact the CRED at nmfs.pic.credinfo@noaa.gov. 

Right: School of breams (Gnathodentex aureolineatus). Photo by Dave Burdick (www.guamreeflife.com) 
24

FISH BIOMASS AND DIVERSITY

The Archipelagic Fish Condition Index, based on towed-diver surveys, demonstrates a north–south pattern of ranks declining from high to low. The highest 

values of biomass of large fishes (≥ 50 cm in total length), calculated as weight 

per unit area, in the Mariana Archipelago were observed around Farallon de 

Pajaros and Asuncion in 2005 and 2007. Around the southern islands, where 

human populations and exposure to anthropogenic stressors are greater 

than in the northern islands, towed-diver surveys of forereef habitats recorded generally lower levels of large-fish biomass in 2005 and 2007, compared 

to the northern islands, or decreasing values between these survey years. 

Observations from these surveys of large fishes (Fig. 11) were dominated by 

jacks and sharks, both piscivores (fishes that prey primarily on other fishes), 

with mean large-fish biomass estimates more than 4 times higher around the 

northern islands than around the southern islands: 0.92 kg/100 m2 (SE 0.18) 

versus 0.22 kg/100 m2 (SE 0.04). The density of sharks encountered during 

towed-diver surveys was nearly 10 times higher around the northern islands 

than around the southern islands: 174.5 individuals/km2 versus 17.6 individuals/km2

. 

Site-specific results from REA surveys that included all fishes of all sizes indicate that the overall mean total fish biomass for the 9 northern islands 

was more than 3 times higher than the total fish biomass for the 5 surveyed 

southern islands: 12 kg/100 m2 (SE 2.37) versus 3.63 kg/100 m2 (SE 0.53). Biomass values from REA fish surveys were higher around the northern 

islands for all trophic groups, but these south–north differences were greatest for piscivores (Fig. 12), for which mean total fish biomass around 

the northern islands with 4.04 kg/100 m2 (SE 1.38) was more than 13 times the value recorded around the southern islands with 0.30 kg/100 

m2 (SE 0.03). Fish assemblages at the southern islands were dominated by herbivores (54% of total biomass), including surgeonfishes and parrotfishes. These patterns in total fish biomass and trophic composition are consistent with the patterns found along anthropogenic gradients 

in other archipelagoes in the central Pacific (Williams et al. 2008; Stevenson et al. 2007; Friedlander et al. 2008; Friedlander and DeMartini 2002).

LARGE-FISH BIOMASS (kg/100 m2)

PARROTFISHES

SURGEONFISHES

SHARKS

BARRACUDAS

SPADEFISHES

JACKS

SNAPPERS

TUNAS

PORCUPINEFISHES

WRASSES

OTHER

ARA

PAT

STI

TAT

ANA

ALA

PAG

AGR

ASC

MAU

SUP

FDP

SRR

GUA

ROT

AGU

TIN

SAI

SAR

GUG

0 1 2 3 4 5 6

Figure 11. Estimates of islandwide mean biomass (kg/100 m2) of large fishes 

(≥ 50 cm in total length) based on pooled data from the 3 MARAMP cruises in 

2003, 2005, and 2007. Large-fish biomass is derived from towed-diver surveys 

of forereef habitats and divided into taxonomic family groups. The dotted line 

distinguishes the West Mariana Ridge from the Mariana Arc. See Page 1 for 

island codes. Error bars indicate standard error (±1 SE) of the mean.
25

HERBIVORES PLANKTIVORES

PISCIVORES

PERSONS

TOP PREDATORS

#

154,805

62,392

6

3,283

3,540

ARA

PAT

SRR

GUA

ROT

AGU

TIN

SAI

ANA

SAR

GUG

ALA

PAG

AGR

ASC

MAU

FDP

ARA

PAT

SRR

GUA

ROT

AGU

TIN

SAI

ANA

SAR

GUG

ALA

PAG

AGR

ASC

MAU

FDP

TOTAL FISH BIOMASS (kg/100 m2)

0 5 10 15 20 25 30 35 40 45

Figure 12 (above). Estimates of (right axis) human population from the U.S. Census 2000 (not to scale) 

and (left axis) mean total fish biomass (kg/100 m2). Values of total fish biomass from REA fish surveys 

were pooled from the 3 MARAMP cruises in 2003, 2005, and 2007 and divided into 4 trophic groups. The 

dotted line distinguishes the West Mariana Ridge from the Mariana Arc. See Page 1 for island codes. Error bars indicate standard error (±1 SE) of the mean.

