Invasive Species - Spiny Water Flea

Barely visible are tiny little black dots on the surface of clear water
Hundreds of tiny spiny water flea, nearly invisible, sit on the surface of this water.

NPS/Paul Brown

The spiny water flea (Bythotrephes cederstroemi) is not an insect but a crustacean with a long, barbed trail. They are small and transparent, making it difficult to see them unless they occur in very large numbers.

Spiny water fleas are native to Eurasia and were probably brought into the Great Lakes in the ballast water of ocean-going freighters. Their spiky tail keeps them from being consumed by smaller fish, which allows for high populations that outcompete native fish and zooplankton for food resources.

Spiky white junk is stuck to a fishing line
Spiny water flea stuck to a fishing line.

Jeff Gunderson, Minnesota Sea Grant

Spiny water fleas have been found in Lake Superior waters at Isle Royale, but the park is trying to keep them out of inland lakes, which are among the most pristine waters in North America. Visitors to the park can help keep our inland lakes free of spiny water flea by following a few simple procedures:

  • Prior to entering inland lakes, paddlers should wipe down all boats and gear that have come into contact with Lake Superior waters, using old rags that can be stored in a sealed, plastic bag, or pouring boiling water over the gear.
  • Hikers and paddlers should carry two water filters - one for use in Lake Superior and one for use on inland lakes. Alternatively, the filter line that makes contact with Lake Superior water should be cleaned and backflushed to remove water fleas.
  • Angers should try to plan their trip so that fishing on inland lakes is completed prior to fishing on Lake Superior, or boil gear used on Lake Superior to kill any aquatic "hitchhikers" before using the same gear on inland lakes.

Last updated: October 10, 2019

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