and Human Disturbance
Coral reefs are centres of marine biodiversity. Reefs are constructed by a
host of reef-building (hermatypic) coral species, but also are home to ahermatypic,
or non-calcium carbonate depositing corals such as soft corals, black corals
and gorgonians. Much like their terrestrial counterparts the tropical rainforests,
reefs combine a host of micro-habitats and a diverse array of life-forms that
are still being discovered and described. Coral reefs are distributed throughout
the tropics, and a large proportion are located in developing countries.
To understand how human activities affect coral reefs, it is necessary to
briefly review their basic life history. Coral reefs are made up of numerous
coral colonies. These colonies are made up of thousands of minute coral polyps
each of which secretes a calcium carbonate skeleton. The deposition rate for
individual coral species varies, but is generally considered to range between
0.1 mm and 10.0 cm per year. The accumulation of these skeletons over an enormously
long period of time results in massive, three dimensional geological structures.
The actual living tissue however, is only a very thin layer lining the surface.
polyps feed by filtering plankton, aided by tentacles tipped with stinging-cells
(nematocysts); they also receive organic matter through their symbiotic relationship
with minute algae (dinoflagellates) called zooxanthellae. These small algal
cells use sunlight to photosynthesise carbonates and water into organic matter
and oxygen, both of which are used by the polyp.
Coral reefs support complex food and energy webs that are inter-linked with
nutrient inputs from outside sources (such as those brought with ocean currents
and run-off from nearby rivers) and from the reef itself (where natural predation
and die-off recirculate organic matter). These complex webs mean that any
effect on one group of individuals will ultimately impact another, and single
disturbances can have multiple effects on reef inhabitants. For example, the
complete eradication of the giant Triton Charonia trinis through overfishing
usually results in outbreaks of Crown-of-Thorns starfish Acanthaster planci.
This in turn leads to massive coral mortalities as the starfish reproduce
and feed on the coral polyps. Habitats and food sources for reef fishes are
then reduced leading to declines in the population of larger predatory fishes.
The following sections provide a brief review of human disturbances and their
general effects on coral reefs.
Collection of corals
Corals have been mined for construction purposes in several countries including
Sudan and Saudi Arabia, broken into manageable sizes or crushed for the manufacture
of cement and lime.
Corals are also collected for use in the ornamental trade as curios, souvenirs,
or as jewellery. The aquarium industry is also responsible for the coral collection,
either for direct sale as live colonies or through the process of fish collecting.
removal of coral colonies decreases the shelter and niche areas available
to numerous other reef inhabitants. Juvenile stages of fishes that seek shelter
among the branching species of corals, and worms and ascidians that take up
residence on massive life-forms, are deprived of protection and refuges and
may become prey to other reef organisms. Furthermore, removal of entire colonies
reduces the overall structural stability of the reef, and increases rates
of erosion from wave damage.
Destructive fishing pressures are taking their toll on coral reefs. Today
this is done with the use of home-made explosives composed of fertiliser,
fuel and fuse caps inserted into empty beer bottles. Blast-fishers hunt for
schooling fish such as sweetlips and fusiliers, which aggregate in groups
in the open or hide under large coral heads. The bombs are usually dropped
into the centre of an area judged to have many fish and after the bomb has
exploded the fishermen use dip nets to collect the stunned and dying fish.
Blast fishing also damages corals. Lightly bombed reefs are usually pockmarked
with blast craters, but many reefs in developing countries have been reduced
to a continuous band of coral rubble instead of a reef crest and upper reef
slope. The blasts change the three-dimensional structure of reefs, and blasted
areas no longer provide food or shelter to reef inhabitants. Further, once
the reef structure has been weakened or destroyed by blast fishing it is much
more susceptible to wave action and the reef is unable to maintain its role
in coastline protection. Larvae do not settle on rubble and thus replenishment
and rehabilitation is minimal. The recovery of such areas has been measured
in decades, and only then with complete protection and cessation of fishery
pressure of any kind.
One of the most destructive fishing methods involves the use of cyanide. An
aqueous solution of sodium cyanide or other chemical is squirted at fish to
stun them, after which they are collected and sold to the live-fish trade.
