1. The Effects of Anchor Damage and Coral Disease on Sea Fans in the British Virgin Islands
Jessica Perreault1 and Graham Forrester2
1Department of Biological Sciences, 2Department of Natural Resources Science, University of Rhode Island
As the defining feature of the Caribbean’s
underwater infrastructure, coral reefs face a
multitude of physical and environmental
threats1. Gorgonia ventalina, or the common
sea fan, is no exception.
Coral disease: Since the early 1990s, a terrestrial
fungus known as Aspergillus sydowii has ravaged
sea fans throughout the Caribbean, wiping out
huge portions of the population2. When exposed,
healthy sea fans typically increase their
production of melanin - a chemical responsible
for their purple coloring - in an effort to combat
the disease2. This reaction creates a distinct, dark
purple ring around the infected area, effectively
isolating the disease before it can spread.
Aspergillosis will compromise its reproductive
capabilities and eventually kill it2.
Anchor damage from boats: Soft corals like
Gorgonians are fairly delicate and therefore
susceptible to physical disturbances from anchor
damage. The British Virgin Islands are a popular
sailing destination that host over 350,000 tourists
annually, with the majority of guests arriving
between November and March3. Many of these
tourists choose to charter a boat without the
guidance of a captain or a crew, a phenomenon
known as “bareboat chartering”. With little to no
sailing experience, people can rent boats for days
or weeks at a time and sail around the islands.
Many popular sites provide moorings for boats
to stay for the day or overnight. However, the
increasing popularity of these charters has
skewed the ratio of boats looking to moor versus
the number of moorings available. Instead,
charters are forced to anchor at these sites,
causing an irreparable amount of damage to the
reefs4. Although anchoring on coral reef is illegal
in the British Virgin Islands, there is hardly
government effort to enforce the policies.
Most charters simply pick up their anchors and
move to their next destination unscathed,
though the same cannot be said for the reefs
they leave behind.
Objective: We examined the effects of
anchor damage on Gorgonia ventalina
populations in the British Virgin Islands
and tested whether anchor damage
exacerbated the impact of Aspergillus
sydowii on sea fans.
Figure 1. A healthy sea fan (left) is easily distinguishable
from a damaged sea fan (right) based on their coloring. The
damaged fan is a darker shade of purple as a result of
increased melanin production. This is a common response
for a threatened immune system much like the human
body produces a fever in response to the common cold.
Disease Dynamics Unchanged Despite a Strong Anchoring Impact
The graphs below show the direct effect of anchoring on sea fans and on
the symptoms of sea fan disease. High and low anchoring sites were
compared using ANOVA, with environmental variation minimized by
grouping geographically and physically similar sites (a “block” effect).
To Everyone Who Contributed
For funding, the National Science Foundation; For fieldwork support, Lianna Jarecki and Rebecca Flynn; For expert opinions, many
BVI boat captains, dive instructors and environmental professionals; For logistical support, the Guana Island staff and Dive BVI.
For photo credit, David Gleeson and Rebecca Flynn. Thank you all for your support!
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DiseasePrevalence
Anchor Damage
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SeaFanSize(cm)
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PopulationDensity
Figure 6A.
Population Density –
Number of sea fans per m2
Areas of low anchorage
experienced a higher density of
sea fans, while sea fans found in
areas of high anchoring were
much more sparsely distributed.
Figure 6B.
Sea Fan Size –
Average height per fan
Areas of low anchoring had
significantly larger sea fans. The
average heights differed from 39.5
cm in highly damaged sites to 55.2
cm in less damaged areas.
Figure 6C.
Disease Prevalence –
Percent of sea fans infected
Prevalence of Aspergillosis was
fairly consistent across field
sites, regardless of damage
condition. This contradicts our
initial hypothesis.*
Threats to Sea Fans on Coral Reefs
Assessing the Damage
1.0m
30.0 m
Sea fan Algae Coral
Site Area Transect
Figure 4A. (Below) A diagram of a standard
hypothetical 30-meter transect, looking from
above. All sea fans within a 1 x 30 m strip
were recorded. Figure 4B. Transect
tapes were placed
haphazardly around
each dive site. Up to
four tapes were placed
at each site.
Figure 4C. Sea fans
exhibiting varied levels
of injury from anchors
were recorded at each
site. Fans knocked over
or uprooted were
classified as dead.
Figure 3. To isolate the
effects of anchoring, we
chose 21 sites to collect
the initial data. Each
site was then assessed
for their level of anchor
damage. Dive sites
were assessed based on
topographical
similarities. Sites in
close proximity
typically featured
similar underwater
landscapes and were
scored in the same
groups.
