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Threats
to Coral Reefs
Aquarium Trade
The aquarium trade is an important income-generating opportunity
for many coastal communities. . However, the excessive collection
of marine ornamental species such as reef fishes, star fish and
sea anemones from our reefs may lead to a rapid decrease in populations
of these species. The use of de- structive collection techniques
such as moxy nets to collect ornamental fish also causes extensive
dam- age to the reef habitat.
Demand
for marine curios
In many reef areas certain marine species are in danger
of extinction due to excessive collection for the curio trade. Shells
and corals taken for the curio trade are used in a variety of ways:
large and attractive shells and pieces of coral are kept intact
and sold as individual items, whereas others are made into items
such as jewellery and ornaments. Precious coral and black coral
are used in the production of trinkets. There are a number of other
marine species used in the curio trade: these are primarily dried
echinoderms (e.g. starfish and sea urchins), dried fish (seahorses
and puffer fish) occasionally dried crustacea (e.g. spiny lobster)
and sea fans and sponges.
Coral
Mining
Traditionally, at various locations in the Indo-Pacific
(Pacific Islands, Sri Lanka, Maldives, India and the Philippines)
coral has been, and still is, used as a building material for the
construction trade. Hard coral skeletons are composed of limestone
and it is used for a number of purposes: burnt lime serves as a
binding agent in plaster and mortar; and raw blocks or boulders
are used as building blocks and as a substitute for clay bricks.
Coral limestone is a much sought after and valuable resource.
The coral colonies
are usually broken off manually with a crowbar and then transported
back to land. When coral is removed or killed, complex habitat features
are destroyed and the balance of reef species altered. The value
of the reef for tourism and fisheries is lost and the coastline
becomes more exposed .to wave action and thereby subject to an increase
in coastal erosion. Coral mining in the sea is illegal in Sri Lanka.
Natural
Pressures Placed upon Coral Reefs
Flooding and Surface Runoff Coral reefs are affected when
soil and surface runoff from inland areas is washed down to the
sea during heavy rains and flooding. Flooding and surface runoff
can bring sediments which smother coral polyps and even kill them.
The impacts of these natural pressures on corals are aggravated
due to human influences such as deforestation and farming on hilly
terrains. Further, pollutants from land and agricultural products
such as fertilisers, pesticides will also be washed to the sea with
the surface runoff.
Tropical
Storm damage
Severe monsoon rains and cyclones may churn up waters,
thereby killing corals. Storm damage also results in coral being
broken.
Predator
Plagues
Coral skeletons are invaded and eroded by the burrowing activities
of a wide range of reef organisms ( e.g. molluscs, worms and sponges)
and the coral polyps themselves may be attacked by many predators.
Eroders and predators influence the structure and growth of a reef,
but generally do not cause serious damage, except when found in
plagues. The only predatory species known to occur in plague proportions
and cause serious damage are ‘crown-of-thorns’ starfish
and coral snails of the genus Drupella.
Crown-of-thorns starfish (COTS) Acanthaster planci:
This large starfish,
reaching upto 60cm in diameter, has up to 23 arms and a body covered
with strong sharp spines. It occurs widely in the Indo-Pacific region.
Usually only one or two COTS occur on any particular stretch of
reef, but sometimes they can occur in plagues. However, corals do
have allies that help prevent predator attacks (e.g. small crabs
and shrimps that live among the coral branches). The immediate impact
of COTS plagues is coral death (mortality). An outbreak of COTS
in the Maldives (Northwest North Male Atoll) in the mid 1980’s
killed up to 85% of reef building corals on many reefs. In the 1970’s,
vast areas in the northwest including the Bar Reef, and in the east
coast of Sri Lanka were affected due to a COTS outbreak.
Why
do outbreaks occur?
This question is still unanswered even after many years
of research. Outbreaks have been occurring for many years, however
some scientists believe that the increase in frequency and size
of outbreaks could be triggered by human activities. Nutrient enrichment
of coastal waters, over-fishing of natural predators are a few of
the possible causes for increased frequency and size of these outbreaks.
Coral
Diseases
There are a number of diseases that can occur and cause
degradation of reef ecology and health. It is not known whether
coral diseases are entirely ‘natural’ or due to human
activities. Often the diseased corals are found on reefs close to
human activity and sources of pollution. The discharge of ballast
water from ships is also known to contribute towards coral diseases.
White Band
Disease (WBD) is also known as white plague and white death. Scientists
have . suggested that it is a ‘stress related necrcosis’
(SRN). A distinct white band -about lcm wide, ,” appears around
the coral. This white band represents the zone where coral tissue
is dying, leaving the white skeleton visible beneath.
Black Band Disease
(BBD) starts as a small dot, which then spreads in ring-like fashion
across the coral colony. It leaves the bare skeleton behind which
rapidly becomes colonised by algae and other organisms. Pink Band
Disease (PBD) The area where the disease is present spreads in a
ring-like fashion across the coral colony.
Coral
bleaching
Coral bleaching occurs when corals are under stress, for
example due to high sea surface temperatures and high levels of
sunlight (especially UV), which cause a whitening effect, or ‘bleaching’
.This loss of colour is due to the expulsion of algae (zooxanthellae)
upon which the coral polyp depends for much of its food. Long periods
of bleaching conditions can eventually lead to the death of the
coral polyp.
Small scale
bleaching is generally due to stress caused locally, but large scale
bleaching is thought to be due to stresses on a global level. It
is also possible that mass bleaching may be connected with a general
decline in reef health due to human activities. At present, evidence
shows that sea surface temperature rise is a cause of bleaching
and therefore global warming is a suspected culprit.
Bleaching does
not always lead to coral death. Some corals can survive for weeks
or even months without their zooxanthellae. However their health
will deteriorate due to a lack of organic material from photosynthesis
-which provide most of the corals food and nutrients. When bleaching
occurs, corals can stay alive only for short periods of time, especially
if they are in shallow water. In Sri Lanka, some corals at a depth
range of 15-20m stayed alive for about 6 months, regained colour
and had recovered to their former state.
A major bleaching
event occurred in 1998 associated with high water temperatures linked
with the El Nino weather pattern and global warming. This was one
of the worst recorded coral bleaching incid~nts in the world and
it caused a great deal of damage to coral reefs in the Indian Ocean.
Sri Lanka’s corals were severely damaged and corals in most
shallow reef areas of Sri Lanka were destroyed to depths of 3-5m
and at 42m depths off the East Coast. The most severe impacts were
seen close to the surface.
lmost all bleached
corals below 10-15m recovered after about 6 months. In shallower
waters, corals showed different levels of decline and recovery from
bleaching, depending on their location and the type of coral.
Surveys
in 1999 and early 2000 showed:
Live coral cover at Bar Reef Marine Sanctuary was near
ZERO in shallow reef habitats (eg.3m depth)
Live coral cover at Hikkaduwa Marine Sanctuary was reduced to only
7%
Weligama live coral cover was reduced to 28%
Rumassala live coral cover was reduced to 20%
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