Ofu & Olosega Islands

View of the south side of Ofu (left) and Olosega (right)
(Photograph by Kerry Grimshaw)

By Kerry Grimshaw

We are currently working near the islands of Ofu and Olosega, which are part of the Manu’a group of islands (which also includes Ta'u). They lie approximately 100 km northeast of Tutuila. Although geographically separate, these islands are often referred to together because they are only separated by a narrow straight (approximately 75 m) that is bridged by a shallow coral reef. The twin islands of Ofu (on the west), and Olosega (on the east) are formed by two sharply eroded, overlapping shield volcanoes which gives these islands a dramatic landscape.

Ofu and Olosega are inhabited with the majority of their population (approximately 500 people according to the 2000 census figures) living in the 2 main villages of Ofu and Olosega. An interesting fact I learned from the National Park of American Samoa’s website is that the To’aga archeological site near Ofu Beach has evidence of more than 3,000 years of continuous human occupancy and some modern descendants still live nearby in Ofu Village.

The south-coast beach of Ofu-Olosega with it’s the 4km (2.5mi) stretch of white sand is one of the most beautiful in the South Pacific. Much of the southern coast is also part of the National Park of American Samoa. Along this stretch there are excellent opportunities to snorkel and see some of the 300 species of fish and 150 species of coral that can be found there. Through our shallow water multibeam mapping in 2004 we learned that Ofu and Olosega had a previously uncharted bank top that is less than 300m deep and extends between 0.2 – 2km offshore before dropping to abyssal depths.

Our work this year will continue efforts to monitor fish, coral, algal, invertebrate, and microbial communities at depths ranging from 3 - 30m (10 - 100 feet) deep, as well as a suite of oceanographic observations to better understand the processes influencing these organisms. This data will be compared with that from the other islands we've visited to get an understanding of overall reef health of this area of the world.

Exploring South Bank

by Cristi Richards

Exploratory map of South Bank, American Samoa
(ARC GIS map created by Tomoko Acoba)

South Bank is a sub-surface rise in the ocean floor, or seamount, located approximately 37 miles south of the island of Tutuila. Until recently, there has been little scientific knowledge about the depths, habitats, or living communities of South Bank. Reported minimum depths varied widely and proposed minimum depths from 10 meters (30 feet) to 30 fathoms. Fisherman have known about and frequently visit South Bank in search of wahoo, tuna, and other pelagic fish that are attracted to the shallower depths. However, it appears that South Bank has only very rarely been observed underwater.

South Bank is probably not part of the Samoan chain, in geologic terms, due to its location and age. The Samoan chain has been building from the Pacific plate moving over a hotspot, creating islands in a similar fashion to the Hawaiian chain. The older islands are found to the west with the youngest islands in the east. However, South Bank may be greater than 10 million years old, much older than the other Samoan islands in the area. This is similar to how Swains and Rose Atolls are geologically not part of the Samoan chain either, despite their proximity.

Heliopora coerulea, Blue Coral at South Bank
(Photograph by Cristi Richards)

In an effort to understand more about this area, we spent several nights last week mapping South Bank using the multibeam sonar installed in the hull of the NOAA Ship Hi’ialakai. For the first time, we found that not only is South Bank a shallow spot in the ocean floor, but it is a former coral atoll which drowned at some point. We can tell that it is a former atoll by the submerged barrier reef, a ring of shallower depths, surrounding a deeper lagoon with a minimum depth of approximately 25 meters (85 feet). There was only one previous known dive to the area that reported the presence of a rubble flat and high currents. Based on this, we planned a series of reconnaissance dives with members from the fish, benthic, oceanography and towed-diver survey teams aboard the Hi’ialakai. Our survey techniques had to be modified to accommodate the deeper habitat and reduced dive times. We were able to complete a total of 36 person dives and approximately 5 km of towed-diver surveys along the raised rim, encircling the lagoon. We encountered mostly rubble flats with a high abundance of macroalgae and low coral cover and diversity. The area seemed highly scoured and although we experienced only moderate currents, it is probable that the area is subjected to high currents.

South Bank appears to be a reef that has not kept up with sea-level rise, the sinking of the atoll due to the weight of the original island at its center and the sinking of the Pacific plate. It is unclear what the original reef ecosystem was like and it is a mystery why this reef wasn’t able to keep up with these processes. Rose and Swains Atolls experience similar conditions, yet continue to have thriving reef ecosystems. South Bank is an area that will require more investigation to fully understand the history and processes of this submerged atoll. It is exciting that the new investigational maps and surveys may provide more information to aid in future explorations.

