Coral reef monitoring surveys completed around the islands and atolls of American Samoa

By Bernardo Vargas-Ángel
Operating area of the HA-15-01 ASRAMP Legs II and III.

Operating area of the HA-15-01 ASRAMP Legs II and III.

With work complete in the U.S. territory of American Samoa, the NOAA Ship Hi‘ialakai stopped in the port of Pago Pago Harbor for a short pause between Legs III and IV of PIFSC cruise HA-15-01. Led by the PIFSC Coral Reef Ecosystem Division (CRED), this mission marks the seventh monitoring cruise in the American Samoa region by PIFSC staff and partner agencies since 2002.

Activities to monitor the coral reef ecosystems of American Samoa began on February 17 and concluded on March 30, completing Leg I and comprising Legs II and III of this longer Pacific Reef Assessment and Monitoring Program (Pacific RAMP) expedition. Around Tutuila, Aunu‘u, Ofu-Olosega, Swains, and Ta‘u Islands, and Rose Atoll, the CRED scientists conducted ecosystem surveys of fishes, benthic and coral communities, and microbes, along with the deployment of oceanographic instruments and biological installations.

Shallow coral reef communities at Rose Atoll, conspicuously dominated by the pink-colored encrusting coralline algae.

Shallow coral reef communities at Rose Atoll, conspicuously dominated by the pink-colored encrusting coralline algae.

A pair of the reticulated butterflyfish (Chaetodon reticulatus) at Swains Island.

A pair of the reticulated butterflyfish (Chaetodon reticulatus) at Swains Island.

At Rapid Ecological Assessment (REA) sites, surveys for reef fishes and benthic coral communities documented the richness, abundance, density, and sizes of the biota and assemblages as well as the percent composition of bottom-dwelling organisms and the health conditions of coral colonies. Broad-scale towed-diver surveys recorded observational data on large-bodied fishes (>50 cm total length), percent composition of the seafloor, conspicuous macroinvertebrates, and coral stress.

In addition, teams studied microbial communities, diversity of cryptic invertebrates, water temperature, salinity, and carbonate chemistry. They are also working to assess the potential early effects of ocean acidification on cryptobiota (e.g. small, hidden organisms) and the rates of reef carbonate deposition, bioerosion, and coral calcification.

Across the Territory of American Samoa, this mission completed more than 60 towed-diver surveys totaling more than 130 km of coastline, 325 fish surveys, and 180 benthic surveys. The Ocean and Climate Change team deployed four climate monitoring stations around Tutuila, and four around Ofu-Olosega and Ta‘u, containing arrays of subsurface temperature recorders (STRs), calcification accretion units (CAUs), autonomous reef monitoring structures (ARMS), and bioersion monitoring units (BMUs). Critical findings during this mission included observations of coral bleaching, local warm water temperatures, and the number and distribution of corallivore crown-of-thorns sea stars (COTS).

Bleached and partly dead staghorn Acropora outside Fagatele Bay, Tutuila, American Samoa.

Bleached and partly dead staghorn Acropora outside Fagatele Bay, Tutuila, American Samoa.

Bleaching of scleractinian corals, averaging 10% of colonies, was reported in shallow (3-6 m) reef habitats of Tutuila Island—particularly within Fagatele and Fagasa Bays—as well as the southwest coast of the island and primarily affected species of branching and table Acropora, Isopora, Montastrea, Porties, and Pocillopora. Although bleaching conditions did not appear to be widespread, current NOAA Coral Reef Watch forecasts predict persistent warm conditions, which could potentially result in more severe and extensive coral bleaching across the region. CRED scientists recorded only occasional sightings of COTS and their feeding scars on corals, despite the ongoing outbreak conditions reported by staff of the National Park Service and the National Marine Sanctuary of American Samoa. In contrast to other regions where COTS outbreaks have been reported by CRED scientists, including Guam, the Commonwealth of the Northern Mariana Islands, and Kingman Reef, it appears that in American Samoa, the sea stars prefer to feed at night and hide under ledges and overhangs during the day, making them inconspicuous during daylight surveys.

Preliminary results from surveys conducted by CRED fish team divers, during PIFSC cruise HA-15-01, are provided in the fish monitoring brief below.

