New Analysis of Survey Images Shows Changes in Benthic Community on Guam Coral Reefs

Aggregated track lines (red) show location of towed-diver surveys conducted in 5 regions around Guam during 2003-2011.
Aggregated track lines (red) show location of towed-diver surveys conducted in 5 regions around Guam during 2003-2011.

Since its first Pacific Reef Assessment and Monitoring Program (Pacific RAMP) research expedition in 2000, NOAA Fisheries has used towed-diver surveys to provide large-scale assessments of benthic habitats in coral reef ecosystems across the Pacific Islands Region. The towed-diver survey method involves towing scuba divers behind a small boat at a constant speed (~ 1.5 knots) and depth (~ 15 m standard target depth), with individual surveys lasting ~ 50 min and covering ~ 2 km of habitat. A geographical positioning system (GPS) track of each survey launch path is recorded, and digital still photographs of the benthos are captured every 15 seconds along the survey track. Visual observations by the divers record other aspects of the benthic community, such as the abundance of crown-of-thorns sea stars (COTS, Acanthaster planci), which prey on corals.

Towed-diver surveys are part of an integrated, multidisciplinary suite of research activities during Pacific RAMP expeditions, which are led by the PIFSC Coral Reef Ecosystem Division (CRED). The research is funded by NOAA's Coral Reef Conservation Program.

New data from the photo images collected during Pacific RAMP cruises is improving our understanding of island-wide trends in the benthic fauna around Guam. By analyzing digital images collected during towed-diver surveys from 2003 to 2011, CRED scientists have estimated benthic cover (% of habitat occupied) around the island for each major functional group of benthic organisms—live hard corals, algae (fleshy macroalgae and turf algae combined), and crustose coralline red algae. Image data from about 100 towed-diver surveys covering 5 geographic regions around Guam provide summary information on the overall island-wide trends in benthic cover by each functional group. These new benthic image analyses provide important baseline and trend data that are pivotal to implementation of NOAA Fisheries' Habitat Blueprint initiative for Guam.

The Guam benthic images indicate a slow, steady, island-wide decline in live coral cover during the period of surveys, from 19.4% (SE 2.6) in 2003 to 10.4% (1.4 SE) in 2011 (Fig. 5). The greatest change in coral cover (~ 20%) was observed between surveys in 2005 and 2007, the same time period in which visual observations from towed-diver surveys noted a steep increase in the abundance of COTS island-wide.

Benthic images from towed-diver surveys at Guam from 2003-2011 showed changes in percent cover of live hard corals, crustose 
               coralline red algae (CCA), and fleshy macroalgae and turf algae combined (algae) over the 8-yr period.  Concurrent data on density 
               of crown-of-thorns sea stars (COTS, Acanthaster planci) suggest the negative effect of COTS on coral cover and associated 
               growth of algae.  Mean density of COTS is given as number of individuals/100 m^2.  Error bars indicate ± standard 
               error of the mean.
Benthic images from towed-diver surveys at Guam from 2003-2011 showed changes in percent cover of live hard corals, crustose coralline red algae (CCA), and fleshy macroalgae and turf algae combined (algae) over the 8-yr period. Concurrent data on density of crown-of-thorns sea stars (COTS, Acanthaster planci) suggest the negative effect of COTS on coral cover and associated growth of algae. Mean density of COTS is given as number of individuals/100 m2. Error bars indicate ± standard error of the mean.