Storms that produce strong waves, such as hurricanes and typhoons or other naturally occurring disasters like tsunamis, can severely damage a reef. The strong waves generated by these events can pound a reef and break off coral branches.
The ideal water temperature for coral is between about 73 and 84 degrees Fahrenheit. In some parts of the world, the ocean temperature is rising. This results in the coral polyps expelling zooxanthellae from their systems, which leads to coral bleaching. Zooxanthellae is the source of the coral's energy, and with nothing in their systems to produce energy, the coral will eventually die.
Pollution from man-made sources has become a serious threat to coral reefs. Pollutants such as runoff from farms, sewage, or chemicals, all can harm or destroy coral. These pollutants can aid the growth of algae, which competes with coral; or they can reduce the amount of oxygen in the water, essentially suffocating the coral.
There are many methods of fishing practiced around the world that have harmful impacts on coral reefs. Blast fishing, which uses underwater explosions to capture fish, destroys coral reef structures. Some fishermen use cyanide in the water to stun fish so they can be caught alive. Coral polyps can be killed by the cyanide. Other styles involve pounding on the reef with sticks to scare fish into nets. This practice can severely damage a reef.
The effects of overfishing near coral reefs may not be as clear as other threats to coral, but it can be just as destructive, as it affects the coral reef's ecological balance. As an example, if the population of one type of fish --- a type that eats plants and algae which live near or on a coral reef --- is depleted, it can lead to an increase in that fish's food source once the "predator" has been removed. These plants and algae can multiply out of control; they essentially take the coral over, and smother it. Without the presence of the predator fish, the ecosystem is thrown out of balance.
