If you want to create a welcoming neighbourhood, you've first got to get rid of the trouble-makers. Then, with any luck, the folks who play nice will move in.

After a century and a half of eating everything in sight and bothering its fellow fish in the waterways around the Greater Toronto Area (GTA), the common carp is being pushed out the door. Conservation groups dedicated to increasing biodiversity in the area's wetlands have declared the common carp as public enemy #1.

"Around here we jokingly them call them the cows of the lake because they graze on everything and stir up sediment. Once the carp is established, it's almost impossible for any native species to establish," says Meg St. John, coordinator of Aquatic Habitat Toronto for the Toronto and Region Conservation Authority.

In the next few months, St. John expects that the authority will start installing carp exclosures (as opposed to enclosures) near the mouth of the Humber River, liberating the wetlands from the aquatic bullies. (A similar gate was installed in Tommy Thompson Park about three years ago. It's so effective it sometimes creates a traffic jam of frustrated carp who can't get in.) The effort will be the first of many larger-scale interventions in the river watershed to restore it to a more natural state. The work will be guided by an environmental assessment of the Humber River that started back in the early 1990s, emerging out of a joint U.S.-Canada initiative to improve the health of the Great Lakes. A long time in the making, when the final Humber assessment is released later this year by the Ontario Ministry of Natural Resources, it will help direct the many governmental, environmental and community organizations in their rehabilitation work.

As with other watersheds in the GTA, the problems in the Humber River wetlands go way back. And it's not just the fault of the common carp, which are believed to have been released here unintentionally in 1879. (Although these carp came originally from Asia, they are not the invasive Asian carp species that have more recently caused concern, says St. John.) Around the time the carp arrived, mills were lining up the riverbanks, making it impossible for spawning fish, like the Atlantic salmon, to get up to the headwaters to reproduce. The first mill, King's Mill, near Bloor Street West, was built in 1796. By the 1860s, there were hundreds along the Humber River.

Meanwhile, humans claimed more and more land for themselves. As the city and its suburbs have grown, Toronto has lost more than 500 hectares of coastal wetland.

"We know that 90 percent of the wetlands across the Toronto waterfront have disappeared and we are left with 10 percent," says Mark Heaton, a biologist for the Ministry of Natural Resources. ""These watersheds are under a lot of development pressure and have been for a long time."

Reducing the influx of people into the GTA seems unlikely. So efforts have focused on making the best of what still exists. The environmental assessment plan for the Rouge River was completed about a decade ago, giving the watershed has a good head start on the Humber. Some former marshlands on the Rouge have been restored and in one case, Barkey Woods in North Scarborough, wetlands have been sculpted wholesale out of retired farmland.

"Instead of creating a small area of wetlands, we're creating a larger area of habitat connected to other areas. It creates a corridor for animals to be able to move safely," says Doug Forder, field supervisor for Ontario Streams, a not-for-profit environmental organization that's helped with watershed restoration across the province.

Much of the work has been done by volunteers -- coming from almost 50 different clubs and associations -- who pick up garbage, plant native flora and help build berms to secure riverbanks. Some solutions are relatively straightforward. In the Rouge, a gravel parking lot near the river was paved to prevent gravel from being washed into the river where it created sediment. Sediment -- whether it's caused by gravel, carp or construction -- is fatal to many small fish, including the tiny redside dace, which is a threatened species in Ontario.

"When you're weak in wetland cover is when you'll find you have weakness in terms of species of wildlife," says Heaton. "You don't see waterfowl, you don't see deer, you don't see the birds, amphibians and reptiles in those areas of wetland loss."

Since work began on the Rouge, deer, beavers, muskrats, snapping turtle and several species of frogs and toads have returned or increased their numbers, as have redwing blackbirds, geese, mallard ducks and teal, says Forder. As work on the Humber progresses, St. John expects birds like the least bittern, moorhen, American coot, egret, night heron and Virginia rail to repopulate the watershed.

Environmental organizations have not sat on their hands waiting for the ministry's final environmental assessment. The remaining weirs on Humber have been notched and fishways have been created, allowing spawning fish to make their way upstream. (One of the things they learned from earlier projects was that culverts aren't the answer -- fish don't like to swim through dark tunnels.) This summer, 100,000 Atlantic salmon were released into the river.

"If you know how to jump like an Atlantic salmon, you can swim from Lake Ontario all the way to Highway 9. In 1995, fish couldn't do that," says Heaton.

When anglers start reporting salmon catches over the next few years -- that's when Torontonians will know that the native fish has at last pushed out the common carp.