Seahorses are in serious decline, with some populations thought to have been cut in half in just five years and one species in South Africa now officially listed as endangered, according to researchers at the London Zoo. Pollution and development of coastal areas have taken their toll on the critters, but the biggest threat is fishing by humans. Hundreds of thousands are captured each year for the aquarium trade or for tourist souvenirs, and at least 20 million a year are taken for use in traditional Chinese medicine, where they are prized for treatment of asthma and impotence.