Figure 5. To measure the
presence of disease, we
recorded height, width,
damage status, disease
severity (percent cover of
disease), and indication of
immunoresponse (melanin
ring width) of sea fans
within each 30-meter
transect.
References
1 "Global Coral Reef Monitoring Network (GCRMN)." International Coral Reef Initiative. United Nations Environment Programme, 1 Jan. 2014. Web. 1 Nov. 2014. <http://www.icriforum.org/gcrmn>.
2 Kim, Kiho, and C. Drew Harvell. "The Rise and Fall of a Six‐Year Coral‐Fungal Epizootic." The American Naturalist 164.S5 (2004): S52-63. Web of Science. Web. 19 July 2014.
3 "Latest Statistics 2013." One Caribbean. Caribbean Tourism Organization, 26 June 2014. Web. 1 Nov. 2014. <http://www.onecaribbean.org/wp-content/uploads/6AUGLattab13.pdf>.
4 Forrester and Flynn, Unpublished manuscript
5 Jolles, A., Sullivan, P., Alker, A., & Harvell, C. (2002). Disease Transmission Of Aspergillosis In Sea Fans: Inferring Process From Spatial Pattern. Ecology, 83(9), 2373-2378.
6 Troeger, Victoria J., Paul W. Sammarco, and John H. Caruso. "Aspergillosis in the Common Sea Fan Gorgonia Ventalina: Isolation of Waterborne Hyphae and Spores."Diseases of Aquatic Organisms 109 (2014):
257-61. Print.
7 Toledo-Hernández, C., Yoshioka, P., Bayman, P., & Sabat, A. (2009). Impact of disease and detachment on growth and survivorship of sea fans Gorgonia ventalina. Marine Ecology Progress Series, 393, 47-54.
Low
Why is Aspergillosis equally successful across sites of
varied population dynamics?
To date, researchers have determined that the prevalence of this disease is most
successfully spread via two potential pathways5:
Primary contact Secondary contact
Diseased sea fans must be very Disease spores can detach from
close or touching in order to transmit infected fans and spread to
the infection. fans within a short range.
How does Aspergillosis spread in high anchoring areas?
This study revealed that the disease is equally prevalent across the BVIs
regardless of damage status (Figure 6C). However, based on what we know
about the spread of Aspergillosis, sparsely distributed sea fans should not be as
susceptible to hosting this pathogen.
One potential explanation for this gap in
understanding may be linked to the presence
of the waterborne disease spores6.
Secondary transmission may still occur at
high anchoring sites, but instead of naturally
detaching from infected fans, disturbance from
anchors could be a mechanism for detaching
the live spores more readily.
A future study could investigate the hypothesis that anchoring accelerates the
secondary transmission of Aspergillosis.
Is anchoring or disease responsible for smaller fans?
Most studies on this subject have concluded that this pathogen typically targets
larger fans, and therefore should not be as successful in areas with smaller
targets. But many of these studies use sea fan mortality as a proxy for disease
severity over time, which can be misleading7. The reality is that sea fans, large
or small, are equally susceptible to Aspergillosis. Small fans have an enhanced
immune response relative to their larger counterparts, so they are able to fend
off the disease for longer. Large fans die off more quickly as a result.
This study observed that areas of high anchoring have a lower mean fan size.
We now hypothesize that the effect of anchors killing off larger fans negates the
size-dependent rate of mortality from Aspergillosis. The timeline below
suggests one potential explanation for the size discrepancy found in areas of
high anchoring exposure.
The option of either primary or
secondary transmission provides a
potential explanation for the success of
Aspergillosis in areas of low anchor
damage, where fans are distributed
fairly close together (Figure 6A). This
spatial arrangement is ideal for the
spread of this disease.
Aspergillosis
spores are
disturbed by
anchors and
secondarily
transmitted to fans
across the reef.
Neighboring
fans of all sizes
become
infected.
Smaller fans have a
stronger immune
response relative to
larger fans. Larger
diseased fans die off
more quickly.
High anchoring
areas experience
an eventual shift
in mean size of
sea fan
population.
Height
Width
%Cover of
disease
Melanin
ring
width
Figure 2. Bareboat charters anchored in Muskmelon
Bay on Guana Island. Most of landscape beneath
them is made up of coral reefs.
*Although not represented graphically, other symptoms of the disease (severity,
immunoresponse, etc.) all showed similar consistencies when compared to sites of
high and low anchor damage.