Blame us, not ducks, for spreading spiny water fleas

New study finds fishing line highest among potential pathways spreading the invasive species.

A glob of spiny water fleas that collected on fishing line. New research shows fishing line among the most likey pathways for the invasive species to gather and spread. Minnesota Se Grant photo.

Val Brady and Donn Branstrator, University of Minnesota Duluth scientists, were at a boat landing on Island Lake north of Duluth when one of Brady’s neighbors happened to show up to go fishing.

The conversation moved to what the scientists were doing that day, research on how invasive spiny water fleas can accumulate on anglers’ fishing gear, when the neighbor weighed in with his opinion.

Why bother, he said, everyone knows spiny water fleas are spread by ducks.


There’s little evidence that birds play any major role in the movement of spiny water fleas. Waterfowl are usually near the surface and near shore, while spinys are down deep and in the middle of the lake. It’s also unlikely spinys could survive the gut of a bird.


But multiple studies have shown that humans unintentionally moving invasive species from lake to lake is a major factor in their spread — that the presence of public boat landings is the biggest factor in whether a lake is infested.

And now new research from Brady and Branstrator has found that fishing line may be one of the most likely way that spiny water fleas can hitchhike on our gear.

In the first major study of its kind, the researchers tested monofilament, fluorocarbon and braided fishing line as well as anchor ropes, bait buckets, downrigger cables and livewells to see which were the most likely to pick up and hold spiny water fleas. All of them held some amount of spiny water fleas, but one stood out with the most.

“It’s fishing line, by far,’’ Bantstrator noted of the findings. There seemed to be little difference in the type of fishing line, he said, spinys could ride along on all of them.

Researchers trolled fishing line, deployed anchor ropes and towed bait buckets through Mille Lacs Lake and Island Lake Reservoir near Duluth — 36 different sessions in each lake — and found the same results. Half the sessions were in daylight, when spinys are down deeper, and half at night when they often rise toward the surface.

“I really expected anchor ropes to be the worst offenders, but they weren't even close’’ to fishing line, Brady noted.

The difference may be because anchor ropes are usually stationary, while fishing line covers more water with more opportunity to snag spinys down deeper. Bait buckets are towed alongside boats on the surface and water flows through them, making them less able to hold spinys. Livewells, too, touch only surface water.

Taking action

The upshot of the research is that anglers now know they can take relatively simple steps to help slow the spread of spiny water fleas. Simply wiping down fishing line, reels and bait buckets at the end of the day can greatly reduce the chance of any spread.


“Maybe make that last cast of the day a really long one, and then bring the line back in through a dry towel,’’ Brady said. “And make sure there aren’t any spiny water fleas on the rod tip.”

Spiny water fleas can only survive for about six hours when totally out of water. But because fishing tackle and boats can hold small amounts of water in nooks and crannies, some spiny water fleas may survive much longer between fishing trips.

Researchers even tested multiple types of towels to see which was the most effective in removing spiny water fleas from fishing line. While any dry towel will work, they had good luck with so-called Swedish dish towels — so good in fact they ordered 4,000 of them to pass out free to anglers with the new message to wipe down fishing equipment, especially fishing line.

“The emphasis of invasive species prevention — clean, drain and dry — has always been on the boat. And that’s the way it should be... But we suspected that fishing gear plays a role as well,’’ Branstrattor said. “It’s going to take more education now, and more effort by anglers, but wiping down fishing gear needs to be part of the effort.”

Funding for the Island Lake research came from the St. Louis County Aquatic Invasive Species Prevention Aid program with money from the Minnesota Legislature. The Lake Mille Lacs research was funded by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Minnesota Aquatic Invasive Species Research Center.

Most likely to hold spiny water fleas

Number of spiny water fleas per test session

  1. Fishing line attached to downriggers - 28.5

  2. Fishing line trolled - 25

  3. Livewells - 3.6

  4. Downrigger cables - 3.4

  5. Bait buckets - 0.0015

  6. Anchor ropes - 0.0015

How you can help

Start by draining all water from your boat and gear, including the bilge, livewell and bait buckets. It’s not only the law in Minnesota, it’s the right thing to do.

Then, use a dry towel or cloth to:


  • Wipe fishing lines, reels and any downrigger gear.

  • Drain and then wipe dry livewells.

  • Drain and then wipe dry bait buckets.

  • For more information go to

Does it help?

Invasive species are confirmed in only about 8 % of all Minnesota lakes and rivers, leading experts to say that efforts to slow their spread — actions taken by boaters and anglers — have helped tremendously. Spiny water fleas have been verified in less than 1 % of Minnesota’s 11,842 lakes.

What’s the big deal about little spiny water fleas?

Spiny water fleas (Bythotrephes longimanus) are a microscopic freshwater zooplankton that invade lakes and can take over the bottom of the food chain, disturbing the ecology of the lake. In some lakes they decimate populations of native zooplankton leaving little food source for native fish and spurring an increase in algal blooms. They have a single long tail with multiple barbs which causes many fish to avoid eating them. One study showed walleyes in lakes infested with spiny water fleas grow slower than they did before the infestations.

Native to Europe and Asia they were first found in Lake Superior in 1987, arriving in the ballast water of ships, and first discovered in an inland water in Island Lake Reservoir north of Duluth in 1990. Now they have infested Mille Lacs Lake, Lake of the Woods, Lake Vermilion and about three dozen other lakes and rivers in the state.

Part of spiny water fleas’ success is due to their ability to reproduce rapidly — they mature and reproduce within about one week. They reproduce both asexually and sexually. Females can produce up to 10 young every two weeks without mating. In the fall, males and females reproduce sexually and produce resting eggs that settle in lake sediments where they overwinter in a dormant state, hatching the next summer.

Spiny water flea is a member of the Crustacea, a large taxonomic group that includes crayfish, shrimp and crabs. Adult spiny waterfleas grow to be about one centimeter long. Populations fluctuate greatly from week to week in the summer. At their peak, their populations can be as high as 100 individuals per cubic meter, sometimes taking over the biomass of the lake.

Source: University of Minnesota

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at
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