THREATS TO CORAL REEFS

Increasing SST's (Sea Surface Temperatures)

Corals often live at the top end of their natural range. This means that even a small rise in temperature (1º or 2ºC) can severely stress or kill them. Heat stress is considered one of the main causes of recent coral 'bleaching' events. When corals are severely stressed they lose their symbiotic algae or the color (pigmentation) within them. They then appear white or "bleached". Whilst corals can survive some degree of bleaching, bleaching events that go on for a long while or are particularly intense will lead to wide scale coral death.

Increased Nutrients

Nutrient rich (eutrophic) water stimulates plant growth. In extreme cases populations of microscopic free-living plants in the plankton (phytoplankton) can increase dramatically in numbers or "bloom", turning the water green and blocking out sunlight. Additionally corals compete with other animals and plants for living space on coral reefs. Marine plants (algae) and other animals (such as sponges) are able to grow much faster than corals. Nutrient-rich water allows algae to grow unchecked and they may then quickly overgrow and out compete corals in the race for space. The presence of abundant algae also makes it difficult for corals to colonize new areas.

Sedimentation

Corals cannot thrive in water that contains high levels of sediment (earth, rock or sand particles). Sediment makes water cloudy, blocking out sunlight. It also smothers or buries corals. Sediment laden water often carries high levels of nutrients too. Sediment is washed into the sea by rivers and in rainwater. The amount of sediment in the water around coral reefs may increase dramatically as a result of activities such as coastal construction, or by land clearance for roads, agriculture or forestry (especially logging). Dredging the seabed can also result in heavy sedimentation. Poor land use practices are considered the greatest long term threat to coral reefs in many parts of the world

Over-exploitation

Humans harvest many of the animals that are part of the coral reef community. When harvesting reaches levels that are too high for the remaining individuals to keep population numbers at a sustainable level, it is called overexploitation. Slow growing species and top predators are most vulnerable to overexploitation. Humans selectively harvest all kinds of marine animals, including fish, shellfish, crabs and lobster, giant clams, sea cucumbers and even corals. Proper control of harvesting is known as sustainable management. Unfortunately, many forms of harvesting are not managed sustainably.

In some areas coral reefs are severely impacted by destructive fishing practices, such as fishing with dynamite (blast fishing) or using bleach or cyanide. These are not sustainable uses of coral reefs. They do not just remove fish, but can destroy the structure of the reef itself and killing many other animals in the process such as untargeted fish species and juveniles.

Overfishing is a worldwide problem affecting many of the fish and marine animals we like to eat. Stocks of many commercially valuable fish and marine creatures are either becoming low in numbers, or have been so heavily fished that there are very few left and they are considered "endangered". This can spell trouble for the whole reef system. Fishing needs to be managed so that enough animals remain on the reef to replace themselves and to fulfill their roles in the coral reef system.

Marine animals are not only exploited for food. The collection of corals and other marine species for other purposes - such as for sale to the aquarium trade or as souvenirs - is a major threat to coral reefs. Corals are also mined for use in road building and as construction material. The unregulated collection of coral reef animals such as seahorses for use in traditional medicine has been a problem in some parts of the world.

Physical Damage

Coral polyps are very delicate animals. Although some corals use fragmentation (growing a new colony from broken off tips of corals) as a way of building new colonies, most die when they are broken or crushed. Broken pieces of coral are rolled around by waves, killing the coral animals that live on the surface. It takes only a second to break a small piece of coral off but it can take up to 25 years for it to grow back.

Physical damage to coral r