Fishermen have been pulling net after net of jellyfish out of the ocean off the coast of Kokonogi, Japan recently.

By ELISABETH ROSENTHAL By Elisabeth Rosenthal The New York Times BARCELONA, Spain Blue patrol boats crisscross the swimming areas of beaches here with their huge nets skimming the water's surface.

Blue patrol boats crisscross the swimming areas of beaches here with their huge nets skimming the water's surface. The yellow flags that urge caution and the red flags that prohibit swimming because of risky currents are sometimes topped now with blue ones warning of a new danger: swarms of jellyfish.

A new study helps explain a cyclic increase and decrease of jellyfish populations, which transformed parts of the Bering Sea--one of the U.S.'s most productive fisheries--into veritable jellytoriums during the 1990s.

A slimy jellyfish weighing as much as a sumo wrestler has Japan's fishing industry in the grip of its poisonous tentacles. Vast numbers of Echizen kurage, or Nomura's jellyfish, have appeared around Japan's coast since July.