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Spiny and Fishhook Waterflea

Spiny (Bythotrephes longimanus) and fishhook (Cercopagis pengoi) waterfleas are small predacious crustaceans that threaten aquatic ecosystems and fishing by competing with native fish for food and fouling gear. Both arrived in ships' ballast water from Eurasia. Spiny waterfleas were discovered in Lake Ontario in 1982, then spread to all of the Great Lakes, and some inland lakes including Lake Champlain. Fishhook waterfleas were first discovered in Lake Ontario in 1998, then spread to parts of lakes Michigan, Erie, and the Finger Lakes of New York.

Anglers often discover new infestations. Both waterfleas collect in masses on fishing lines and downrigger cables (see cover). These masses can clog the first eyelet of rods, damage a reel's drag system, and prevent fish from being landed. They can spread to inland waters when fishing gear is contaminated with egg-laden females. While females die out of water, under certain conditions they produce eggs that resist drying, remain viable, and can establish a new population. Eradicating established infestations is impossible. Your help detecting and reporting new infestations is vital for preventing their spread.

Identify Invasive Waterfleas

Identify Invasive Waterfleas

General Characteristics

  • Difficult to distinguish without magnification, 1/4 - 5/8 in (5-15mm) total length
  • Clumps look and feel like gelatin or cotton batting with tiny black spots
  • Prefer deep lakes, but can establish in shallow waterbodies and rivers
  • Abundant during summer (June-September) depending upon water temperatures

What You Can Do

Follow Minnesota Aquatic Invasive Species Laws:

  • Clean all aquatic plants, animals and mud from watercraft, trailers, docks, lifts, anchors and other recreational equipment before leaving access. Waterfleas may appear as gelatinous or cotton batting-like material on lines, especially where lines meet a swivel, lure or downrigger ball connection.
  • Drain water-related equipment (boat, ballast tanks, portable bait containers, motor) and drain bilge, livewell and baitwell by removing drain plugs before leaving water access. Keep drain plugs out while transporting watercraft.
  • Dispose of unwanted bait, worms and fish parts in the trash.

Also Recommended:

  • Spray watercraft and equipment with high-pressure water, or
  • Rinse with very hot water, or
  • Dry for at least 5 days.

Report New Sightings — note exact location; place specimens in a sealed plastic bag or store in rubbing (isopropyl) alcohol; and call a Minnesota DNR Invasive Species Specialist (see www.mndnr.gov/invasives/contacts.html), 1-888-MINNDNR or (651) 259-5100; or the Minnesota Sea Grant Program in Duluth, (218) 726-8712.

Know the Rules!

Specimens are needed to confirm sightings, but some jurisdictions prohibit possession and transport of invasive aquatic plants and animals. Contact your local natural resource management agency for instructions. Unauthorized introduction of plants, fish, or invertebrates into the wild is illegal. Protect your property and our waters.

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This page last modified on May 04, 2016     © 1996 – 2019 Regents of the University of Minnesota     The University of Minnesota is an equal opportunity educator and employer.
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