They say one breed in particular - the lion's mane - showed up about a month earlier than usual. The biologists blame everything from breeding conditions and climate change for the abundance of jellyfish so early in the season.
Cornell University biologist Mark Bain says there is "widespread evidence of increasing jellyfish around the world."
The lion's mane are blamed for stinging athletes who jumped into the Hudson River on Sunday during the swimming stage of the New York City Triathlon. The stings are not considered deadly.
One triathlete died after being pulled from the water. An autopsy failed to determine a cause of death.
