Enemy at the gates

The waters may have been choppy at first, but the carp exclusion project at Manitoba’s Delta Marsh is having a real and positive impact.

After this winter’s repairs, the project is better prepared to take on common carp, an invasive species that’s wreaked havoc on this important freshwater wetland since the 1950s.

As bottom feeders, carp rake the sediments of the marsh as they search for food. In their wake, they leave a cloudy trail that makes it almost impossible for the sun to penetrate the water. This prevents aquatic plants from growing, and in turn, reduces the biodiversity once found at the marsh.

In 2013, DUC, in partnership with Manitoba Conservation and Water Stewardship, undertook an infrastructure project that introduced dikes and carp exclusion screens in three areas where channels connect Delta Marsh to Lake Manitoba.

Each spring, fish migrate to the marsh from the lake to spawn and feed. DUC staff place exclusion screens in the water after native fish have arrived, and just before the carp appear. These strong steel bars prevent carp from entering Delta Marsh. Dikes built in and around the screens also keep the invasive species from sneaking around the structures in times of high-water levels and wind.

Unfortunately, Mother Nature had other plans. In 2014, high water combined with a violent Canada Day storm, washed out two dikes. In spring 2015, a dike on the east marsh was temporarily repaired with sandbags. Then, on the Victoria Day weekend, high winds sent waves of water crashing in and out of the wetland from the lake, washing a hole in the sandbag wall, and punching several exclusion screens from their holdings. Although carp were able to access the wetland over a two-day period, their victory was short lived.

DUC staff replaced the screens, and this past winter, rebuilt the dikes and reinforced them with rock riprap. The repairs are expected to protect the structures from all but the most extreme high-water levels.

“Even if we’ve had issues…we’ve still kept lots of carp out of the marsh,” says DUC research scientist Dale Wrubleski.

Thanks to their efforts, Delta Marsh continues to be an essential wetland for waterfowl, including canvasbacks. “It’s an important migration place for birds, so we want to make sure they have it,” says Wrubleski.