More cool critter sightings

We are currently at the dock in Pago Pago Harbor, waiting for deliveries so that we can continue our work eastward to Ofu / Olosega and Ta'u. Until we are able to leave dock, these are a few more photos of critters that we see while on the reef. All of these pictures were taken in a reef environment between 10 and 20 meters (30 and 60 feet). Enjoy!

Gomophia sp., a type of Sea Star
(Photograph by Molly Timmers)

Crinoid (Photograph by Erin Looney)

Dardanus sp., a type of Hemit Crab
(Photograph by Molly Timmers)

Gymnothorax sp.,  a species of Moray Eel
(Photograph by Erin Looney)

Octopus sp. (Photograph by Molly Timmers)

Herbivores!

by Kaylyn McCoy

Acanthuras triostegus, Convict surgeonfish forming a feeding
aggregation (photograph by Cristi Richards)

As my dive buddy and I are ascending from a dive, she points behind me and puts her hand to her forehead, making the sign for “shark.” I whip around, full of anticipation and hoping to catch a glimpse of a 12 foot Tiger Shark (Galeocerdo cuvier), preferably swimming away. But, it’s just a three foot Black-Tip Reef Shark (Carcharhinus melanopterus), cruising around below us. Big fish like sharks and jacks are exciting and important, but we can’t forget about the little guys! Above is a picture of a school of Convict surgeonfish (Acanthurus triostegus). These fish are herbivorous, and feed on the algae that grows on the reef. Certain species like these Convict Tang form dense feeding schools possibly to overwhelm smaller, but incredibly aggressive damselfish defending their territories.  Herbivorous fish play an important role in maintaining equilibrium in an ecosystem. Without these fish, certain species of algae can grow out of control, smothering the corals of the reef.

Scarus xanthopleura, Red Parrotfish
(Photograph by Paula Ayotte)

Another algae muncher that we see on the reef is the red parrotfish (Scarus xanthopleura). These fish have a specialized “beak” or dental plate used for scraping the algae off of the reef. Some parrotfish simply scrape the algae off the surface while other, usually larger species, bite of sizable chunks of the reef. Sometimes when we are counting fish, we can actually hear them feeding. Much of the sand you see on a coral reef may have passed through the belly of a parrotfish at one time or another.

Some areas of the Pacific are considering protecting specific herbivorous fish to help control invasive algae. So while it’s exciting to think about a shark snacking on a poor unsuspecting fish, don’t forget about the importance of the herbivores, the lawn mowers of the reef!

Sighting the rare Guitarfish

by Marie Ferguson

A Shovelnose Guitarfish (photograph courtesy of www.Elasmodiver.com)

A few days ago, while conducting a fish REA (Rapid Ecological Assessment) survey my dive buddy, Rusty Brainard, and I enjoyed a rare sighting of a Rhynchobatus djiddensis, Whitespotted Guitarfish, along the northwest side of Tutuila. The guitarfish was spotted while conducting a deep SPC (Stationary Point Count) survey, at approximately 70 feet. We had just completed our final SPC and were on our way to the surface for our safety stop when the 4 foot long guitarfish swam by us with a remora (‘shark sucker’) attached to its underside. Up until this point, a Whitespotted Guitarfish has never been observed or recorded by our research team in American Samoa or other locations during the many thousands of surveys we have conducted across the Pacific Islands over the past decade.

Rhynchobatus djiddensis belongs to the Rhinobatidae or Guitarfish family. It is unique in that it resembles a cross between a shark and a ray with the anterior or front half of its body looking like a ray while the posterior or rear half looking like a shark. Like other rays, guitarfish have small mouths with teeth that are flat and pavement-like and generally prey on crabs, cephalopods and small fishes. Most guitarfish species have been known to occur on continental shelves or insular shelves of large islands in roughly 2 to 50 meters of water depth. Due to the variation over its range, this type of guitarfish has been divided into approximately 5 to 6 species. Little is known about the biology of this species, however data collected has suggested that it does have a low fecundity and very slow growth rate.

According to the ICUN Red List of Threatened Species, the large size and nearshore areas that this species inhabits make it highly susceptible to gillnet and shallow-water trawl fishing. In several parts of the world, such as Tanzania, Rhynchobatus djiddensis is being exploited mainly for its fins and is being commercially fished in bottom-set gillnets. Data recorded has also shown that this species is caught as bycatch in prawn trawls. Other documented areas where this species of guitarfish is either fished intentionally or as bycatch include shores off of Kenya, Mozambique, East Africa and the Middle East in the Western Indian Ocean, many areas in which policing and regulatory enforcement is often limited. The ICUN Red List of Threatened Species has evaluated this species as ‘vulnerable’ due to the “commercially high value and growing demand for its fins, restricted nearshore habitat as well as its limiting life history characteristics”.

If you are out diving or snorkeling and see this elegant and fascinating creature, then make sure to grab a photo and relish in the moment of a rare sighting!

