Overfishing

As a result of a growing human population and the increasing tourism industry there is a high demand for fish as a source for food. Popular fishes are mostly large predators, this leads to an overpopulation of prey which cause a disbalance in the ecosystem. However, the numbers of predators decline, so the fishers have to move their preference to smaller ones. This leads to an algae overgrowth because heavily fished reefs have a shortage of algae grazing herbivores. If fishers just catch one group of fish the food chain is affected which have a great impact on the complex coral ecosystems. 

The fishing methods themselves could also be destructive. There is a fishing method where fishers use dynamite to kill fishes. The explosions destroys parts of the coral as a side effect. Although this method is illegal in many countries, fishers still use it, especially in South-East Asia. Another method is to poison fish, fishers use cyanide to stun the fish, this can bleach the corals and kill polyps.