Huge swarms of stinging jellyfish and similar slimy animals are ruining beaches in Hawaii, the Gulf of Mexico, the Mediterranean, Australia and elsewhere, U.S. researchers reported on Friday.
The report says 150 million people are exposed to jellyfish globally every year, with 500,000 people stung in the Chesapeake Bay, off the U.S. Atlantic Coast, alone.
Another 200,000 are stung every year in Florida, and 10,000 are stung in Australia by the deadly Portuguese man-of-war, according to the report, a broad review of jellyfish research.
The report, available on the Internet here, says the Black Sea's fishing and tourism industries have lost $350 million because of a proliferation of comb jelly fish.
The report says more than 1,000 fist-sized comb jellies can be found in a cubic yard (meter) of Black Sea water during a bloom.
They eat the eggs of fish and compete with them for food, wiping out the livelihoods of fishermen, according to the report.
And it says a third of the total weight of all life in California's Monterey Bay is made up of jellyfish.
Human activities that could be making things nice for jellyfish include pollution, climate change, introductions of non-native species, overfishing and building artificial structures such as oil and gas rigs.
Creatures called salps cover up to 38,600 square miles (100,000 sq km) of the North Atlantic in a regular phenomenon called the New York Bight, but researchers quoted in the report said this one may be a natural cycle.
"There is clear, clean evidence that certain types of human-caused environmental stresses are triggering jellyfish swarms in some locations," William Hamner of the University of California Los Angeles says in the report.
These include pollution-induced "dead zones", higher water temperatures and the spread of alien jellyfish species by shipping.