Figure 13 (right). Size-class distribution (length in cm) for (top) groupers, (middle) parrotfishes, and 

(bottom) surgeonfishes from REA fish surveys conducted in 2007 on forereef habitats around the northern and southern islands of the Mariana Archipelago.

GROUPER SIZE-CLASS DISTRIBUTION (cm)

0.0

0.5

1.0

1.5

2.0

NORTHERN ISLANDS SOUTHERN ISLANDS

0–5 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45 45–50 50–55

DENSITY (FISHES/100 m2) 0.0

0.2

0.4

0.6

0.8

1.0

NORTHERN ISLANDS SOUTHERN ISLANDS

DENSITY (FISHES/100 m2)

PARROTFISH SIZE-CLASS DISTRIBUTION (cm)

0–5 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45 45–50 50–55

0

2

4

6

8

10

12

14

16

18

DENSITY (FISHES/100 m2)

NORTHERN ISLANDS SOUTHERN ISLANDS

SURGEONFISH SIZE-CLASS DISTRIBUTION (cm)

0–5 5–10 10–15 15–20 20–25 25–30 30–35 35–40 40–45 45–50 50–55
26

INTEGRATING ECOSYSTEM COMPONENTS

The Archipelagic Coral Reef Condition Index provides a simple tool to compare the general conditions of coral reef ecosystems across the Mariana Archipelago. This index illustrates striking differences in the conditions of both benthic and fish communities: conditions become lower 

moving from the northern islands to the southern islands (Figs. 14 and 15). These spatial patterns coincide with gradients in several potential 

causative factors, including human population, latitude, and geologic age and structure. A comparison of the 2005 and 2007 ranks for geographic regions in the Archipelagic Coral Reef Condition Index demonstrates that the index ranks (high, medium, or low) for about two-thirds 

of the geographic regions in this archipelago did not change between years. The other one-third of geographic regions, then, are identified as 

areas of change by the shift in their index ranks between years. With continued long-term monitoring, and additional years of data beyond 2005 

and 2007, changes illustrated with these condition indices will reveal ecologically relevant patterns.

This analysis of MARAMP survey data shows that our observations are in agreement with other studies that suggest higher levels of macroalgal 

cover, lower fish biomass, smaller fish sizes, and lower percentage of coral cover are likely to occur in association with high densities of human 

population and high use of resources. In addition, the overall abundance of marine debris was greater around the southern islands compared to 

the rest of the Mariana Archipelago, with the greatest number of sightings of derelict fishing gear and other man-made objects, such as anchors, 

ground tackle, mooring blocks, and metal scrap, found around Guam and Tinian.

Levels of species richness, measured as the number of fish species encountered per 100 m2, were averaged across the 3 survey years of 2003, 

2005, and 2007. Species richness was higher at all northern islands with a range of 30.6–43.6 species/100 m2, except for Anatahan with 14.1 species/100 m2

, than at any of the southern islands with a range of 20.2–30 species/100 m2. The low species richness and biomass around Anatahan 

was likely a consequence, at least partially, of the low visibility encountered as a result of ash and debris in the water column from volcanic eruptions in 2003 and may also reflect low sampling effort there (just 3 REA sites were surveyed there during the MARAMP cruise in 2003).

