Many Fisheries and tourist destinations are becoming cluttered with these Cnidarians. Areas that have been hit hard by these swarms include Hawaii, the Gulf of Mexico, the east coast of the U.S., the Bering Sea, the Mediterranean Sea, Australia, the Black Sea, the Sea of Japan, the North Sea, and Namibia.
Some of these swarms cover hundreds of square miles. Many water enthusiasts have been injured and a few have died. These swarms have also wreaked havoc on fisheries, fish farms, marine mines, desalination plants, ships, and nuclear power plants. It is estimated that jellyfish swarms have cost fishing and tourism industries hundreds of millions of dollars since the 1980s.
Some swarms consists of jellyfish the size of refrigerators such as the ones found in the Sea of Japan and other swarms off the coasts of Australia consists of jellyfish the size of peanuts. The cause of these swarms is unknown but it is hypothesized that they are human caused. Pollution, climate change, introductions of non-native organisms, overfishing, and the presence of artificial structures, such as oil and gas rigs have all been put to blame. Whether or not it was one of these or a combination has yet to be determined.
The article can be found at http://www.macroevolution.net/jellyfish-swarms.html