The Effects of Overfishing and Water Pollution
Scientists agree that the most widespread threat to coral reefs is overfishing. Researchers believe that the excessive taking of fish, particularly large species, can cause significant and potentially damaging changes to a reef. For example, heavy fishing of herbivorous (algae- or plant-eating) fish may cause a reef to become overgrown with algae. A blanket of algae can smother the underlying corals and prevent young corals from attaching to the reef and growing into adults. Moreover, in some regions, cyanide, bleach, and even dynamite are used to take large quantities of fish. These practices cause great damage by poisoning reef organisms and destroying the structure of a reef.
Another serious threat to coral reefs is pollution from land-based wastes, such as sewage and agricultural runoff. Such wastes are typically nutrient-rich, containing high concentrations of nitrogen and phosphorous. When excess nutrients enter the sea, a blooming (rapid population explosion) of algae can occur. The dense growth of algae can smother the slower-growing corals and substantially reduce the amount of sunlight available for photosynthesis by the algae within the corals' tissues. In addition, the runoff from land sometimes contains toxic chemicals and sediment that can poison or bury the corals.