Many coastal waters in Spain, New York, Australia, Japan and Hawaii are filled with more jellyfish now than ever before, The New York Times reported Saturday.
"These jellyfish near shore are a message the sea is sending us saying, 'Look how badly you are treating me,'" said Dr. Josep-María Gili of the Institute of Marine Sciences of the Spanish National Research Council in Barcelona, Spain.
Overfishing of predators such as tuna, sharks and swordfish has resulted in increased numbers of jellyfish, the Times said.
Scientists say because jellyfish survive best in damaged habitats, pollution and global warming are also contributing to their increase.
"Human-caused stresses, including global warming and overfishing, are encouraging jellyfish surpluses in many tourist destinations and productive fisheries," the U.S. National Science Foundation said.