Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
Coral communities and reefs are dynamic and changing ecosystems subject to natural and humaninduced
disturbances. Natural disturbances that have had significant impacts on coral communities and
reefs in the Mariana Islands include storm-related damage caused by frequent typhoons, ENSO events,
outbreaks of the crown-of-thorns starfish (COTS) (Acanthaster planci, a corallivorous predator),
freshwater runoff, recurrent earthquakes, and volcanic activity (Richmond 1994; Birkeland 1997; Paulay
2003; Abraham et al. 2004; Bonito and Richmond Submitted). Human-induced disturbances on reefs in
the Mariana Islands include erosion, sedimentation, polluted runoff (input of nutrients), exposure to warm
water (global warming and thermal effluents) leading to bleaching, overfishing, anchor damage, tourismrelated
impacts, ship groundings, and certain military activities (Birkeland 1997; Houk 2001; Richmond
and Davis 2002; Starmer et al. 2002; Paulay 2003; Quinn and Kojis 2003; DoN 2003a; Abraham et al.
2004).
Natural Disturbances—Coral communities and reefs on the eastern, windward side of the islands are
exposed to dominant winds, strong wave action, and storms (including typhoons). Corals found above the
30 m isobath on windward coasts are conditioned to withstand heavy wave action and will recover if
damaged (Randall 1985; Birkeland 1997; Paulay personal communication). Typhoons can cause
substantial damages to corals on windward coasts (Paulay personal communication). Corals in this
exposed area of the reef typically include encrusting or massive growth forms of corals as well as
columnar, platy and branching growth forms. Exposed windward reef fronts are dominated by three
growth forms of Acropora: corymbose (colonies are composed of horizontal branches and short to
moderate vertical branchlets that terminate in a flat top), digitate (colonies are composed of short, nonanastomosing
branches like the fingers of a hand), and caespitose (bushy, branching, possibly fused
branches) (Paulay personal communication). There are currently more acroporids on reefs at Unai
Dankolo than in sheltered bays of Lau Lau Bay (southeastern Saipan) or Sasanhaya Bay (southwestern
Rota) (Houk personal communication). Reef growth in the CNMI at wave exposed sites is more
conditioned by the availability of a suitable habitat and an underlying substrate than by wave action
(Randall 1985; Houk personal communication).
The disruption of the trade wind pattern during ENSO events has caused sea level to drop in the Mariana
Islands and exposed shallow corals and other reef organisms over a prolonged time which has caused
mass mortality (Birkeland 1997). Further, ENSO events have produced unusually high seawater
temperature which may have caused coral bleaching (Richmond and Davis 2002). The bleaching of
corals has been recorded in the Marianas since 1994, and some bleaching events have caused coral
mortality (Paulay and Benahayu 1999; Richmond and Davis 2002; Starmer et al. 2002). In 1994, corals
bleached on all reefs of Guam (Paulay and Benahayu 1999). While pocilloporids and acroporids incurred
severe bleaching on Guam in 1994; Paulay and Benayahu (1999) observed no stony coral mortality
during that bleaching event. The chronic outbreaks and predation of COTS on corals (including Acropora,
Montipora, and Pocillopora) have also caused coral mortality. In the fore reef zone in sheltered areas,
massive corals (Porites and Favia) that are more resistant but not immune to A. planci have replaced the
corals decimated by A. planci (Quinn and Kojis 2003; Bonito and Richmond Submitted; Paulay personal
communication). Weather and wave action-exposed reefs (e.g., Unai Dankolo, Tinian) appear to be more
resilient to COTS outbreaks compared to reefs in sheltered bays (e.g., Lau Lau Bay, Saipan; Sasanhaya
Bay, Rota) (Houk personal communication).
Other sources of coral mortality and degradation are freshwater runoff and seismic and volcanic activity.
Freshwater runoff naturally affects reefs in the study area during the rainy season (Richmond and Davis
2002). Areas particularly affected by sedimentation following heavy rainfall include the Ugum River
watershed (southeast Guam), the south coast of Guam, Lau Lau Bay (southeastern Saipan), and Opyan
Beach (southern Saipan) (Houk 2001; Richmond and Davis 2002; Abraham et al. 2004). Reefs in the
islands north of FDM are likely to have been impacted by frequent and recent seismic and volcanic
activity (Birkeland 1997; USGS 2004, 2005b). The southern islands (Guam to FDM) have not been
impacted by recent volcanic activity but by recurrent seismic activity as witnessed in 1993 in Guam (EERI
1993; Birkeland 1997).
Human-Induced Disturbances—The increased land-clearing and construction of coastal roads, housing,
and tourism-related facilities have caused the increased erosion, sedimentation and runoff (particularly
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