Perspectives Of Underwater Flight: Towed-Diver Surveys Around The Line Islands

By Jake Asher and Molly Timmers

Towed-diver Kevin Lino surveys the fish of Jarvis Island

How can scientists get a better sense of what’s living on the bottom or swimming above coral reefs on an island-wide scale?  Detailed surveys examining benthic and fish assemblages at specific sites are one way; however, if you're interested in a fast, effective, and extensive method for assessing and monitoring coral reef health over a large spatial scale, towed-diver surveys are for you.
 

The towed-diver survey methodology is a unique and integrated data collection method for mesoscale assessment of benthic coral reef habitats.  The method utilizes SCUBA divers pulled behind a small boat at depth, covering enormous areas of terrain each day, sometimes surveying close to 18 hectares (18 kilometers x 10 meter survey swath).  Multiply that out over a 30-day cruise and you can imagine what the towed-diver team sees!

Towed-diver forward-facing view; top panel; Typical photograph from the benthic towed-diver.

What’s on a towed-diver board? Benthic divers have a bottom-mounted camera that collects still photographs of the benthic habitat every 15 seconds, while fish divers have a video camera that records forward-facing video for the duration of the 50-minute survey. Temperature and depth are recorded every 5 seconds throughout the survey (cylinder on the left side).  Gauges/timers tell the diver how long  the divers have been down for, how deep they are, and sound a 5-minute alarm when each survey segment is completed.  Finally, both benthic and fish observations are tallied on the datasheet located on the right-hand side of the board.

Towed-divers typically fly around the entire forereef perimeter of the smaller islands, and stagger their surveys along larger ones.  In some cases, divers also survey backreef or lagoon habitats (e.g. at atolls) or terraces.

Towed-diver observational data can be processed relatively quickly in order to get a general picture of what the reefs are comprised of (e.g. hard and soft coral cover, stressed coral, algae, etc.) and what fishes are present, while the processing of  photographic and video data sets occurs back in the lab in Honolulu.  Given the spatial extent of surveys conducted on this cruise, It would be impossible to convey everything recorded thus far; however, here are a few of the benthic highlights from each of the island ecosystems:

Jarvis Island

• Jarvis was largely dominated island-wide by the species of hard coral Montipora aequituberculata.
• The west side has an extensive population of Sinularia (soft coral) found nowhere else around the island, extending ~ 300 meters north-south at the 50 foot survey depth, and covering nearly 100% of the bottom.  
• Live, branching Pocillopora and Acropora coral fragments were found along the south-facing shore, suggesting a recent weather/wave event.
• All macroinvertebrates (crown-of-thorns sea stars, sea cucumbers, giant clams, urchins) counts were low.  While the reasons for this remain unclear, potential causes include predation pressures or lack of suitable benthic habitat. 

Images obtained from towed-diver surveys of Jarvis Island: Montipora aequituberculata , left panel; Sinularia dominance on the western side of the island, upper right panel; Broken Pocillopora colonies, lower right panel

Palmyra Atoll

• While towed-diver surveys recorded localized proliferation of a number of hard coral genera, the majority of benthic segments were dominated by a species of Porites along the forereef and western terrace.
• Low levels of bleaching were observed within numerous genera around Palmyra;  additional analysis of towed-diver photographs will further explore the extent of coral bleaching around the atoll..
• Visible macroinvertebrates (crown-of-thorns sea stars, sea cucumbers, giant clams, urchins) were nearly absent from our surveys. 

Images obtained from towed-diver surveys of Palmyra Atoll. Partially bleached coral, left panel; forereef, left side; the forereef benthic and fish community, upper right panel;
Missing macroinvertebrates, lower right panel

Kingman Reef

• Hard and soft coral cover varied between habitats, and varied depending upon depth and exposure to wave energy.  However, overall hard coral cover for all pooled surveys was nearly identical as all pooled surveys around Palmyra.
• The southeastern backreef continues to harbor the highest concentration of giant clams (Tridacna spp.) of anywhere we surveys around the Pacific.
• The east-side backreef adjacent to the shipwreck showed a dramatic increase in cyanobacteria at 50’ – 60’ since the previous 2008 surveys, along with the presence of a fish aggregation device (FAD) not seen before. 

Images obtained from towed-diver surveys of Kingman Reef. Fish Aggregation Device (FAD) seen from below, left panel; Cyanobacteria bloom near the shipwreck , middle panel; Giant Clams along the southeastern backreef, right panel