Revitalizing Greenfield with Sterile Fish

Wilmington, NC, April 5, 2006 – Mitchell Morton's truck is full of fish. Five hundred sterile grass carp paid for by the city of Wilmington and eight freebies who wriggled their way along for the ride, which ends here at the Greenfield Lake boat ramp. They're packed so close together in big plastic trash cans that they've worked themselves into a froth. Standing among the barrels and nets, Morton explains the his next moves - first he'll lower the water level even more, before replacing it with buckets from the lake. The process will give the fish a few minutes to acclimate the new temperature, before he tips them in.

Morton is the fisheries manager for Foster Lake and Pond Management, contracted by the city to stock Greenfield with carp. The first batch, a thousand strong, went in a year ago. Today is round two. Grass carp eat algae and other vegetation, and vegetation is precisely Greenfield's problem. Fertilizers and pet waste wash into the lake from surrounding neighborhoods, and when those nutrients hit its shallow warm waters, and you get a bloom of algae that's landed Greenfield a spot on the state's list of impaired water bodies. For the ordinary user, that just translates into gross - a stinking mat of vegetation that can stretch across the lake's surface. Jennifer Butler coordinates outreach and education for the city's storm water services, and says algae got so bad that RiverWatch, which rents paddle boats and canoes on the lake, found itself with unhappy customers on its hands. There was a point, she says, where they were having paddleboats returned because of the algae, people were having a hard time paddling through it.

Enter the carp. These guys can eat three times their weight a day in vegetation, and they group upwards of thirty pounds. They don't bother the other fish. And although they're not native, they are guaranteed not to become permanent residents.

Morton explains the process: Basically, when the fish are very small, when they've just been hatched, they take those little fry and subject them to intense heat or pressure... The end result is essentially that their chromosomes divide one more time, so they have three sets of sex chromosomes so they're triploid. Sterile, basically. They're neither male or female. These fish are tested by a lady from the Fish and Wildlife Services. They actually take blood samples from these fish and analyze them to make sure that they're triploid. They travel from the hatchery with documentation. So there's long process to get them from the hatchery to the lake.

That process does make these some pretty pricey fish. Each foot-long flopper costs a little more than six dollars. But they're only one part of the city's investment to clean up Greenfield Lake. It's installed four water circulation systems to help increase oxygen levels, applied herbicide, and even taken to hand-weeding invasive species. But according to Butler, that's only part of what's needed; the real challenge is public education.

We're always going to have to let people know the polluted run-off doesn't go to a treatment plant. It flows directly into the lake from the Greenfield Lake watershed, so that's why we have some of the problems that we have in the lake

It's a familiar dilemma for UNCW biology professor Lawrence Cahoon. He compares managing an urban watershed to rolling a rock uphill and says the situation is especially difficult for Greenfield lake since that one is not flushed very well the nutrients tend to be trapped and they stay there and stimulate the algal growth, so it's a tough situation.

The real test of the carp will come later in the year, when the water heats up and the algae starts booming. But some locals out enjoying the lake recently say things have already improved.

It's been better than it used to be, according to Donna, who didn't want to give her last name. 'cause it used to be real filthy. I couldn't bring my grandchildren out here. Now I can bring them. Now it's more cleaner, the smell is pleasant, and you see a little more people out here than you used to.

Fisheries manager Morton says those people may eventually notice the carp, too. The fish he's added to Greenfield have an eight-to-twelve-year lifespan, making them a feature of the lake for years to come.

As people walk around these shallow areas, if they see large swirls swirling away from them, there's a good chance they're grass carp. these fish are very skittish. They tend to jump at the first ripple that disturbs them.

Not always skittish enough, though. Minutes after Morton emptied his final bucket, a waiting raptor made a sudden dive and came up with a fish that looked suspiciously carp-like. Seven dollars of city money turned into bird-sushi. How well the rest of that investment goes remains to be seen.

Megan Williams, WHQR News