Reef corals are 99% skeleton, covered by a very thin layer of animal tissue, containing millions of single-celled algae. Cyanide causes the corals to release these algae into the water. This stress reaction causes the coral to turn white, or “bleach”, because it is the algae that give most corals their color. The coral tissue often dies within a few days or weeks quickly rotting away and exposing the coral skeleton. Boring organisms gradually attack the skeleton, and it slowly disintegrates.
Other environmental costs are less understood but it is clear that small fish, corals and other organisms are killed in unknown numbers and with unknown ecological effects.
Reefs may take decades to recover from the impact of cyanide fishing. Researchers have found that in laboratory or in the sea, corals exposed to small dosages of cyanide die within a short time.
The MUCK diving paradise of Mabul and Kapali are a result of cyanide fishing (wikipedia article). With no big fish, all the small fish and invertebrates have increased in numbers. As a result blue ring octopus, flamboyant cuttlefish and other unique creatures are more abundant and therefore easier for divers to find. Recent 2010 surveys show the Mabul reef has suffered a ecosystem change compared to other less overfished reefs, many fewer fish, fewer corals and more diversity of small species. Good for divers, bad for fishers and the long term impacts for the ecosystem are unknown.