Study looks at CNMI corals over a 12-year period

A study examining coral reefs in the CNMI over a 12-year period looked at how natural disturbances, local stressors, and existing environment factors played into the long-term dynamics of the reefs here.

The study, published in August this year, found more coral decline on Saipan than on Rota, which was may likely be explained by the poorer water quality, runoff-caused nutrient enrichment on Saipan as well as smaller fish size on the island.

The study focused on the “significant coral loss” in the CNMI between 2003 and 2006 with the passing of several typhoons, and high densities of crown of thorn starfish, which are known to prey on coral.

No “potential synergies or linkages” between storm activity and COTS densities were proven, though. The study said the failure of reef recoveries were best explained through local stressors. These stressors transformed reefs into “persistent stands of turf and macroalgae,” making it less conducive for coral recovery, the study said.

The study found that declining trends were strongest for gently sloping reefs, which largely occur in areas of low wave exposure. These reefs are also “centers for reef based tourism,” the study said, and highlights a need to improve upon the “compromised” fish, and grazing urchin populations, among other things, where water quality problems occur, according to the study.

COTS was the “primary driver” of disturbances during the mid-2000’s, the study claimed. However, while COTS densities were similar during these years for Saipan and Rota, more coral decline on Saipan was found.

The study said this could be because Rota had less coral or prey to begin with, or fish on Rota had larger biomass or body-size, which reportedly is associated with reduced impacts from COTS disturbances.

Also, water quality on Rota was consistently better over the past decade compared to Saipan, and nutrient enrichment from runoff has contributed to localized COTS populations on Saipan, the study said.

The study also highlighted what it called “an interesting and emerging association” between high COTS impact, smaller-bodied fish populations, and low wave exposure.

It explained that even if similar biomass of fish existed, fish made up of smaller-bodied species have a reduced ecological function in coral reef food webs. It referred to a study that showed large fish species grazing “disproportionally” on algae, while smaller fish grazed on detritus or substrate.

The study said these large fish species represented “highly desirable food fish” in the CNMI. It said local market studies showed the fishes’ “declining size and abundance” over the last couple of decades.

It said size and functional diversity of the fish were influential to coral reef recovery patterns, which made their improved management desirable.

Scientists who led the study were from the University of Guam, Pacific Marine Resources Institute, and CNMI Bureau of Environmental Quality.

The study is titled “Coral Reef Disturbance and Recovery Dynamics Differ across Gradients of Localized Stressors in the Marianas Islands.” It can be found online.