Observations of size-class distribution for 3 families of fishes from REA surveys show that densities of large size classes were higher in the northern islands of the Mariana Archipelago than in the southern islands (Fig. 13). Groupers of all size classes were more abundant in the northern 

islands. Large size classes (≥ 25 cm) were commonly encountered in the northern islands, but these size classes were rare in the southern islands. Parrotfishes displayed more of a bimodal distribution with large individuals more common in the northern islands and small individuals 

more common in the southern islands. While the surgeonfish community was dominated by small size classes, its overall size range was greater 

around the northern islands, where densities were higher in all but the size class of 15–20 cm.
27

0 25

km

0 1

km

SINGLE-YEAR SURVEY OR NO SURVEY

FARALLON 

DE PAJAROS

MAUG

ASUNCION

AGRIHAN

PAGAN

ALAMAGAN

GUGUAN

AGUIJAN

ROTA

GUAM

SAIPAN

TINIAN

SARIGAN

PATHFINDER SANTA ROSA REEF

ARAKANE

ARCHIPELAGIC CORAL REEF CONDITION INDEX 2005 & 2007

HIGH

MED

LOW

LOW MED HIGH

2005 Index

2007 Index

2005

Value

2007

Value

midline

Figure 14 (left). The Archipelagic Coral Reef Condition Index, based on 

towed-diver surveys conducted in 2005 and 2007, reflects the integrated 

condition of the benthic and fish communities for each geographic region, relative to other regions across the Mariana Archipelago. The regions 

around Maug, for example, have high index ranks for both 2005 and 2007. 

Thus, they are all assigned the bright-green color that corresponds to the 

top-right square in the legend. The position of the horizontal bar above 

the midline in this square also reflects that those geographic regions maintained a high rank for both 2005 (y-axis) and 2007 (x-axis).

NORTH SOUTH

FISH RICHNESS

FISH BIOMASS

MACROALGAL COVER

MACROALGAL DIVERSITY

CORAL DIVERSITY

CORAL COVER

COTS DENSITY

FISH ASSEMBLAGES

Figure 15 (above). General comparison of parameters between the 9 

northern islands and 5 southern islands. A green arrow indicates superior 

condition, a red arrow represents inferior condition, and the direction of an 

arrow shows whether a condition was higher or lower relative to the other 

island group. The blue arrows indicate that conditions were similar.
28

CORAL REEFS OF THE SOUTHERN ISLANDS
29 Opposite page: A massive Diploastrea heliopora colony with enlarged corallites. The cause of this skeletal growth anomaly is unknown. NOAA photo by Jean Kenyon

This section includes management-relevant analyses from observations collected in the southern islands of the Mariana Archipelago, where 

human populations are more concentrated and there is high interest in issues of marine resource management.

BENTHIC COMPOSITION

The southern islands and nearby reefs include Guam, Rota, Aguijan, Tinian, and Saipan as well as Santa Rosa Reef, near Guam, and Tatsumi Reef, 

close to Tinian. The composition of the benthos around these islands was dominated by macroalgae. Mean macroalgal cover from towed-diver 

surveys conducted on forereef habitats in 2005 and 2007 was 47% with islandwide mean values ranging from 31% (SE 1.2) around Saipan to 

50% (SE 1.5) around Tinian. The highest macroalgal cover in the Mariana Archipelago was recorded on Santa Rosa Reef with 70.5% (SE 3.1). Mean 

live-coral cover, from towed-diver surveys conducted during the 3 MARAMP cruises in 2003, 2005, and 2007, was 11% with values ranging between 7% (SE 0.5) around Rota and 19% (SE 0.6) around Guam. A mean coral cover of 7.7% (SE 0.2) was recorded for the 2 nearby offshore reefs, 

Tatsumi and Santa Rosa. Cover of crustose coralline red algae did not vary much among the southern islands, with mean cover ranging from 8% 

(SE 0.4) to 12.3% (SE 1.3) and the highest mean values recorded around Aguijan. At Santa Rosa Reef and Tatsumi Reef, crustose-coralline-red-algal cover was slightly lower with 4.9% (SE 1.2). Looking specifically at hard-bottom areas, results from REA surveys conducted only in 2007 show 

lower estimates of macroalgal cover (6%–24%) and similar values of coral cover (8%–16%) compared to islandwide estimates from towed-diver 

surveys encompassing all substrate types. Size-class distribution of coral colonies did not vary substantially among the southern islands in 2007. 

