Overharvesting and destructive fishing
When people overharvest fish on a reef, whether for food or the aquarium trade, the entire food web is affected. If too many herbivorous fish are taken, macroalgae (seaweed) can overgrow and suffocate reefs. Taking too many large fish with the best reproduction potential can result in a much less healthy and robust fishery over time. Overharvesting of sharks, a top level predator on the reef, can start a cascade of effects in which populations of other fish collapse.
When fishing is done carelessly, non-target species often end up as bycatch. Destructive fishing techniques, anchors, and gear (like nets) can damage reefs physically; one of the more harmful practices, dynamite fishing, can destroy an entire reef in one act. Sometimes fishermen use cyanide to stun fish, then capture them. While large fish can metabolize cyanide, smaller fish and other marine animals, including coral polyps, are poisoned by the chemical cloud created by this activity. Read more about CORAL’s work to prevent unsustainable fishing.
Coral reefs thrive in clean, clear water. Pollution from both point (specific places) and non-point sources (runoff from the land containing many pollutants from many sources) can affect the health of reefs. Too many nutrients (usually from agricultural runoff or discharges of treated wastewater), excess sediment (from activities like agriculture, deforestation, and development), or stormwater runoff can stress reefs, impeding their growth and reproduction, or even kill them. Pollution can also harm or kill sensitive species and alter the ecological functioning of the reef.
Marine debris, trash that makes its way into the sea, can also harm or kill coral reef animals. Floating trash hooked on reefs can block the sunlight reefs need for their algae tenants to photosynthesize. Sea turtles often mistake plastic bags for jellyfish and eat them. Plastic can block the turtles’ digestive tracts and cause them to starve. Lost or discarded fishing nets sometimes snag on reefs and strangle fish, sea turtles, and marine mammals. Read more about CORAL’s efforts to improve water quality.
Unsustainable tourism can take many forms; resorts, marine recreation providers, and tourists can all harm reefs. Resorts sometimes discharge treated wastewater near coral reefs or inject it into groundwater wells. The wastewater can seep from the wells into the ocean. This water contains high levels of nutrients, which allow seaweed to flourish and suffocate the reef. Tour boats dropping anchor on coral or interfering with marine mammals and other wildlife can affect reef ecosystems as well. Tourists who grab, kick, or walk on coral can destroy coral habitat; stirring up sediment on the bottom affects water quality. And by purchasing jewelry or other souvenirs made from coral or other once-living marine life, consumers are contributing to the destruction of reefs. Read more about our work to ensure that tourism is sustainable.
Coastal development and sedimentation
Development to accommodate the growing human population along coasts has put additional stress on reefs. At one time, big cities such as Hong Kong, Singapore, Manila, and Honolulu had thriving coral reefs. Long ago, these reefs were destroyed by impacts from growing human populations. Now, reefs near other coastal communities are experiencing the same pressures. In many places, developers have constructed piers and other structures directly on top of coral reefs. Building and road construction, mining, logging, and farming can all lead to erosion. As a result, sediment particles end up in the ocean and cover coral reefs, smothering the coral by depriving it of sunlight. Some recent studies have found that sediment from deforestation is a bigger threat to coral reefs than impacts from climate change. Mangrove trees and sea grasses, which normally act as filters for sediment, are also being rapidly destroyed. This has led to an increase in the amount of sediment reaching coral reefs. Mangrove forests are often cut for firewood or removed to create open beaches. They are also destroyed by prawn harvesters to open up areas for creating artificial prawn farms. Read more about our work to improve water quality.
Mining can also destroy coral reefs. Sometimes coral pieces are removed for use as bricks or road-fill; sand and limestone derived from coral reefs are used to make cement for new buildings. Corals are also mined to make souvenirs and jewelry.