Researchers from the Minnesota Aquatic Invasive Species Research Center (MAISRC) recently released the results of a four-year research project looking at one of the tiniest yet troublesome threats to waterways in northern Minnesota – the spiny water flea (SWF).

With those results come new ways to try and prevent them from spreading any further than they already have.

The microscopic freshwater zooplankton – which are native to Europe and Asia – were first found in Lake Superior in 1987 and first discovered in inland Minnesota lakes in Island Lake Reservoir north of Duluth in 1990.

Today, they are found in about 40 bodies of water in the state including, in our neck of the woods, including Lake Vermilion (since 2015) and Pike River, and around Ely in Shagawa and Burntside (since 2010) Lakes, as well as the Dead River and Burntside River.

They are also found in Lake Mille Lacs and Lake of the Woods.

Spiny water fleas invade lakes and take over the bottom of the food chain, effectively disturbing the ecology of the food web and presenting what experts consider a serious potential threat to the lake.

More specifically, according to information on the MAISRC web site, “they can decimate populations of Daphnia and other native zooplankton resulting in a decreased food source for native fish and an increase in algal blooms. They can also clog the eyelets of fishing rods, causing problems for recreationalists.”

“Many young fish depend on zooplankton until they get big enough to eat other foods,” Valerie Brady, who headed up the MAISRC research project, said this week by email. “This includes fish we really care about like yellow perch, bass, and walleye, but can also affect the fish that our favored fish need to eat when they become adults - the forage fish. So, SWF have the potential to affect much of the food web.”

How that affects fishing is still being researched, brady added.

“What we know is that high populations of spiny water fleas eat the native zooplankton that feed young fish and planktivorous fish. This can affect fish populations but is difficult to prove scientifically. A research team led by Dr. Gretchen Hansen showed that young walleye were smaller in lakes with SWF,” Brady said. “Smaller fish are more vulnerable to predation by larger fish and may not survive the winter as well as young fish that go into winter a larger size. No one has yet proven that this changes walleye fishing, though.”

What is known is the SWF can reproduce and spread quickly so preventing the spread is imperative. There’s also no way to get rid of them once they move in.

“There is nothing available at this time to eliminate spiny water fleas from an infested lake. The problem is that their densities can become so high - up to 50 or more individuals in every cubic meter of lake water during midsummer - that in order to eliminate them we would need either a chemical treatment or physical removal scheme that was applied to almost every drop of water in the lake, from the shallow regions to the deepest regions,” said MAISRC researcher Donn Branstrator. “This is different from zebra mussels whose adult distribution tends to be concentrated in the shallow bottom areas of lakes that are both more visible and more accessible to chemical treatments and physical removal of the invader.”

Branstrator added that spiny water fleas can reproduce asexually – which is similar to a cloning process – and so even a single living female could repopulate an entire lake.

There are also fewer predators on spiny water fleas than on native zooplankton because small or young native fish can’t consume their sharp, barbed spine.

Spiny water fleas were first introduced to the Great Lakes through ballast water. Today, recreational boaters and anglers can inadvertently move them or their eggs in any residual water in boats or gear and on fishing line, bait buckets, live wells, or fishing nets.

The research project, which started in 2017, was funded by St. Louis County and the Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Committee on Minnesota Resources, was an attempt to better understand how the flea spreads and how to better prevent that spread.

Previous studies had shown that over 40% of northern Minnesota lakes provide suitable habitat for spiny water fleas, and human recreational activity is believed to be the primary vector of spread.

To learn more about spread and prioritize prevention efforts, researchers measured the relative risk of spiny water flea attachment on commonly used recreational equipment including stationary anchor ropes, trolled fishing lines, trolled bait buckets, trolled downrigger cables, and trolled simulated live wells.

They sampled in the middle of the day and in the evening to account for spiny water fleas’ tendency to migrate closer to the water’s surface at dusk and ranked the threat of each type of gear tested to help recreationalists prioritize their cleaning efforts to prevent further spread of spiny water fleas.

They conducted 36 sampling events on Island Lake Reservoir (near Duluth) and 36 sampling events on Lake Mille Lacs and then compared the number of spiny water fleas on the deployed recreational equipment to the natural abundance of the fleas in the surrounding lake water.

In total, researchers processed 216 anchor ropes; 72 bait bucket samples; 72 live well samples; 72 downrigger steel cable samples; 72 downrigger fishing line samples (monofilament); and 216 shallow-running fishing line samples (72 monofilament, 72 fluorocarbon, and 72 braided).

“We wanted to mimic anglers trolling surface and downrigger lines, so we set up a fishing boat to troll three surface lines and a downrigger line. We would troll about 1 km (⅔ mile), then reel in each line and carefully collect and count how many spiny water fleas were on each line,” Brady said by email. “We also mimicked water running through a live well and towed a bait bucket beside the boat. We repeated this multiple times on two lakes, Island Lake near Duluth, and Lake Mille Lacs. A second research boat followed the first boat and sampled how many spiny water fleas were actually in the water.”

Researchers found that the downrigger and shallow-running fishing lines accounted for 87-88% of all ensnared spiny water fleas on the gear tested. They did not find a difference between twilight and daytime ensnarement of the fleas on gear. As expected, higher numbers of spiny water fleas in the lake water resulted in greater ensnarement of spiny water fleas on gear, particularly angling line.

While few spiny water fleas were found in bait buckets or simulated live well samples, these items are still risky because they can retain residual water in which the spiny water fleas could remain alive longer than on other gear. Residual water in any gear or boats carries this risk.

Researchers found almost no spiny water fleas on anchor ropes. However, these ropes were left stationary in the water for several hours. They were not exposed to currents or flowing water. Researchers do not know if flowing water containing spiny water fleas would result in greater ensnarement of these fleas on anchor ropes.

As a result of the research, officials now can provide a more specific message to boaters and anglers.

“Everyone knows that when they come off a lake, they need to clean off their boat and trailer, drain all the water, and let everything dry in the sun; the ‘clean, drain, dry’ message,” Brady said. “But our research showed that there could be spiny water fleas on fishing line, particularly, and possibly on other gear that has been in the water. So, our research shows that anglers need to pay particular attention to cleaning their lines and other fishing gear in addition to thinking about their live well, boat, and trailer.”

To reduce the risk of spreading spiny water fleas, researchers recommend that when fishing in infested lakes, anglers wipe down their line as they reel it in from their final cast of the day or as they reel in their downrigger line and cable. A small sturdy cloth or towel works well for this.

At the dock, recreationalists should be very careful to drain all water from their boat and all gear and wipe out water holding areas such as live wells and bait buckets.

From other studies, researchers know that spiny water fleas and their eggs cannot survive being completely dry for longer than six hours.

Thus, researchers also recommend that all gear and the boat are dried in the sun until everything has been completely dry for more than six hours before moving to another water body.

More information can be found at


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