by Andrea Smith

Beneath the surface of many an Ontario lake lurks a tiny organism that is wreaking havoc on our aquatic ecosystems. The spiny water flea (Bythotrephes longimanus) is less than two centimetres long, with a barbed tail over half its length. Originally from Europe, the flea was introduced to the Great Lakes in ballast water in the early 198 0s. Since then it has spread to inland waters from Kingston to Thunder Bay.

For the past two summers, scientists have been studying the spiny water flea in Muskoka. As part of the Canadian Aquatic Invasive Species Network (CAISN), researchers from York University have worked with the Ontario Federation of Anglers and Hunters (OFAH ) and the Ontario Ministry of the Environment to track the distribution and dispersal of this aquatic invader in cottage country. More than 300 lakes were sampled in 2005 and 2006, a period in which the number of known invaded lakes in Muskoka doubled from 23 to 46.

This is in part because the organism is easily transferred from lake to lake on contaminated fishing gear. A thick slimy layer, consisting of thousands of individual animals, attaches to fishing line used in trolling an invaded lake. Although the animals die once exposed to air, many carry viable eggs that may hatch when introduced into new lakes.

Spiny water fleas affect lake ecosystems in several ways. As voracious predators of smaller zooplankton, they outcompete larval and juvenile fish for food. The fleas also cause their prey to avoid the warm upper layer of water (the “epilimnion”) during the day, dramatically lowering zooplankton growth rates. This, combined with direct predation, is reducing the diversity and changing the structure of native zooplankton communities in both small and large lakes on the Canadian Shield.

CAISN researchers will analyze the data collected in the Muskoka study to create models predicting which Canadian Shield lakes are susceptible to spiny water flea invasion, and the role humans play in its spread.

Bythotrephes is the greatest threat to the biodiversity and structure of native Canadian Shield zooplankton communities since acid rain,” says Dr. Norm Yan, professor of biology at York University and a principal researcher in the monitoring project. “[The species] will end up in thousands of lakes on the shield if nothing is done, and the effects on zooplankton will be substantial.”

For more information on preventing the spread of the spiny water flea, visit OFAH ’s Invading Species website at