It's a far cry from Cajun country, but a U.S. crayfish used in Southern cooking is now eating its way across Africa, scientists say. Without any native predators to keep it in check, the Louisiana crayfish, also known as the red swamp crayfish, is gobbling up small freshwater fish, fish eggs, mollusks, crustaceans, and aquatic plants. The 6-inch-long (15-centimeter-long) invader is already widely distributed in lakes and other bodies of water throughout Kenya, as well as in Rwanda, Uganda, Egypt, Zambia, the Seychelles, Mauritius, and South Africa.
Conservationists are now concerned the crayfish will reach the East African lakes of Malawi, Tanganyika, and Victoria, which are home to hundreds—and probably thousands—of species found nowhere else. "By removing animals and plants from wetlands, [the crayfish] can upset the balance of ecosystems and reduce valuable ecosystem functions," said Geoffrey Howard, global coordinator for invasive species for the Species Programme of the International Union for Conservation of Nature (IUCN).
Louisiana crayfish were first imported in the 1970s into Kenya and South Africa, where the species was grown in aquaculture operations. People bred the species in Kenya's Lake Naivasha and sold the delicacy to Scandinavian buyers after that region's native crayfish had been wiped out by disease. "They are rarely seen or recognized as a threat," Howard said, "but they have certainly affected the fishery in Naivasha." That's because, "by eating fish eggs and fingerlings, [crayfish] can reduce the populations of fishable fish." Though some people have benefited from selling the crayfish, it's a bit of a boom-and-bust venture, said Arne Witt, invasive-species coordinator for Africa at the Centre for Agricultural Bioscience International (CABI). Crayfish numbers can quickly explode and then collapse after eating the available prey in a given area, he said.
The crayfish were also introduced into dams around the Kenyan cities of Nairobi, Kiambu, and Limuru to rid those areas of parasite-carrying snails. But by burrowing into the edges of dams, rivers, and lakes to make their nests, the crayfish have damaged local infrastructure and landscapes. For instance, their burrowing has caused water canals to leak, earth dams to collapse, and banks of rivers and lakes to erode. "Walking" crayfish will eat anything. The species also has adaptations that make it an especially efficient invader. Unlike most aquatic animals, for example, the crustacean can live out of water for hours at a time, sometimes walking several kilometers—especially at night and on wet days, IUCN's Howard said. "They can also walk and swim upstream in rivers that feed lakes, and can even more easily move downstream in rivers and streams," Howard said. "In addition, they have been moved by people using them as fishing bait and possibly for food, [as well as] those collecting specimens for aquariums."
Secondly, Louisiana crayfish can change or switch diets based on whatever food is available, from plankton to amphibians, CABI's Witt noted. The crayfish "has been held responsible for the disappearance of many aquatic plant species," he said. Its wide-ranging diet also reduces the amount of prey items available for fish, birds, and other predators. With no funding available to detect and map their spread, it's unknown just how deeply the crayfish has infiltrated Africa, IUCN's Howard noted.
Trapping and possibly poisoning could control the species in small bodies of water, Howard said. Physical barriers could also stop the crustaceans' movements—but only if a crayfish population's precise distribution is known. Also, removing water hyacinth and other aquatic weed species from water bodies such as Lake Naivasha may allow birds more access to catch crayfish, CABI's Witt said. In the long-term, scientists may need to introduce a disease that will specifically target the crayfish, he said. For now now, people should stop moving the crayfish around the continent, he emphasized. That "should be made illegal and punishable by law."
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