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Fishhook Water flea

(Cercopagis pengoi)

Photo of fishhook water flea

Fishhook water fleas are part of a group of animals known as crustaceans that include crabs, shrimp and crayfish. However, unlike these other crustaceans, the spiny and fishhook water fleas are very small creatures known as zooplankton. The fishhook water flea closely resembles the spiny water flea. The fishhook water flea arrived in the United States via ballast water in ships traveling from Eurasia, and was discovered in Lake Ontario in 1998.


Other names for this animal include:
  • Common names: Fishhook water flea
  • Scientific names: Cercopagis pengoi
Ecological threat:
  • The fishhook water flea is detrimental to Wisconsin water because it alters the composition, structure and function of the ecosystem by eating smaller zooplankton, putting them in direct competition with other large zooplankton and juvenile fish for food. Young fish have difficulty eating them because of their long tails, meaning that there are small numbers of natural predators that can keep the population manageable.
  • Fishhook water fleas can decrease the biodiversity in an ecosystem by consuming and eliminating the native plankton species. This may lead to a decrease in populations of young fish, as they prey upon the same plankton eaten by the fishhook water flea.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited/Restricted

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for fishhook water flea was based upon this literature review [PDF] developed by the department.


Body: This species has a translucent body that measures 0.4" (1 cm) on average. The most noticeable part of this species is the black eyes that boldly stand out against the light colored body. Typically found in clusters of 10+ individuals, the cluster will resemble wet cotton lumped on fishing line, ropes, cables and other items in the water.

Similar Species: Spiny water flea


See the reported locations of fishhook water flea in Wisconsin.

Do you know of additional populations? Send us a report.


Mechanical: Recreational fishing and boating are the primary means of transport to new waters, so this species is best combated with boater education and steps to prevent their spread. Once in a body of water, the fishhook water flea is almost impossible to eradicate.

Prevention Steps:
  • Inspect and remove aquatic plants, animals, and mud from the boat and equipment before leaving the boat launch.
  • Drain water from your boat and equipment before leaving the boat launch.
  • Throw away unwanted bait in the trash.
  • Spray or rinse your boat and equipment with high pressure and/or hot tap water, especially if moored for more than a day, or, dry your boat and equipment completely for at least 5 days.


Links for more information: