Spiny Water Flea – Oneida County Aquatic Invasive Species Program

​Spiny Water Fleas  (Bythotrephes cederstroemi)

​Oneida County’s healthy fisheries will be a thing of the past if the invasive spiny water fleas jumps ship into our waters!   Despite their small size, this invasive species has the potential of doing a great deal of damage in the aquatic food web and wreak havoc to both fish and anglers.

With your help, Oneida County’s waterbodies can continue to be free of spiny water fleas.

​Identification:   The spiny water flea is a tiny, predatory crustacean (a relative of crayfish and shrimp) that is recognizable by its ¼ to ½ inch long translucent body and possesses a long, sharp, barbed, tail-spine that is twice as long as its body, which prevents it from being eaten by many small fishes.  It possesses one large eye black, a pair of swimming antennae and four pairs of legs, of which the first pair is used to catch prey.  They swim in a rapid jerky manner by flipping their antennae and appendages. Its tiny size and translucent body make it hard for the human eye to spot unless gathered in a large cluster.

Impact to the Food Web:   The Spiny water flea is a predator that devours native zooplankton, more so than all the fish and other invertebrate predators in a lake. Its favorite target species is Daphnia, a native zooplankton that are prominent in the freshwater food pyramids, and are an important food source for small fish and juvenile sport fish such as bass, walleye, and perch. This puts the spiny water flea in direct competition with juvenile fish for food. Additionally, spiny water fleas consume Daphnia at a faster rate than the Daphnia can reproduce. A decline in Daphnia will cause a decline in the food web and reduce the number of young-of-the-year fish.  Additionally, when the bottom of the food web is disrupted, the stability and sustainability of a lakes ecosystem will decline.

Defensive Tail-Spines:   Although the water fleas can fall prey to fish, their unique tail-spine and thorn-like barbs at the base of the tail frustrates most small fish, which tend to choke on the tail-spine as well as experience difficulty swallowing and ingesting the tail-spine. The tail-spine can also puncture the internal organs of the small fish. Spiny water fleas are not a risk to humans, dogs, larger fish, and wildlife.

Recreational Impacts:   Anglers are usually first to detect and report new infestations of spiny water fleas, after experiencing frustrating issues caused by an odd gelatinous clump gumming up their tackle. This is a result of their long tail-spine sticking to fishing line, lures, nets and cables trolling through the water. As their densities accumulate, they form gummy clumps that clog the first eyelet of fishing rods, which causes damage to a reel’s drag system, and prevents fish from being landed. Anglers have to cut their lines because they are unable to reel in a fish. That spells disaster for the fish at the end of the line!

Robust Reproduction and Resilient Eggs:   The majority of the Spiny water flea population is female and is capable of asexual reproduction as well as sexual reproduction. Asexual reproduction facilitates explosive population growth and sexual reproduction facilitates genetic clones.  They reproduce from spring until early fall.  Female water fleas will die out of water. However, under certain conditions, their eggs can resist drying and freezing and will remain dormant for long periods. In fact, the eggs can overwinter, hatch the following spring, and establish a new infestation.  Eggs consumed by fish survive passage through the digestive tract.
High-seas Hitchhikers:  Native to Europe and Asia, they were introduced into the Great Lakes in the early 1980’s, because of ballast water discharge. Now free from its natural predators and competitors, the spiny water flea has spread like wildfire throughout the Great Lakes and has invaded numerous inland waterbodies in both the U.S. and Canada. 
Dispersal Vectors and Distribution:  The Spiny water fleas rapid spread is the result of adults and eggs hitching a ride in ballast transfers, water currents, recreational boats, bait buckets, livewells, ropes, cables, fishing lines, downriggers coated with both eggs and adults, and other sources of water dumped from boats. They can even invade a new waterbody if an infested fishing line is cast into a new lake. Additionally, while female water fleas die out of water, under certain conditions the eggs resist drying and freezing, and can establish a new infestation.

You Can Help: With your help, Oneida County’s waterbodies can continue to be free of spiny water fleas.  Before launching and before leaving you must:

  • Inspect your boats, trailers, and all equipment including fishing gear, anchors, anchor ropes, nets, and inside bait buckets.
  • Remove any attached plants, animals, sand, mud, gelatinous clumps, and organisms.
  • Drain all water from boats, motors, livewells and equipment. 
  • Never Move plants or live fish away from a waterbody! You may take leftover minnows away from any state water and use them again on that same water only if no lake or river water, or other fish or organisms were added to their container.

If you have been on a waterbody that has a spiny water flea infestation, please consider taking extra precautions (mentioned below) to ensure the eggs are killed, eliminate cross-contamination, and reduce the risk of transporting invasive species from lake to lake.

  • Wash your boat and equipment with a hot, high-pressure washer at a car wash or at home,
  • Make sure all water has evaporated from your boat and equipment, including live wells, for at least twenty-four hours before visiting another lake,
  • Please report suspected infestations to the Wisconsin Department of Natural Resources.