The biggest of the jellyfish, purple, highly poisonous, invade the waters of England. They arrived in England as a result of temperature increase of sea water. This sort of Medusa, 4 inches (about 10 cm) wide, multiply rapidly in warm water. They don't often come to the waters of England, because they prefer to live in the warm waters of the Mediterranean and central Atlantic. But in the past three years hundreds of millions of jellyfish have arrived in the northeast Atlantic and the English coast, because the temperature of the high coast of England since 2002 in a centigrade. In 2007 large quantities of jellyfish poisoned about 100,000 salmon in a fishing area in Northern Ireland, causing a loss estimated at one million pounds sterling. The jellyfish that invade the waters of England represent a danger for tourists on the beaches, but the biggest loss has been of other related industries. Overfishing at sea is a difficulty to maintain the natural enemy of Jellyfish which is tuna.