Threats to Coral Reefs – Human Impacts

Coral Reefs—Part 4

Anthropogenic Impacts

Collection of coral for construction and use in the curio trade

Coral has a multitude of uses as a construction tool. It can be utilised for the production of lime kilns, house foundations and embankment of streets, canals and fish ponds. Huge businesses also thrive on collecting coral and selling it as souvenirs or exporting it for sale in the aquarium trade.

Chemical Pollution

Chemical Pollution. © NameAgriculture and industry release a variety of chemicals into coastal waters. Pesticides and fertilisers used in agricultural development projects are carried in run off to sea and have been known to take part in coral reef destruction. Pesticides can destroy or damage zooplankton or reef communities. They cause further damage by accumulating in animal tissue and may affect physiological processes. Herbicides may interfere with the basic food chain by destroying or damaging zooxanthallae in coral, free living phytoplankton, algal or sea grass communities.

Exposure to chemicals (namely hydrocarbons) released by spillage from oil tankers, harbours and pipelines has been known to affect reproduction, growth rate and feeding, defensive responses and cell structure in corals. Industrial practices such as mining, dredging and refining all release heavy metals into coastal waters. Some coral species are sensitive to these, although the extent is not yet known.

Nutrients Loading/Sewage

The discharge of fertilisers, waste feed and other materials from aquaculture and agriculture into coastal waters can result in nutrient loading. The introduction of organic compounds results in eutrophication and subsequent oxygen depletion. Europhication is where the nutrient load gets to an extent that the community becomes dominated by algal/seaweed, exceeding the capacity to control by grazing organisms. This leads to oxygen and light reduction and perhaps death of the communities living there.

Coral reef ecosystems have been described as oases in the oceanic desert due to the lack of nutrients present in their environment. When faced with large quantities of nutrients, they are easily overcome by algae and severely damaged, if not killed. Sewage from coastal developments and local communities can supply these nutrients providing food for algae, which go on to overgrow the coral.


Pile of fish. © NameOver exploitation affects the vast majority of the world’s reefs. This leads to an average decrease in the size of the fish and a reduction in predatory target fish. Removal of key herbivores and predator species may result in large scale ecosystem change. If grazers are removed from reefs, algae are quick to take over and dominate, especially if the area is also suffering from organic pollution.

Destructive fishing and boating practices

Blast Fishing. © NameFishing methods in the tropics can be particularly destructive especially those of dynamite ‘blast’ fishing, cyanide or poison (duva) fishing and fish hunting with gum boots. Other adverse fishing practices include disintegration of the reef structure in order to weight traps and remove hiding places and beating coral surfaces to herd fish into nets. Anchor damage and accidental grounding of boats can be a serious threat to reefs. For example, one cruise ship destroyed 3150km2 anchoring on one occasion. These practices lead to habitat destruction and disintegration of the reef ecosystem.

For example: here are photos of the broken coral heads left behind when Tiger IV hit the reef next to Malolo Island in 2006.

Broken Coral Head

Broken Coral Head

Broken Coral Head

Construction and Sedimentation

Coastal Sedimentation. © NameSedimentation (losing soil from upland areas) is an extremely important cause of coral reef destruction. Coastal construction and shoreline development often result in heavy sediment loading. Further effects are caused by inadequate land management and deforestation where soil run off from farms and settlements delivers sediments to the reefs. Watersheds cleared of their forests and other vegetation cover is vulnerable to erosion and flooding, resulting in increased levels of sediments reaching the reefs. Chemicals applied to upland agriculture also make their way down to the reefs via run off from land and rivers.

Dredging has many very serious implications for reefs. The most dramatic effects are caused by suspension of silt, sedimentation, turbidity, oxygen reduction and the release of bacteria and toxic matter. A large quantity of either coarse or fine particles will bury the corals, which are unable to withstand cover for more than one or two days.

Sedimentation covers the coral reef. © NameSedimentation causes corals to secrete the protective mucus mentioned earlier, in a bid to rid themselves of the sediment. This process requires increased energy levels, which have to be diverted away from essential processes. If the problem is exacerbated by other external stresses e.g. temperature change, then the corals become over stressed and can die. The secretion of this sediment clearing mucus also makes the coral more susceptible to bacteria and therefore more likely to suffer from disease.

Excessive sedimentation exceeds the clearing capacity of some filter feeders and smothers the substrate. It reduces light penetration and can alter the vertical distribution of plants and animals on reefs. Sediments can also absorb and transport other pollutants.

Mangrove cutting

The destruction of mangroves by clear cutting or pollution has resounding consequences on coral reef ecosystems. To remove mangroves, is to remove the main source of leaf litter, a food resource for the variety of coral reef animals. Mangroves also provide the nutrient rich feeding grounds for many marine species, which if removed would eliminate fishing grounds. Furthermore, mangroves protect the shoreline against cyclones and storms and stabilise it against erosion and land loss.


An eel showing out of a can left underwater. © NameRubbish and litter is one of Fiji’s biggest problems. All these artificial products take a long time to decompose. Plastic bags breakdown in 50 years, plastic bottles in 150 years, and cigarette buts in 75 years, paper in 1 year and batteries in 200 years. These all take so much time to breakdown to the detriment of creatures that live around us. If a turtle encounters a plastic bag, which looks similar to jellyfish, he may swallow the plastic bag and choke on it. Batteries leak poisons as they breakdown and can contaminate the fish we eat, as well as kill corals and other marine life.

Rubbish should be disposed of properly, by recycling or taking it back to the mainland dump. If rubbish is left lying around, it can easily get blown into the sea. take it away with you after picnics and inform tourists where they can dispose of their rubbish if you are on a boat.


Snorkellers standing on the reef. © NameTourism is a great industry as it provides people with jobs and income. However, tourists are not always as educated as we hope. A lot of them come from large cities and towns and have spent very little time around natural environments. Tourists come to visit and experience an environment they do not have at home. We therefore need to take the responsibility to inform them how to respect and treat that environment. Putting some guidelines for tourists to follow in place helps Fiji to protect their marine environment in particular. Giving a tourist environmental briefing has been shown to reduce damage to the environment by 95%. Awareness and education is the key to protecting the precious resources of Fiji.


The feeding behaviour of reef fishes, eels, sharks and even rays has come to a selling point through commercial fish feeding dive tours and ‘interactive diving’. However, many do not realise the harmful effects this activity has on these animals. Studies done around the world have indicated that fish feeding significantly alters behavioural patterns by “training” these wild creatures with human food handouts. In addition, fish feeding causes health problems for the fed animals and disrupts the natural processes within the marine community. Here in the Mamanucas, particularly at sites where fish feeding occurs, there has been an increase in aggressive behaviour within schools of surgeonfish, fighting amongst themselves and causing injury, even to the point of destroying their own reef habitat by breaking hard corals. Triggerfish have also been observed biting and destroying the reef structure. Sergeant Damselfish swarm around snorkellers or divers expecting to be fed. The fish that are fed often ‘peck’ at the snorkellers or divers entering the water, taking away the pleasure of observing the reef and its inhabitants in a calm and inoffensive manner. By feeding the algae eaters that control algae growth, they become handout feeders that soon neglect their important role of eating algae, which in turn can overgrow coral. Major conservation organisations, including UNEP, DAN , WWF and Environmental Defence, encourage passive interaction with marine life and avoiding feeding and petting, which may lead to accidental injury.

This part completes the Coral Reefs series.