In the process of stunning the fish, the cyanide affects corals, small fish
and invertebrates. A solution, which is narcotising to large fish, is often
lethal to smaller ones.
has also been shown to limit coral growth, cause diseases, bleaching, and
ultimately, in many coral species, leads to death.
Also among destructive fishing practices are normal trawl and purse fishing
operations, when these take place close to or over reefs. Trawlers try to
operate close to reefs to take advantage of the higher levels of fish aggregated
around them, but often end up with their trawls caught on the reefs. Many
of these then have to be cut away and discarded, becoming further entangled,
breaking corals and smothering others. Spearfishing may also damage corals
if fishermen trample and break coral to get at fish that disappear into crevices.
Mankind also has an impact on corals through the uncontrolled and often unregulated
discharge of a number of industrial and domestic effluents. Many of these
are 'point-source' discharges that affect localised reef areas, rather than
causing broad-scale reef mortality. Discharges may release chemicals that
are debilitating, toxic, or lead to a change in the environmental conditions.
The release of fluids high in organic matter or nutrients, such as sewage
or abattoir refuse can lead to a phenomenon known as eutrophication. Plague
quantities of algae may grow, stimulated by the high nutrient levels. When
these die, the bacteria that cause decomposition can deplete the water of
oxygen to such an extent that it becomes impossible for corals and other animals
to survive. Untreated and partially treated sewage is often discharged where
fringing reefs are located close to shore, such as the reefs that fringe the
length of the Red Sea. Raw sewage can also result in tumours on fish, and
erosion of fins from the high concentrations of bacteria that develop.
Petroleum hydrocarbons from the oil industry have also caused widespread damage
to coral reefs. The levels of oil and its derivatives in the marine environment
(many of which are persistent carcinogens) have been correlated with coral
diseases in the Red Sea, especially Black Band Disease. Drilling activities
frequently occur near reefs, such as along the Saudi Arabian shoreline in
the Arabian Gulf. Drilling mud can suffocate reefs and it also contains compounds
that disrupt growth and cause diseases. Discharges result in an increase in
diatoms (algae) and a decrease in marine fauna in their vicinity.
Many millions of tonnes of oil are transported through the region each year.
Oil spills affect coral reefs through smothering, resulting in a lack of further
colonisation, as occurred in the Gulf of Aqaba in 1970 when the coral Stylophora
pistillata was found not to re-colonise oil-contaminated areas. There have
been more than 20 oil spills along the Egyptian coast since 1982, some which
have smothered and poisoned corals and other organisms.
Industrial effluents also impact coral reefs and their associated fauna and
habitats. Discharge of heavy metals may give rise to elevated levels of lead,
mercury or copper in bivalves and fish, or elevated levels of cadmium, vanadium
and zinc in sediments. Larval stages of crustaceans and fish are particularly
affected, and effluents often inhibit the growth of phytoplankton, resulting
in a lack of zooplankton, a major food source for corals.
outflow from desalination plants is typically 5-10 ºC higher in temperature
and up to 3?10 ppt higher in salinity than normal seawater. These discharges
may also include chorine and anti-scaling chemicals. The higher temperatures
decrease the water's ability to dissolve oxygen, slowing reef processes, and
usually result in localised bleaching of coral colonies. The higher salinity
of the discharges increases coral mucus production and results in the expulsion
of zooxanthellae and eventual bleaching and algal overgrowth. Often these
waters are also chlorinated and contain compounds that are non-biodegradable
and circulate in the environment for years. These compounds are absorbed by
phytoplankton and then by filter-feeding corals. Through the complex food
webs on the reef these compounds concentrate in carnivorous fishes, which
may become poisonous to mankind.
Many airborne particles are also deposited over coral reefs, such as fertiliser
dust, or dust from construction activities. At Ras Baridi, on the Red Sea
coast of Saudi Arabia, a cement plant operating without filtered chimneys
discharges more than 100 tonnes of partially processed cement over the nearby
coral reefs each day. The reefs are now smothered by over 10 cm of fine silt.