Coral and coralline-algal diseases were observed around all 5 of the surveyed southern islands. The highest prevalence of coral disease of 1.4% 

(SE 0.4) was recorded at Guam, and the lowest prevalence of 0.1% (SE 0.03) was observed at Rota. Many of the cases of coral disease observed 

around these 5 islands disproportionately affected important reef-building taxa, including hard corals in the families Poritidae, Acroporidae, 

and Faviidae, as well as the carbonate-producing crustose coralline red algae. Excluding Aguijan, where only one site was surveyed, islandwide 

mean density of coralline-algal diseases was greatest at Tinian with 2.8 cases/100 m2 (SE 1.5). Although prior field surveys (Starmer et al. 2008; 

Goreau J, pers. comm.) have identified the presence of coralline-algal disease in this region, the results from MARAMP surveys conducted in 

2007 represent the first attempt to quantitatively evaluate the occurrence of these diseases across the Mariana Archipelago. In this context, patterns of coralline-algal disease indicate that some disease states were abundant and widespread and others were uncommon and rare. Of the 

124 coralline-algal disease cases enumerated during MARAMP surveys in 2007, > 87% of cases corresponded to coralline lethal orange disease.
30

REEF FISHES

Mean large-fish biomass, estimated from towed-diver surveys conducted on forereef habitats in 2003, 2005, and 2007, was highest around Rota 

with 0.4 kg/100 m2 (SE 0.1) and lowest around Guam with 0.1 kg/100 m2 (SE 0.03). Mean total fish biomass, estimated from REA surveys conducted in 2003, 2005, and 2007, was highest around Aguijan with 5.2 kg/100 m2

 (SE 0.6) and Rota with 4.1 kg/100 m2 (SE 0.9) and lowest around 

Santa Rosa Reef with 1.5 kg/100 m2 (SE 0.1).

INTEGRATED OVERVIEW OF SAIPAN & GUAM

The Coral Reef Ecosystem Monitoring Report of the Mariana Archipelago: 2003–2007 devotes individual chapters to Guam and 13 islands of the 

CNMI, discussing results from each of several interdisciplinary surveys. For the 4 populated, southern islands of Guam, Rota, Tinian, and Saipan, 

this report also provides comparisons of the benthic and fish communities around each island relative to the conditions around only these 4 

islands. This overview section focuses on Saipan and Guam, which have the largest human populations in this archipelago.

Despite the lower levels of live-coral cover along the east coast of Saipan relative to the west coast, the general condition of the benthic communities in most survey areas around Saipan appeared above average compared to other survey areas in the 4 populated, southern islands in 2005 

and 2007, based on the calculated Benthic Condition Index for Guam, Rota, Tinian, and Saipan. The relatively low coral cover on the east coast may 

reflect the natural character of the reef, including geologic, topographic, and oceanographic factors, rather than the impact of human activities. 

Integrating towed-diver benthic and fish data with modeled wave conditions reveals that coral cover was higher along Saipan’s sheltered west 

coast than in other areas, but large-fish biomass was relatively constant around this island (Fig. 16).

Ranks for survey areas in the Coral Reef Condition Index for Guam, Rota, Tinian and Saipan, as well as in the associated Benthic Condition Index, varied around Guam. The survey area near the Guam National Wildlife Refuge on the northwest corner of this island had coral cover of 

11%–20% (Fig. 17) and a high rank in the Benthic Condition Index in 2005, but this rank declined to medium in 2007. The large population near 

and frequent use of Tumon Bay on the northwest coast of Guam and the numerous associated stressors that can impact high-use areas may 

have resulted in deteriorating conditions of the benthic communities on the forereef off Tumon Bay: the rank for this survey area in the Benthic 

Condition Index changed from medium in 2005 to low in 2007. It should be pointed out that many possible land-based impacts likely take place 

predominantly on the shallow backreef that was not surveyed as part of the MARAMP. Within the Tumon Bay Marine Preserve, regulations pro-
31

¥

!

!

S

LARGE-FISH

BIOMASS*

(kg/100 m2)

BOAT LAUNCH

SEWAGE OUTFALL

IMPAIRED WATER

> HARBOR OR MARINA

MARINE PROTECTED AREA

2005

2007

BOTH YRS

2005

2007

BOTH YRS

STRESSED

CORALS

WAVE CONDITIONS

L: LOW

M: MEDIUM

H: HIGH

0–5

6–10

11–20

21–30

> 30

CORAL

COVER* (%)

0

0.01–0.16

0.17–0.34

0.35–0.75

0.76–1.65

>1.65

!

ambient

episodic

WATERSHED BOUNDARY

*Based on means from towed-diver 

 data collected in 2005 and 2007

 POTENTIAL COTS

OUTBREAK

0 2 4

km

S

S

S

S

S

S

H

H

H

H

L

L

L

M

H

L

L

H

SAIPAN

!

