Published August 20, 2012, 11:04 AM

Freshwater jellyfish back in warm Northland waters

They are harmless, mysterious and fascinating to watch, according to those who have seen them, and freshwater jellyfish are back in some Northland lakes.

By: John Myers, Duluth News Tribune

They are harmless, mysterious and fascinating to watch, according to those who have seen them, and freshwater jellyfish are back in some Northland lakes.

The tiny, opaque jellyfish have been reported recently in Everett Lake and Mullenhoff Lake near Iron River in Bayfield County.

“I swam right through them. They were bouncing off of me,” Andy Einspanier, owner of Delta Lodge on Everett Lake, said. “I swim the lake in the evening and I’ve been seeing them since about mid-July. They don’t bother anyone. They don’t sting.”

One family vacationing on Mullenhoff Lake said bluegills appeared to be feeding on the jellyfish, said Catharine Khalar at the Brule River State Forest headquarters.

In 2010, thousands of the small jellyfish were spotted by Steve Geving, a Lake Superior area fisheries specialist for the Minnesota Department of Natural Resources who was kayaking on Namakan Lake in Voyageurs National Park.

Biologists have speculated that water temperature and abundance of zooplankton — jellyfish food — may play a role in allowing the jellyfish to grow to a noticeable size that floats to the surface.

In 2006, another warm summer, Chad Polecheck of Esko, a former Minnesota conservation officer, told the News Tribune that he saw “thousands and thousands” of the jellyfish while fishing on Little Sturgeon Lake north of Hibbing in late July. At first he thought it was pollen in the water, but he captured several of them in a jar and watched them “pulse” through the water.

Another 2006 report came from Dodo Lake near Duluth. Those jellyfish were on display for a time at Fisherman’s Corner bait shop outside Duluth.

Though freshwater jellyfish have been around for decades, most people have never seen them because they spend most of their lives as underwater polyps that live on or near lake bottoms.

Only occasionally, and scientists aren’t sure why, do the polyps develop into dime- to quarter-sized jellyfish that can be seen floating and pulsing near the surface. Most sightings are in August and September, experts say. The “blooms” usually last only a few days.

DNR biologists in past years said there is little information on freshwater jellyfish in Minnesota. They often go unreported for years, and then several reports come in about the same time.

It’s still safe to go into the water, officials say. While freshwater jellyfish have tiny tentacles to sting and capture zooplankton, they are too small to sting people like their larger saltwater cousins can.

Freshwater jellyfish — Craspedacusta sowerbii — have been in Minnesota and Wisconsin lakes for decades. They are naturally occurring now, but they are an invasive species just like Asian carp or zebra mussels. So far, however, there’s no sign of any ecological disruption or damage from jellyfish.

Some scientists speculate there could be more jellyfish now as northern lakes get warmer earlier in the year and stay warmer later. Northland lakes on average now have two weeks less ice cover than they did 50 years ago, and this has been the warmest summer in decades. As warmer water reaches the polyps attached to the bottom of lakes, more of the polyps may mature into jellyfish “blooms.” The jellyfish-stage critters then reproduce asexually and tiny eggs attach to the bottom of the lake and become polyps, restarting the cycle.

Usually, when in the jellyfish form, the creatures are white or green and nearly gelatinous — 99 percent of their bodies consist of water. The jellyfish lacks a head, has no skeleton and contains no organs for respiration or excretion.

Freshwater jellyfish are found throughout the world, but are originally natives of China. They were found in England in 1880 and the U.S. in 1908, according to the University of Michigan’s Museum of Zoology. They were first reported in Wisconsin in 1969. It’s believed they were moved around the globe along with exotic fish and flowers for water gardens popular at the time. They then slowly spread into waterways, according to Sandy Engel, a retired water-quality biologist in Wisconsin.