In cyanide fishing, fishermen release liquid cyanide, a very dangerous and deadly chemical, into the reef. __21__, the fish become stunned. This makes them easy to collect. The fishermen can then sell them for big money to the aquarium market, or for __22__in restaurants. Meanwhile, the reef is damaged by the cyanide which kills large numbers of coral polyps, and by the fishermen who __23__ the reef looking for the stunned fish.Another threat to a reef is water __24__. When floods in Australia covered the Great Barrier Reef with dirty freshwater, the quality of the water changed, and chemicals killed the reef life. This is happening to many reefs around the world. In addition, __25__global warming, many reefs have become sick, turning white in a process known as coral bleaching.