Saturday, January 28th, 2023
Saturday, January 28th, 2023

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Are grass carp doing the tango in the Tittabawassee?

Mike Schoonveld

Great Lakes Log Book


Excessive amounts of submerged vegetation has probably been a problem in small lakes and ponds since the glaciers retreated. Excessive pond weeds are likely more prevalent now, since many wetlands have been drained so rainwater hurries downhill faster and contains more nutrients than had it been slowed and filtered by wetlands. 


If that pond or small lake is yours it’s personally troublesome. Weeds make it unswimmable, hard to fish, and can be so thick they actually hurt the fish population. Trying to cut the weeds mechanically is expensive, time consuming, and requires special equipment. Then, like a run-away lawn in June, miss a mowing for a week or so and the growth is out of control. Chemical herbicides can be the answer for private ponds, but that’s costly, too. It’s best done by trained applicators. And who wants to catch fish or swim in a lake tainted by herbicides?  


So the search was on for some other, more natural, less expensive and less time-consuming method of keeping ponds weed-free. One of the things discovered is a fish called a grass carp. They got their name because they feed on submerged “grass” and other underwater plants. 


Like other kinds of carp – including bighead and silver carp, which have invaded the Midwest’s great rivers and threatens the Great Lakes – they‘re native to Asia. 


To be fair, the issues associated with invasive species a few decades ago weren’t as “front burner” as they are these days. Environmentalists were rightly concerned with acid rain, nuclear waste, and persistent pesticides. So importing a few grass carp to eat weeds from private ponds seemed a likely, biologically safe control solution. 


Most DNRs, however, had a few forward-looking biologists in their ranks who were smart enough to think, “What if the carp escape?” 


Dams fail. Ponds overflow. Even 100-year floods show up every 10 or 12 years, or so it seems.


For those reasons most states banned or regulated the importation of grass carp. Then researchers learned to heat-treat fertilized grass carp eggs at a certain point of development, which would change their chromosome count from two to three. The resulting carp were then sterile and unable to reproduce. Put a couple of sterile carp in your pond and let them munch away. 


It mostly worked. 


These carp, called “triploid” due to having three chromosomes, are in such demand that dozens of private hatcheries crank out hundreds of thousands annually to sell to pond owners. 


“Mostly” is the operative word. To make the triploids, there has to be a few diploid, non-sterile grass carp at each hatchery. What if those fertile, brood-stock carp escape? What if one egg in a million or so isn’t rendered sterile with the treatment? What if some yay-hoo imports or produces a batch of non-sterile grass carp accidentally – or on purpose?


The result is any number of lakes, rivers, and reservoirs – mostly in the southern states – contain non-sterile grass carp populations and biologists now have to contend with them and factor these fish into their management plans. Fortunately, that’s not a problem here in Michigan, at least until recently. 


The Michigan DNR never jumped on the triploid grass carp bandwagon nor allowed private pond owners to import them. That’s not to say hundreds (if not thousands) of triploid carp haven’t been smuggled in from Indiana or other states where they are legal. They have. 


And they have escaped to rivers and infiltrated by some means other lakes – even the Great Lakes. Since 1989, 13 grass carp have been captured in Lake Huron but all were escaped sterile triploids. Whenever a grass carp is encountered by the DNR, it’s tested to see if it’s sterile or capable of reproducing. 


Recently, a two-chromosome, reproductive-capable grass carp was captured in the Tittabawassee River in Midland County. The Tittabawassee flows into the Saginaw River, which flows into Saginaw Bay, which is part of Lake Huron. 


It’s not yet been determined where or how the grass carp originated. The DNR is working to learn if it was a wild or hatchery fish. Here’s hoping it was a loner and never encountered an opposite sex, fertile female. 


On the other hand, it only takes two to tango.

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