An abnormally large boom in the population of jellyfish in Southwest Florida bays -- including sea nettles that pack a powerful sting -- could be a result of the state's persistent drought.

Lack of fresh water flow from streams and rivers has led to a spike in salinity levels on the coast, where a noticeable increase in jellyfish has spread from Charlotte Harbor to Sarasota Bay.

Jellyfish are no strangers to the coastal environment, but they usually do not proliferate until March or April, near the end of Florida's dry season.

"This is earlier than you would expect because of the drought," said Philip Stevens, a state researcher who heads out on Charlotte Harbor every day.

The current increase could indicate that even more jellyfish will be seen as the region's drought is projected to persist, Stevens said.

He has noticed several types of jellyfish, including sea nettles, larger moon jellies and the upside-down jellyfish that anchor themselves to the sea bed in mangrove swamps.

The same phenomenon is occurring in Sarasota Bay, said Meghan Mikesell, marine biologist for Mote Marine's educational boat tours.

Factors other than drought can contribute to jellyfish abundance, said Catherine Corbett, senior scientist with the Charlotte Harbor National Estuary Program.

High levels of nutrients, such as phosphorus and nitrogen, can lead to algal blooms that provide food for the little creatures that jellyfish like to eat.

The absence of top predator fish may also prompt higher concentrations of jellyfish, Corbett said.

But the most notable change in the bay this year is salinity, Stevens said.

"It's definitely a saltier year, one of the saltiest on record," Stevens said.

With less fresh water flowing from inland waterways, salt water moves in from the Gulf, carrying with it salt-loving creatures.

Jellyfish are a particular nuisance to swimmers because their tentacles contain stinging poisons. The sting from nettles and moon jellies can hurt like a burn, but it is not debilitating or life-threatening.

Bill Godek, a naturalist with Grande Tours in Placida, said he has noticed large numbers of sea nettles on kayak tours recently.

And Bill Dunson, a retired biology professor who spends his winters in Englewood, said he has never seen so many jellyfish floating by his dock in Lemon Bay. And last week while walking along the beach at Cayo Costa, on a Charlotte County barrier island, he saw an area where about 20 moon jellyfish, which can grow to more than a foot in diameter, had washed ashore.

Scuba divers also have seen large numbers of jellyfish in the Gulf in recent months.

About two weeks ago, diver and fisherman Joe Cellamare was about 15 miles offshore when he saw several reddish-colored jellyfish that were about four feet in diameter. Neither he nor the old stone-crabber he was fishing with had ever seen anything like it, he said.

The undulating creatures were so large that it would have taken a 25 gallon bucket to contain just one of them, Cellamare estimated.

The Wall Street Journal reported last week that a huge increase in jellyfish -- some weighing 450 pounds -- off the coast of Japan was causing trouble for that nation's fishing industry.

Scientists speculated that nutrient pollution, rising sea temperatures and overfishing of top-predator species could all trigger the jellyfish population growth there.

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