Grass carp do their job, sometimes too well
When I watch a white amur eating vegetation in my little pond, I have to suppress my gag reflex.
These fish, also known as grass carp, have a way of widely opening their mouths, “inhaling” a gob or long strand of vegetation, and chomping down. When actively feeding, they keep repeating and repeating that action and it is all you can do, watching, not to choke. They are eating machines.
Which is exactly why I bought a couple chunky little 10-inch amurs this spring and stocked them in my pond. These Asian fish are as hardy as their roiling, pestiferous common carp cousins that we foolishly stocked here in the 19th century. And they could become just as troublesome.
So why did I plant two of them in my pond? Because my amurs are not going to reproduce. They are sterile, treated at the hatchery stage to have three sets of genes instead of two. Thus they are rendered incapable of reproducing.
I stocked one amur in my pond a few years ago in hopes that it would control rooted pond vegetation and algae, which had grown rampant. It took two or three years of gobbling, but that fish grew like Topsy and eventually became a hulking 30-inch, 8-pound pond hog. It did its job so well my pond was left with no vegetation at all. Not good. The fish even took to rooting the shallows in search of remaining plant roots and muddying up the waters as a result. The bass, bluegills, dragonflies, turtles, snakes, and a host of other aquatic creatures and insects that form a pond ecosystem suffered from too much control. Pond management always is a balancing act.
I had to dispatch that big, old amur; it fed local raccoons and other scavengers, the carcass gone in a couple of nights, down to a few big leftover scales and a rib or two.
It took my pond a couple of summers to recover from being a water-desert. But inevitably it started to regenerate its aquatic plants (it would be ignorant to call them weeds). This spring, the green regrowth was back in spades, too much so. I found myself on the verge of choking the pond again with too much vegetation. I long ago tired of “raking” the pond to remove the thick mats of vegetation. Amurs do it gleefully, for free, without cease. Stocking a couple of them was a no-brainer.
But I’ll keep a close watch on my pond’s two carp newcomers this time, to make sure they do not eat themselves out of house and home. They are entertaining to watch, if you can avoid gagging.
The foregoing experience with my sterile grass carp stands in contrast to developments in northwest Ohio’s Sandusky and Maumee rivers, and the great body of water that they feed, Lake Erie.
There, white amur or grass carp have become a cause for concern, for these fish indeed are fertile.
Back in the 1970s, diploid or genetically sterile white amur were imported, reared, and stocked widely to control vegetation in ponds. Some escaped in flood events and some accidentally or even purposely may have been planted in streams and ultimately ended up in waters like the Lake Erie system.
It was found that the fish are reproducing in low numbers in spawning streams, especially the Sandusky River, and so far, to a lesser extent, the Maumee.
As a result, fisheries biologists from state, federal, provincial, and private agencies have undertaken an annual project on the Sandusky and Maumee to assess their ability to capture grass carp.
Crews joining in came from the Ohio Department of Natural Resources (ODNR), the Michigan DNR, U.S. Fish and Wildlife Service; Department of Fisheries and Oceans Canada; Ontario Ministry of Natural Resources and Forestry; Minnesota DNR; Great Lakes Fishery Commission; The Nature Conservancy; U.S. Geological Survey; Quebec Ministry of Forest, Wildlife, and Parks; New York State Department of Environmental Conservation; Michigan State University; The Ohio State University; and the University of Toledo. It was quite a gathering, which underscored the importance of the mission for all the Great Lakes.
The project, conducted recently during grass carp spawning time, meant three days of looking for and collecting grass carp using electrofishing vessels and nets. In all, 31 grass carp were collected – 28 from the Sandusky River and three from the Maumee River. The project was successful largely because it occurred during a period of high stream flows and at spawning time, when the fish concentrate, biologists said.
Bottom line here is that although present in the system, grass carp populations are considered to be low. One aspect of the project is to reduce if not eliminate even the low populations of grass carp, given their destructive potential. (Just look what one could do in my pond.)
For the record, all species of Asian carp do not have the same negative ecological effects. Grass carp present significantly different risks to the Lake Erie ecosystem compared to highly invasive bighead carp and silver carp.
Adult grass carp commonly weighs more than 20 pounds and can grow up to 48 inches long. The fish are primarily herbivorous. Because they eat such large quantities of aquatic vegetation, they can affect other native fish communities, primarily through habitat modification, along with waterfowl using lowlands and wetlands.
But they stand in sharp contrast to their prolific, highly reproductive cousins, bighead and silver carp. The latter two species infamously have infested the entire Mississippi River system, including the Ohio River and its tributaries, all but destroying native fisheries. A fourth species, black carp, are not at this point considered a significant threat.
Of the 31 fish captured recently, nine were outfitted with tracking devices and released; 21 will be studied to determine their age, origin and if they were sterile stock or not, and one fish that had been captured previously and already had a tracking device was released, Travis Hartman, Lake Erie program coordinator for the ODNR, told The Blade in Toledo.
“From those 21 fish we’ll be looking at, we will look for evidence that they came from the Sandusky River system,” Hartman told the newspaper. “With everyone partnering in this manner, we’re learning so much more about these fish and looking at how we can control or eradicate them. It is tough to assume that we can eradicate them, but now we know that if there is a high-flow spawning event, we can successfully remove them from the waterway.”