The Nutria or Coypu (Myocastor coypus) was first brought to the United States from South America by the McIlheny family, of Tabasco Sauce fame. Their idea was to start a fur farm on Avery Island, Louisiana. Unfortunately, in 1941 a hurricane blew in and wrecked the cages, releasing about 150 nutria into the local marshes.
By 1959, these 150 nutria expanded to a population estimate of 20 million!
Nutria are considered a serious pest. Wherever they have gone, there has been a steep decline in muskrat populations, with whom they directly compete. In addition, nutria are voracious eaters of vegetation and are capable of cutting huge holes in marshes, leading to increased soil loss, shell-fish deaths, lost fish spawning grounds, and lost song and game bird habitat. Not for nothing are they sometimes called "that rat that ate Louisiana".
On the up side, there is little doubt that the introduction of the nutria has had a very beneficial impact on the American alligator, whose numbers were pushed perilously close to the edge in the 1960s, but which are now in superabundance thank to better game management laws and a steady supply of nutria (as well as cats, possums, small dogs, raccoons, turtles and fish).
Efforts are underway to wipe out the nutria in Maryland, where populations have been destroying the Blackwater Wildlife Refuge. In the American South (Louisiana, Florida, Mississippi, Texas, Georgia) it is probably impossible to turn the tide, no matter how many conibear traps are set out.
Nutria were introduced to England sometime prior to 1944, when a few escaped from a Sussex fur farm, but they were exterminated by 1988 thanks to a concerted trapping campaign.