Pacific Reef Assessment and Monitoring Program
Fish monitoring brief: American Samoa 2015

By Adel Heenan

About this summary brief
The purpose of this summary brief is to outline the most recent survey efforts conducted by the Coral Reef Ecosystem Division (CRED) of the NOAA Pacific Islands Fisheries Science Center as part of the long-term Pacific Reef Assessment and Monitoring Program (Pacific RAMP). More detailed survey results will be available in a forthcoming status report.

Sampling effort

  • Ecological monitoring took place in American Samoa from February 15 2015 to March 30 2015.
  • Data were collected at 338 sites. Surveys were conducted at Ofu and Olosega (n=52), Rose (n=47), Swains (n=32), Tau (n=46) and Tutuila (n=162).
  • At each site, the fish assemblage was surveyed by underwater visual census and the benthic community was assessed.

Overview of data collected
Primary consumers include herbivores (which eat plants) and detritivores (which bottom feed on detritus), and secondary consumers are largely omnivores (which mostly eat a variety of fishes and invertebrates) and invertivores (which eat invertebrates).

Figure 1. Mean total fish biomass at sites surveyed.

Figure 1. Mean total fish biomass at sites surveyed.

Figure 2. Mean hard coral cover at sites surveyed.

Figure 2. Mean hard coral cover at sites surveyed.

Spatial sample design
Survey site locations are randomly selected using a depth-stratified design. During cruise planning and the cruise itself, logistic and weather conditions factor into the allocation of monitoring effort around sectors of each island or atoll. The geographic coordinates of sample sites are then randomly drawn from a map of the area of target habitat per study area. The target habitat is hard-bottom reef, the study area is typically an island or atoll, or in the case of larger islands, sectors per island, and the depth strata are shallow (0-6 m), mid (6-18 m), and deep (18-30 m).

Sampling methods
A pair of divers surveys the fish assemblage at each site using a stationary-point-count method. Each diver identifies, enumerates, and estimates the total length of fishes within a visually estimated 15-m-diameter cylinder with the diver stationed in the center. These data are used to calculate fish biomass per unit area (g m-2) for each species. Mean biomass estimates per island are calculated by weighting averages by the area per strata. Island-scale estimates presented here represent only the areas surveyed during this cruise. For gaps or areas not surveyed during this cruise, data from this and other survey efforts will generally be pooled to improve island-scale estimates.

Figure 3. Mean consumer group fish biomass (± standard error). Primary consumers are herbivores and detritivores, and secondary consumers are omnivores and invertivores.

Figure 3. Mean consumer group fish biomass (± standard error). Primary consumers are herbivores and detritivores, and secondary consumers are omnivores and invertivores.

Figure 4. Mean fish biomass per size class (± standard error). Fish measured by total length (TL) in centimeters (cm).

Figure 4. Mean fish biomass per size class (± standard error). Fish measured by total length (TL) in centimeters (cm).

Each diver also conducts a rapid visual assessment of reef composition, by estimating the percentage cover of major benthic functional groups (encrusting algae, macroalgae, hard corals, turf algae and soft corals) in each cylinder. Divers also estimate the complexity of the surface of the reef structure, and they take photos along a transect at each site that are archived to allow for future analysis.

About the monitoring program
Pacific RAMP forms a key part of the National Coral Reef Monitoring Plan of NOAA’s Coral Reef Conservation Program (CRCP), providing integrated, consistent, and comparable data across U.S. Pacific islands and atolls. CRCP monitoring efforts have these aims:

  • Document the status of reef species of ecological and economic importance
  • Track and assess changes in reef communities in response to environmental stressors or human activities
  • Evaluate the effectiveness of specific management strategies and identify actions for future and adaptive responses

In addition to the fish community surveys outlined here, Pacific RAMP efforts include interdisciplinary monitoring of oceanographic conditions, coral reef habitat assessments and mapping. Most data are available upon request.

For more information
Coral Reef Conservation Program
Pacific Islands Fisheries Science Center
CRED publications
CRED monitoring reports
CRED fish team
Fish team lead and fish survey data requests: ivor.williams@noaa.gov, adel.heenan@noaa.gov

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