Negative externalities of Nutrias

The South Korean ecosystem is being distracted by overbred nutria.

http://www.koreatimes.co.kr/www/news/tech/2011/06/113_88098.html

These rodents were imported for the fur industry to South Korea in the 1980s. As South Korea's fur industry became senile, those nutria raised for fur have been released. They can grow as big as 1 meter from head to tail.
In South Korea, now, there is no natural predator to nutrias. First, during the Japanese occupation in the early 20th century, the Japanese colonial government massively hunted large preyers such as grey wolves, black bears, tigers, or leopards. Second, during the Korean War, the U.S. Air Force bombed through the peninsula and it led to even smaller preyers to being extinct. Especially, the Korean inner-land water ecosystems have naturally lacked large predators like grizzlies. Snakes and large birds could be potential predators near lakes or rivers but their populations have decreased recently, too, because of rapid industrialization and subsequent urbanization. Where the tiger is absent, the fox becomes the ruler. In this case, a nutria is the fox.
It is indeed disastrous to the South Korean ecosystem. Since nutrias' main preys are water insects and fish. Outgrown nutria population is leading the natural population of water insects and fish to decrease, which in turn is leading to a even more serious decrease in large birds and snakes populations. They also eat water plants, which are not only diets for fish and other water animals but also oxygen providers to water. A decrease in oxygen in water leads to lower water quality, which is harmful to both humans and other lives. They also attack migratory birds temporarily nesting in Korea during summer or winter, which could cause even greater distraction in environments in East Asia. Recently, they even started attacking the people who came to the water to enjoy leisure.
To reduce their population, municipalities and provinces started incentivizing nutrias hunting by paying citizens per nutria body. I don't think it is too brutal. However, the Korean authorities could have prevented this distraction from happening by levying a large sum of environmental taxes. They might have simply banned fur industry if the tax rate required to prevent the distraction were too high.
Another way that could have prevented the problem is privatization of properties near lakes or rivers owned by government, where nutrias live to the people. Nutrias should lower the values of those properties and the owners would have tried to prevent them from coming in. Then, they would have either force the owners of nutria farms to pay to build screening barriers or the property owners would have built them themselves.
I'm not sure any of those method will work now. However, it is still true these negative externalities could have been averted if Koreans took proper measures suggested above.