!

!

Figure 16. Integrated overview of benthic and 

fish communities around Saipan, illustrated 

with various human and natural factors that 

may influence their condition.

hibit various fishing activities, yet towed-diver surveys of the forereef habitats of Tumon Bay recorded very low biomass of large fishes (≥ 50 cm 

in total length) in all 3 survey years, with generally only a few individuals recorded per survey (the low values from 2005 and 2007 are illustrated 

in Fig. 17). Around southwest Guam, where very low levels of coral cover (0%–5%) were observed (Fig. 17), factors exerting a potential negative 

influence on reef communities included poor water quality along portions of the coastline, a lack of favorable substrate for reef development, 
32

Figure 17. Integrated overview of benthic and 

fish communities around Guam, illustrated 

with various human and natural factors that 

may influence their condition.

and elevated sediment input. The conditions of the reef in the Pati Point Marine Preserve, located around the northeast corner of Guam, were 

among the best observed around the 4 populated, southern islands. Ranks for the survey area closest to the northeastern tip of Guam were 

high in the Coral Reef Condition Index and Benthic Condition Index for 2005 and 2007. The highest numbers of large fishes around Guam, from 

towed-diver surveys conducted in 2007, were recorded in this area.

!

!

!

!

!

!

!

¥

¥

0 2 4

km

GUAM

!

!

!

!

! >

>

>

>

>

S

S

S

S

S

S

H

H

M

L

L

L

L

M

L

H

!

S

LARGE-FISH

BIOMASS*

(kg/100 m2)

 POTENTIAL COTS

OUTBREAK

2005

2007

BOTH YRS

2005

2007

BOTH YRS

STRESSED

CORALS

0–5

6–10

11–20

21–30

> 30

CORAL

COVER* (%)

0

.01–.16

0.17–0.34

0.35–0.75

0.76–1.65

>1.65

*Based on means from towed-diver 

 data collected in 2005 and 2007

¥

!

!

BOAT LAUNCH

SEWAGE OUTFALL

IMPAIRED WATER

> HARBOR OR MARINA

MARINE PROTECTED AREA

WAVE CONDITIONS

L: LOW

M: MEDIUM

H: HIGH

!

ambient

episodic

WATERSHED BOUNDARY
33

Although large-bodied fish species generally were encountered 

more frequently around the northern islands, an exception was the 

rare humphead wrasse (Cheilinus undulatus), also known as the Napoleon Wrasse. This fish species is one of the largest coral reef fishes, 

reaching up to 2 m and 190 kg. It occurs patchily throughout much 

of the Indo-Pacific region, and its life-history characteristics make it 

more vulnerable than most other species of coral reef fishes to overexploitation. Although this species is not common, the CRED recorded 48 sightings of humphead wrasse around the southern islands, 

with a mean density of 10.9 individuals/km2 (SE 7.3), but only 7 sightings around the northern islands, with 2.2 individuals/km2

 (SE 1.1), 

over 3 survey years. The sizes of individual fish were similar with an 

average length of 92.8 cm (SE 9.9) in the northern islands and 90.7 

cm (SE 4.1) in the southern islands. Three of the individuals seen at 

the remote banks on the West Mariana Ridge were slightly larger 

with a mean length of 115.8 cm (SE 9.3).

SIGHTINGS OF RARE WRASSE

The humphead wrasse (Cheilinus undulatus) was seen more frequently around the 

southern islands compared to the northern islands. NOAA photo
34

REMOTE REEF AREAS
35

Before the CRED initiated MARAMP surveys, little information was available on the benthic composition and geologic structure of the remote 

reefs and banks located on the West Mariana Ridge (Fig. 18). The CRED conducted limited reconnaissance surveys at Arakane Reef, Pathfinder 

Reef, and Stingray Shoal in 2003 and at Arakane and Pathfinder Reefs in 2005. Results show, in general, that cover percentages for corals and 

crustose coralline red algae were fairly high (Table 2) at these 3 remote reef areas, relative to other survey areas in the Mariana Archipelago. 

