This animal normally lives between China and Japan, migrating from the Yangtze River to the Yellow Sea. In summer when the water is warmer, they move to the coasts of Japan and Korea. Even with their huge size, a few of these jellies isn't much of a concern. However, their populations have been exploding since 2002, and they are now wreaking havoc on the Japanese fishing industry. They weigh down and damage fishing nets, and their toxins made fish inedible. In 2009, a fishing trawler was sunk when the crew tried to haul in a net that was full of Nomura's jellyfish.
Nobody is one-hundred percent sure what is causing the invasion. Some scientists believe it's because the water temperatures in the area have increased by 1.89 degrees Fahrenheit, which makes for better jellyfish breeding conditions. Overfishing is likely another factor. Reduced numbers of fish means less competition for the jellyfish, and fishing nets kill off large numbers of sea turtles (a predator of jellyfish).