For more pictures of Guitarfish, check out Elasmodiver.com who kindly granted us use of the above photograph.

Lasting Effects of a Shipwreck

text and photographs by Cristi Richards

Typical site at Rose Atoll dominated by coralline algae
(light pink).

Coralline algae or CCA typically dominate the reefs surrounding Rose Atoll, making up 50% - 75% of the benthic cover according to preliminary results of our 2010 surveys. Scleractinian or hard corals comprise another 5% - 30% of the substrate. At the start of each dive, we are greeted by a light pink landscape accentuated by the greens and reds of fleshy macroalgal genera and the browns, purples and yellows of various genera of coral. The coralline algae are a combination of several genera including Mastophora, Porolithon and Peysonnelia.

In general, coralline algae play many important roles in the ecology of a reef ecosystem. They are a food source for many reef inhabitants including parrotfish, sea urchins and mollusks. Coralline algae also act as a stabilizing component or ‘cement’ for the reef and several genera of coral larvae will selectively colonize patches of this algae.

Site of ship wreck at Rose Atoll.
Brown and darker areas are dead coral
and CCA covered by turf algae and
cyanobacteria.

In October 1993, a 135-foot Taiwanese longline fishing vessel ran aground at Rose Atoll, releasing 100,000 gallons of diesel and 500 gallons of oil into the surrounding waters, across the reef flats and into the lagoon. The initial spill killed large numbers of giant clams, urchins, sea cucumbers and the dominant benthic organism, coralline algae, as well as endangering the health of 12 species of migratory seabirds and the threatened green sea turtle.

However there is still damage occurring to the reef ecosystem, 17 years later. Although there are on-going efforts to remove metal wreckage, with 37.5 tons having been removed to date, there are still iron contaminants being released by the remaining portions of the vessel. When we surveyed this site yesterday, it was immediately obvious that the habitat was vastly different than those seen at other areas of the Atoll. The percent cover of coralline algae was up to 50% less than at other areas, with corals comprising less than 1% of the benthic cover. We also noted an increase in the occurrence of cyanobacteria and turf algae.

Cyanobacteria covering a
colony of Favia stelligera

The iron contamination continues to cause a cyanobacteria or blue-green algal, bloom that is detrimental to the health of both the coralline algae and the corals. The cyanobacteria compete for space with coral larvae by settling on the CCA in thick mats, thus blocking the larvae’s access to the CCA. The thick mat also blocks sunlight from reaching the CCA. In addition, strands of cyanobacteria that detach from the bottom can settle and develop directly on corals. The end result is that corals and CCA are smothered and eventually die, thus changing the overall benthic composition of the reef. The presence of the increased levels of cyanobacteria causes a physical and chemical environment that is not suitablefor the growth of the original reef inhabitants.

This scar at the site of the shipwreck can still be seen today as a reminder of how fragile these ecosystems really are. With continued surveys, this will be a valuable documentation of how long it takes areef to recover from a wreck such as this.

Rose Atoll, part 2

by Kerry Grimshaw
photographs by Jean Kenyon

Exposed coralline algae at Rose Atoll

Our day began with a beautiful sunrise and light winds at Rose Atoll, which is one of the smallest atolls in the world and is diamond shaped. The outer reef slope around Rose is steep down to depths greater than 200 meters (~650 feet). The atoll also encompasses 2 small islets named Rose and Sand Islands. Perhaps the most outstanding feature of the atoll is the bright pink color of the exposed reef. The reef gets itís pink hue from the dominant crustose coralline algae Porolithon. This crustose coralline alage is one of the primary reef-building species at Rose Atoll.

Rose Atoll has a coral and fish community different from elsewhere in American Samoa. Currently the US Fish and Wildlife Service reports that there are 113 species of coral and about 270 fish species recorded at Rose. The atoll also supports the largest populations of giant clams, nesting seas turtles and rare reef fish species in the territory. In addition, humpback whales, pilot whales and various species of dolphin have been seen in the waters surrounding Rose Atoll.

Reefs dominated by coralline algae

The 2 islets are not without their own claims of importance. Rose Island is home to a grove of Pisonia trees on it, which is the only remaining Pisonia stands in Samoa. Rose and Sand Islands provide vital nesting habitat to the most important seabird colony in the region, including 12 federally protected migratory seabirds. Some of the birds that utilize Rose Atoll are the Red-footed Boobies, Greater Frigate birds, Lesser Frigate birds, Black Noddies, White Terns, Reef Herons and Red-tailed Tropic birds.

Since Rose Atoll is very remote and extremely unique due to its terrestrial and marine communities it provides an excellent place for scientific research. As such, our days spent at Rose Atoll are always a highlight of our cruise!