New data on benthic cover at Guam now available

Since the first cruise of the Pacific Reef Assessment and Monitoring Program (Pacific RAMP) in 2000, NOAA Fisheries has used towed-diver surveys to provide large-scale assessments of benthic habitats. This survey method involves towing scuba divers behind a small boat at a constant speed (~1.5 kt) and depth (~15 m standard target), with individual surveys lasting ~50 min and covering ~2 km of habitat. A GPS track of the path of each survey launch is recorded, and digital still photographs of the benthos also are captured every 15 s. Towed-diver surveys are part of an integrated, multidisciplinary suite of research activities conducted as part of Pacific RAMP, which is led by the PIFSC Coral Reef Ecosystem Division (CRED) and funded by NOAA’s Coral Reef Conservation Program. Pacific RAMP cruises have been conducted biennially at Guam since 2003.

Figure 1. Map of towed-diver-survey tracks and five geographic regions around Guam from CRED research cruises conducted in 2003–2011.

To improve understanding of islandwide trends, CRED scientists have determined benthic cover around Guam at the functional level—corals, algae (fleshy macroalgae and turf algae combined), and crustose coralline red algae—through analyses of digital images collected during Pacific RAMP towed-diver surveys conducted from 2003 to 2011. The data set generated from these image analyses summarizes the overall islandwide trends on the basis of ~100 towed-diver surveys completed in five geographic regions (north, east, south, west, and northwest; Fig. 1). These new analyses represent an important baseline and trend data set that is pivotal to the implementation of NOAA Fisheries’  Habitat Blueprint initiative for Guam.

Results from these image analyses indicate a slow, steady, islandwide decline in live coral cover from 19.4% (2.6 standard error of the mean [SE]) in 2003 to 10.4% (1.4 SE) in 2011 (Fig. 2). The greatest change in coral cover (~20%) was observed between 2005 and 2007, the same time period in which visual observations from towed-diver surveys noted steep increases in densities of crown-of-thorns seastars (Acanthaster planci) islandwide.

These image-analysis data are available currently upon request from the CRED information services team. Make inquiries to

 By Bernardo Vargas-Ángel

Figure 2: Temporal comparison of mean values of cover (%) of live hard corals, algae (fleshy macroalgae and turf algae combined), and crustose coralline red  algae (CCA) from analyses of benthic images collected during towed-diver surveys conducted at Guam in 2003–2011. Mean densities (individuals/100 m2) of crown-of-thorns seastars (Acanthaster planci; COTS) are provided for reference. Error bars indicate standard error of the mean (± 1 SE).

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