EPA's Vessel General Permit and Small Vessel General

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. In 2005, wide-scale bleaching occurred
throughout the Caribbean with wide-scale mortality, with some areas reaching 95% of coral
colonies affected (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 even slightly more acidic sea
water (thereby eroding the shells which form coral hard parts)(Anthony et al. 2008, De’ath et al.
2009, Wei et al. 2009, Crawley et al. 2010). Acidification also reduces the thermal tolerance of
corals, meaning that bleaching can occur at lower temperatures (Anthony et al. 2008).
Hurricanes can cause wide-scale inhibition of recruitment in years following storm passage as
well as physical damage to coral colonies themselves (Mallela and Crabbe 2009).
Anthropogenic threats
Threats to elkhorn coral also include eutrophication, sedimentation, anchoring, which degrade
coral condition and increase synergistic stress effects (e.g. bleaching).
Critical habitat
NMFS published a final rule to designate critical habitat for elkhorn and staghorn corals on
188