EPA's Vessel General Permit and Small Vessel General

than 12% on many reefs (Rogers et al. 2008). Coral mortality due to the 2005 bleaching event
was more severe than at any time in the last 40 years of monitoring in USVI (Woody et al.
2008). Staghorn corals suffered widespread mortality associated with the 2005 bleaching event
and current monitoring data does not indicate significant recovery (Rothenberger et al. 2008,
Woody et al. 2008). Overall, colonies of Atlantic Acropora have declined by up to 98% and live
colonies were no longer present at many study sites in the USVI following the 2005-2006
bleaching event.
Natural threats
The overriding threats are disease, temperature-induced “bleaching” (loss of zoozanthelle), and
physical damage from hurricanes (Carpenter et al. 2008, Mallela and Crabbe 2009, Baskett et al.
2010). Disease is widespread, episodic, and unpredictable in its occurrence and results in high
mortality. This is primarily due to a disassociation of zoozanthelle from coral tissue. Just prior to
this, coral epithelium and gastrodermis tissue begins to decay and die, likely as a result of stress
to the individual coral (Ainsworth et al. 2008). Optimal water temperatures range from 77º to
84ºF, with mortality observed at 61º and 96ºF (Jaap 1979, Roberts et al. 1982). High light levels
can also induce mortality. Synergistic analyses have found that high temperature increases the
risk of colony mortality under a variety of sediment loading conditions, but excessive sediment
appears to reduce mortality risk under high light and temperature regimes, possibly by reducing
exposure to these stressors (Anthony et al. 2007, Boyett et al. 2007). High sediment with
otherwise good light and temperature conditions appears to increase colony mortality (Anthony
et al. 2007). Elkhorn coral require near oceanic salinities (34 to 37 parts per thousand). High
temperature or rapid heating can result in heat shock and alter cellular metabolism within the
coral as well as possibly hinder immune response or the ability of zoozanthelle to thrive
(Rodriguez-Lanetty et al. 2009, Middlebrook et al. 2010). Bleaching can occur due to adverse
environmental conditions (Ghiold and Smith 1990, Williams and Bunkley-Williams 1990) and is
currently a significant factor in deteriorating coral reef health. The major El Niño/La Niña
Southern Oscillation cycle in 1997-1998 resulted in a large bleaching event in the Caribbean and
the Atlantic, as well as massive losses of corals in the Indian Ocean and Western Pacific
(Wilkinson and Souter 2008). However, the most significant bleaching event to date in the
USVI and other areas of the Caribbean occurred in 2005 when sea surface temperatures
exceeded the 29.5°C coral bleaching threshold for twelve weeks, and maximum temperatures
exceeded 30°C (Woody et al. 2008). Bleaching occurred in twenty-two species, including
Acropora, over a wide range of depths and affected more than 90% of the coral cover, on
average, between July and November in the USVI (Woody et al. 2008). Wide-scale mortality,
with some areas reaching 95% of coral colonies affected, resulted from this event (Wilkinson
and Souter 2008). The US Virgin Islands, a location of Acropora critical habitat, experienced
greater than 50% mortality of corals, the greatest level ever recorded. Puerto Rico and Florida
(additional areas of Acropora critical habitat) also experienced disease rates of 50% of coral
colonies or greater. Bleaching was associated with unusually warm waters in the region.
Encouragingly, bleaching events can lead to increased thermal tolerance in affected reefs,
meaning that subsequent bleaching events are not as severe (Maynard et al. 2008). A record
number of hurricanes also caused extensive damage to coral reefs; the prevalence of hurricanes
and subsequent coral reef damage has been linked to climate change (Wilkinson and Souter
2008). Ocean acidification is also a threat due to the increased solubility of calcium carbonate in
192