How invasive species affect fishing

How invasive species affect fishing: Why invasive carp are bad for fish habitat

Because of their large size and abundance, common carp cause some big problems. They spawn in shallow waters and disrupt wetland habitat by stirring up sediments, uprooting aquatic vegetation and reducing water clarity, which degrades the plants and invertebrates that ducks and many other species depend on.

“Through their feeding habits, they get into these shallow water environments and they almost act like little bulldozers along the bottom, essentially sucking up whatever organic material they can from the sediments, and then they spit out sediments back into the water column. So that mixes up mud into the water column (and) limits light penetration,” Kowal says.

Native fish species like northern pike and yellow perch depend on these shallow, vegetated waters for spawning and as nursery habitat for newly hatched young. When carp degrade these environments, native species lose their spawning grounds and the sheltered areas young fish need to survive their first season.

Because common carp grow quickly and can produce millions of eggs at a time, it’s hard for native predators to keep their numbers in check. Once established, they can become the dominant fish species in a system.

Restoring Delta Marsh with carp exclusion structures

At Delta Marsh on the south shore of Lake Manitoba, invasive common carp have been a problem for fall staging waterfowl since the 1960s.

That’s why, in the early 2010s, DUC initiated a project called “Restoring the Tradition.” The project saw the construction of carp exclusion structures, allowing the installation of bar screens in openings in the structures each spring to reduce the numbers of large carp from accessing the marsh. Over time, this helped restore clear water, healthy vegetation and productive food webs, improving aquatic habitat that both fish and waterfowl rely on.

By 2018, carp abundance inside the marsh was reduced, marsh habitat quality improved and waterfowl populations rebounded to historical (1970s) levels.

After the bar screens are positioned in the exclusion structures, carp unable to pass through the screens congregate on the lake side of the structures. The Province of Manitoba began issuing a limited number of commercial fishing licenses to local fishers, allowing them to harvest the tens of thousands of pounds of carp that congregate on the lake side of the structures.

The success of this project has led to provincial management, ensuring ongoing operation of the carp exclusion structures. (Click here to read more about the fish community response at Delta Marsh.)

The growing threat of invasive Asian carp in the Great Lakes

Like the common carp, the four Asian carp species, which includes silver carp, bighead carp, grass carp and black carp, are also destructive to aquatic habitats and have been wreaking havoc in waterways in the United States for decades. But, Kowal notes, “A number of organizations are actively working trying to keep these other fish from getting established in Canadian waters of the Great Lakes ecosystem.”

How Phragmites is affecting Canada’s wetlands

While invasive carp stir things up under the surface, another invader is quietly transforming shorelines and wetlands across much of eastern Canada. Invasive Phragmites (also known as common reed) is a tall, aggressive grass that crowds out almost everything else along the water’s edge.

“It’s an invasive grass, originally from Europe, that was brought over accidentally in the 1800s. It has become really established across North America, especially on the eastern side,” says Matthew Bolding, conservation programs specialist with Ducks Unlimited Canada in Ontario. In Canada, it can be found as far west as British Columbia and Alberta, but is most well established in Ontario, Quebec and the Atlantic region.

Phragmites thrives anywhere from damp roadside ditches to water more than half a metre deep. That makes the area from shoreline to open water prime real estate for this plant. Once it gets a foothold, it spreads fast.

“What you get is a vegetation community where you have really high biodiversity, like 50 or 60 different species,” Bolding explains, “and that gets reduced down to two or three because it’s all Phragmites.”

Above ground, the plants can tower up to 20 feet; below ground, intense root systems “just [suck] up all the nutrients in that area,” Bolding says, starving out native plants.

How invasive species affect fishing: Why fish struggle in Phragmites-infested wetlands

For fish, and anglers, the result is a serious loss of habitat. Instead of a mosaic of cattails, bulrushes, water lilies and submerged plants that provide food, cover, and spawning and nursery areas, wetlands taken over by Phragmites turn into walls of tightly packed stems.

“In the water where you have fish habitat…when phrag moves in, it’s just super high-density stems, so there’s no space for any of those other plants,” says Bolding.

In addition to choking wetlands and making them inaccessible, invasive Phragmites also reduces water circulation/quality and the invertebrates that fish rely on.

Fighting invasive Phragmites in Ontario wetlands

In Ontario, DUC is tackling invasive Phragmites with a toolbox of solutions. Carefully targeted herbicide treatments and mechanical cutting or mowing help knock back dense stands, followed by repeated treatments to prevent regrowth.

DUC also supports cutting‑edge biological control, including the release of a host‑specific moth whose caterpillars bore into Phragmites stems, weakening the plant over time, eventually slowing or stopping the spread, and helping native vegetation re‑establish.

“It’s a template that has been used for a lot of different plants. A big one is purple loosestrife, which is another project that DUC was involved in. Purple loosestrife was a bad invasive for wetlands, and it’s still around, but it’s not really a problem anymore, because a co-evolved predator has been introduced that is keeping it in check basically,” Bolding says. (Click here to learn more about DUC’s work with Phragmites biocontrol in Ontario.)

How drones and AI are helping stop invasive species before they take hold

At the same time, Ducks Unlimited Canada staff are using drones and AI‑powered image analysis to detect invasive plants earlier, before they’ve had a chance to establish, and map them more efficiently.

At a large wetland in Ontario, DUC staff have been managing a population of European water chestnut (a floating invasive plant that grows quickly and takes over open water spaces, crowding out native vegetation and degrading fish habitat) by paddling to the plants and pulling them out by hand, a slow and labour‑intensive job.

Now, they send a drone over the wetland to capture thousands of high‑resolution images from above. To speed up photo analysis, DUC partnered with a software company to build a machine‑learning program that can recognize the distinctive shape of water chestnut from the air.

“So we can take all these photos and put it into this software, and the software will scan it and identify them, and then give us the GPS coordinates…and we follow them to where the plants are and then we pull all the plants out,” explains Bolding.

“It makes it a lot more efficient, which frees up funding to do more removal.”

Together, these tools are helping restore open water, healthy wetland edges and the food‑rich nursery areas that young fish—and great fishing—depend on.