Estimates of coral cover and large-fish biomass were higher at Stingray Shoal with 54.9% and 5.2 kg/100 m2 than at any other area surveyed in 

the Mariana Archipelago, and macroalgal cover was highest on Arakane Reef with 52.3% (Table 2).

ARAKANE REEF

MACROALGAL COVER (%) 52.3 1.6 45.8 2.3 37.3 1.3 29.0 1.1 33.9 1.9

CRUSTOSE CORALLINE 5.3 0.7 3.6 0.6 5.6 0.8 10.0 1.2 1.5 0.4

RED ALGAL COVER (%) 

CORAL COVER (%) 24.1 1.1 11.8 1.6 24.2 0.9 24.8 1.4 54.9 2.7

– – 0.0 0.0 – – 0.1 0.0 – – STRESSED-CORAL

COVER (%)

0.8 0.5 1.4 1.1 1.4 0.4 0.5 0.1 5.2 1.1 LARGE-FISH BIOMASS

(kg/100m2)

2.1 N/A 6.4 2.8 4.7 N/A 7.2 1.9 – – TOTAL FISH BIOMASS

(kg/100m2)

2003

MEAN SE MEAN SE MEAN SE MEAN SE MEAN SE

2005 2003 2005 2003

PATHFINDER REEF STINGRAY SHOAL

Table 2. Overview of benthic composition and fish biomass at 3 remote, submerged reefs on the West Mariana Ridge.

Opposite page: Benthic composition at Arakane Reef captured during a towed-diver survey. NOAA Photo
36

NEWLY MAPPED REEFS: STINGRAY SHOAL

Though there were undocumented reports from fishermen about Stingray Shoal, 

the best available nautical charts in 2003 indicated shallowest water depth in this 

area of 931 fm (1703 m), with no suggestion of a shallow-water coral reef ecosystem. 

Using shipboard and handheld single-beam sonars, the CRED located this littleknown, submerged seamount and mapped a minimum depth of only 13 m. Ensuing 

diver-based biological surveys found predominantly hard substrate with highcomplexity habitats supporting live-coral cover of 40%–100% (Fig. 18). The coral 

reefs of Stingray Shoal were found to support a high-density reef-fish community, 

with large-fish biomass levels of > 5 kg/100 m2 almost 5 times the values observed 

for the northern islands and 23 times the values found around the southern islands. 

Reef sharks accounted for the highest proportion (50%) of large-fish biomass with 

the greatest contribution from the grey reef shark (Carcharhinus amblyrhynchos). 

Though biomass was high, towed-diver surveys discovered derelict fishing gear and 

other man-made objects, clearly indicating that these reefs had been fished.

PHOTO HERE?

0 0.25 km

DEPTH (m)

10–18

18–25

25–35

35–45

45–65

65–75

75–100

100–120

120–140

140–160

160–180

0.1–1

1.1–5

5.1–10

10.1–20

20.1–30

30.1– 40

40.1–50

50.1–62.5

62.6–75

75.1–100

0

LIVE CORAL 

COVER (%)

20 ̊35’ 20 ̊25’ 20 ̊30’

142 ̊20’ 142 ̊25’ 142 ̊30’ 142 ̊35’

The reef at Stingray Shoal, part of the West Mariana Ridge. NOAA photo

Figure 18. Information about Stingray Shoal (top) prior 

to Pacific RAMP surveys and (bottom) live-coral cover 

(%) from towed-diver surveys and depths (m) from sonar 

mapping conducted by the CRED in 2003.
37

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Stevenson C, Katz LS, Micheli F, Block B, Heiman KW, Perle C, Weng K, Dunbar R, Witting J (2007) High apex predator biomass on remote Pacific islands. Coral 

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PACIFIC ISLANDS FISHERIES 

SCIENCE CENTER

 CORAL REEF 

ECOSYSTEM DIVISION