Spiny and Fishhook Waterfleas

  • Spiny and Fishhook Waterfleas

    (Bythotrephes longimanus and Cercopagis pengoi)

    Spiny Waterfleas and the Fishhook Waterflea are small aquatic predators that were likely introduced to North America through ballast water from ocean-going ships. Native to Eurasia, the two species of zooplankton were first observed in Lake Ontario, the Spiny Waterflea in 1982 and the Fishhook Waterflea in 1998. Spiny Waterfleas prefer large, deep, clear, oligotrophic (poor in nutrients but rich in oxygen) lakes where they migrate vertically from deeper waters during the day to surface waters at night; they collects on fishing lines, downrigger cables and nets as jelly-like masses. Fishhook Waterfleas prefer the upper, warmer layer in deep, open lakes but are able to tolerate a wide range of temperatures; they collect on fishing lines as cotton-like masses.

    Reproducing by asexual and sexual means, both species can multiply very quickly. They can spread to inland water bodies when egg-laden females are caught in fishing equipment. Their eggs can survive winter on lake bottoms and be transported long distances on boats or equipment in moist conditions.


    Spiny Waterfleas are found in the Great Lakes, with very high densities in Lake Erie, moderate to high in Lake Huron, low in southern Lake Michigan and offshore areas of Lake Superior and very low in Lake Ontario, and in more than 100 inland lakes across Ontario. The species has also been observed in Manitoba and Minnesota. Fishhook Waterfleas are found in Lake Erie, Lake Ontario and Lake Michigan.

  • Identification

    • Spiny and Fishhook waterfleas need to be identified with a microscope; they both have a single dark eye, four pairs of legs and branched antennae used for swimming.
    • The Spiny Waterflea is bigger, 1 to 1.5 cm in length while the Fishhook Waterflea reaches 1 cm.
    • The Spiny Waterflea has a straight or slightly angled tail, 1 to 3 barbs and a pointed end while the Fishhook Waterflea has a sharply angled tail, 1 to 3 widely spaced barbs and a loop or hook at the end.
    • The Spiny Waterflea may be orange, blue and green with a red stripe half the length of the tail while the Fishhook Waterflea is transparent.
    • The Spiny Waterflea has a balloon-like egg pouch while the Fishhook Waterflea has a pointed, elongated egg pouch.


    • Spiny and Fishhook waterfleas reduce food supplies for small fish and young sports fish.
    • They can quickly multiply from a few individuals to a large population.
    • They can easily spread to new waterbodies on fishing equipment, bait buckets, live wells and bilge water.
    • Native populations of zooplankton usually decline about 30 to 40 per cent after Spiny Waterfleas are introduced to a waterbody.
    • Both species affect recreational and commercial fishing by catching on to fishing equipment and clogging commercial nets and trawl lines.

    How can you help?

    • Learn to identify Spiny and Fishhook waterfleas and how to prevent their spread to other water bodies.
    • Inspect your boat and equipment and remove any plants, animals or mud before leaving a water body.
    • Drain water from motor, live well, bilge and transom wells while on land.
    • Rinse all recreational equipment with high pressure and hot water or dry in sunlight for 5 days.
    • If you observe Spiny or Fishhook waterfleas or other invasive species, please contact the Turtle Island Invaders hotline at 1-844-872-2348 (1-844-TRACE-IT) toll-free or report the sighting to your local Lands or Natural Resources officer.
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