Solid Waste Dumping
Plastics, metal, wood, rubber, and glass can all be found littering coral
reefs. These wastes are often non-biodegradable, or persist over long periods
of time, causing damage which is primarily of a physical nature. Solid wastes
damage coral colonies at the time of dumping, and thereafter through movements
with natural tidal and surge action.
Construction activities include coastal reclamation works, port development,
dredging, and urban and industrial development. Commercial and residential
property development in Jeddah have filled in reef lagoon areas out to the
reef crest and bulldozed rocks over the reef crest for protection against
erosion and wave action. 'Landfill' activities of this type generally result
in increased levels of sedimentation as soil is nearly always dumped without
the benefit of screens or silt barriers. Coral polyps, although able to withstand
moderate sediment loading, cannot displace heavier loads and perish through
development of ports and marinas also involves dredging deep channels through
reef areas for safe navigation and berthing, and damages reefs through the
direct removal of coral colonies, sediment fallout, and disruption of the
normal current patterns on which the reefs depend for nutrients.
Port activities can have adverse effects on nearby reefs through spills of
bulk cargoes and petrochemicals. Fertilisers, phosphates, manganese and bauxite
are often loaded and offloaded using massive mechanical grabs which spill
a little of their contents on each haul. In Jordan, the death of corals was
up to four times higher near a port that had frequent phosphate spills compared
to control sites. The input of these nutrients inhibits calcification and
increases sedimentation. Algal blooms also develop through input of nutrients
(nitrogen and phosphorous compounds), limiting light penetration and depleting
The recreation industry has caused small but significant localised damage
to coral reefs. Flipper damage by SCUBA divers is widespread, but certain
activities, such as underwater photography, finds divers breaking corals to
get at subjects and trampling reef habitats in order to get the perfect shot.
Anchor damage from boats is another problem at tourist destinations. Experiments
have proven that repeated breakage of corals, such as caused by intensive
diving tourism, may lead to substantially reduced sexual reproduction in corals,
and eventually lower rates of re-colonisation. In the northern Red Sea, another
popular diving destination, efforts are underway to install permanent moorings
to minimise the damage to reefs from anchors.
Most anthropogenic effects and disturbances to coral reefs are easily identifiable.
Blast debris and lost fishing nets can be seen. Pollutant levels and sediment
loads can be measured. However, many other man-made or induced problems have
indirect impacts on coral reefs that are both problematic to link directly
to coral mortality and difficult to quantify. Agricultural practices and logging,
for instance, contribute to coral reef degradation through the runoff of sediment,
fertilisers and pesticides. These result in the smothering of corals, limited
larval settlement and localised nutrient enrichment.
Global warming, resulting from the Greenhouse effect and the build-up of carbon
dioxide in the atmosphere, might also kill corals. The extensive coral-beaching
event that took place in 1998, which was particularly severe in the Indian
Ocean region, is accepted as having been the result of a rise in sea surface
temperature. Bleaching of coral colonies occurs through the expulsion of zooxanthellae
as coral polyps become stressed by adverse thermal gradients.
Coupled with global warming is sea level rise; this is predicted to be 25
cm by the year 2050. If not matched by coral growth, this will mean that corals
will be submerged deeper and will not receive the levels of sunlight required
for photosynthesis by the zooxanthellae. Additionally, the ability of coral
reefs to protect coastlines from erosion will be lost if the waves are able
to wash over the newly submerged reefs.
Mankind has contributed to the widespread destruction of corals, reef areas
and their associated fauna through a number of acute and chronic pollutant
discharges, destructive processes, and through uncontrolled and unregulated
development. These effects are more noticeable where social and traditional
practices have changed with a lack of infrastructure, finances, and educational
resources. Destructive fishing pressures and the development of coastal industry
affects reefs throughout their geographic range. If mankind is to be the saviour
of coral reefs in the coming millennium, there is going to have to be a change
in fishing practices, and adherence to development and shipping guidelines
and regulations, and integrated coastal management programmes that take into
account the socio-economic status of people, the environment, and developmental
Institute of Biodiversity and Environmental Conservation, Universiti Malaysia
Sarawak, 94300 Kota Samarahan, Sarawak, Malaysia. Email: firstname.lastname@example.org
Al Sanbouk 12 (in press)