Asian carp establish ‘finholds’ in Lake Erie, Ontario and Michigan

By Engbretson, Eric [Public domain], via Wikimedia Commons

Prediction is that invasive species will be established in Lake Huron within a decade

GREAT LAKES—Invasive grass carp pose a major threat to the Great Lakes basin; with experts warning the species will become established in Lake Huron in 10 years, opinions are strongly divided as to the best ways to deal with the threat, as well as the tactics agencies are using to manage these fish that belong to the Asian carp family.

Mary Muter, chair of the Georgian Bay Great Lakes Foundation, raises strong concerns in a press release her organization had published on July 25. She criticizes the Ohio and Michigan Departments of Natural Resources (DNRs) for their ongoing efforts to catch Lake Erie grass carp, implant them with tagging devices and release them to study their movements and better understand their patterns.

Instead, she says all agencies in the Great Lakes region need to begin eradicating the fish immediately.

Michigan has been a leader in the efforts to tag and study grass carp under direction of the Lake Erie Commission, a group that consists of representatives from provincial- and state-level environment management organizations. This includes the Ontario Ministry of Natural Resources and Forestry (MNRF), the Pennsylvania Fish and Boat Commission, New York State Department of Environmental Conservation, and the Ohio and Michigan DNRs.

According to Seth Herbst, aquatic invasive species coordinator at the Michigan DNR, the agencies in western Lake Erie have been able to use tagging studies to determine fish movement patterns and what habitats they prefer. “We’ve been using that to inform where and when we use our response actions, Mr. Herbst said, adding that they are completing some eradication efforts while conducting these studies.

An invasive species

Grass carp are native to eastern Asia, with a range from Vietnam to the Amur River near Siberia. They are an herbivorous fish, eating aquatic vegetation in mass quantities. Their voracious appetite is why people began to import them to North America, and is the same factor that makes them such a threat.

Pond owners have been stocking grass carp since the 1960s to manage their weeds without having to use pesticides. Grass carp eat vegetation from the top down, rather than ripping the roots out and increasing the water’s cloudiness.

Grass carp are one of four subspecies of the Asian carp family that also includes bighead, black and silver carp. All four are considered invasive species in North America. Grass carp are considered established in the Mississippi River drainage basin, and studies from the past decade have confirmed new fish are now spawning in the Great Lakes basin. Many of those fish have been traced to the Sandusky River in Ohio.

Two types of the same fish

Grass carp had proven themselves to be an excellent way of dealing with nuisance weeds, but sometimes, they were too good to be true. In Texas, officials stocked 270,000 grass carp into Lake Conroe in the early 1980s. After one year, the lake had lost 3,600 hectares of underwater vegetation and close to half of the native fish species had experienced declines. Some states have placed bans on fertile grass carp for this reason, a decision that had major impacts on pond owners who once relied on the fish.

Researchers found an alternative in 1983: If they gave the eggs a shock of either temperature or pressure at a specific time in the development cycle, the fish would keep an extra, third DNA chromosome that is normally discarded. That variation made them effectively sterile, as they would not be able to pass on the proper two-chromosome genes to future offspring. Those fish are referred to as triploid (three chromosomes) and diploid (two chromosomes) by biologists to represent their ability to reproduce.

Although success rates are close to 100 percent, the process is not perfect. Sometimes, best efforts to produce a batch of triploid fish may still result in some diploid individuals slipping through. Producers have to test every fish and the U.S. Fish and Wildlife Service samples 120 fish from each batch before certifying them as triploid, but there is still a small possibility that a presumed triploid fish could turn out to be fertile.

Currently, many states such as Ohio only allow triploid grass carp for vegetation maintenance, while other regions like Michigan and Ontario have outright bans on all grass carp. There are, however, seven states in the continental U.S. that still allow diploids: Alabama, Arkansas, Iowa, Mississippi, Missouri, Nebraska, and the portion of Colorado east of the continental divide.

State of the union

Approximately 12 grass carp have been captured in the Canadian waters of Lake Huron since the late 1980s. Since DFO started its Asian Carp Program in 2012, only two fish have been caught. Both turned out to be triploid.

“It’s a species that we’re concerned about and we go in with the intention of capturing these species in our early detection locations and removing them from the Canadian waters of the Great Lakes,” says David Marson, senior biologist and field operations lead of Fisheries and Oceans Canada (DFO)’s Asian Carp Program.

Grass carp are not likely to reproduce in Lake Huron for the next five years because of their low populations, as stated in a DFO risk assessment from 2017. However, that same risk assessment states that the numerous wetlands and tributaries around Lake Huron—especially Georgian Bay—would be prime habitat in which grass carp could live and breed. The risk assessment also says if existing Asian carp populations grow, the risk level for Huron and the other Great Lakes would increase considerably.

A population of reproducing grass carp now exists in Lake Erie and is poised to continue growing. Studies have predicted and later confirmed that grass carp spawn in Ohio’s Sandusky River, confirmed both by the capture of grass carp eggs in the river and from chemical analysis of mature fish that matched the compounds in the river.

The responsibility to conduct efforts to control and eliminate grass carp within U.S. waters falls to the state level. Higher-up agencies such as the Fish and Wildlife Service and the United States Geological Survey (USGS) help to coordinate a broader, unified effort and provide scientific backing, but the actual work is done by state agencies.

Waterways like the Sandusky and Maumee Rivers in Ohio and the warm water discharge from a power plant in Monroe, Michigan are proven hotspots for these fish, based on previous capture and study work. And while every agency in the Great Lakes basin has a role to play in managing Asian carp populations, because these fish tend to thrive in the western basin of Lake Erie, the response by Ohio and Michigan will play a major role in determining the future of the Great Lakes as a whole. That view is shared by DFO, Ms. Muter and researcher Nick Mandrak at the University of Toronto.

What’s at stake?

The most direct impact of grass carp is an environmental and ecological risk. According to a 2017 report, an average 15-year-old grass carp can eat between 60 and 200 kg of vegetation per year. That same study’s simulated models showed invaded wetlands lost over half their vegetation after just one year.

Wetlands are key to managing water quality by filtering out pollutants and providing opportunities for animal life to thrive. Wetlands are already under threat from invasive plant species, such as the phragmites infiltration on which The Expositor has previously reported—phragmites appears to be one of the few aquatic plants that grass carp do not eagerly eat.

Wetland loss also directly impacts fish populations, as wetlands provide numerous fish species with spawning grounds, food and cover from predators. As habitats are destroyed and available food becomes increasingly scarce, populations of native species would begin to decline.

That has economic impacts, too. If fish stocks decrease, Canada’s fishery industry stands to lose a great deal. As of 2018, commercial and recreational fishing, wildlife viewing and lakefront use total more than $1.2 billion in economic activity. That number could reach nearly $30 billion in 20 years, according to a DFO report.

Managing established invasive species is both challenging and expensive. Canada alone had spent $8.1 million on its sea lamprey program in 2016-2017, an invasive species that is established throughout the Great Lakes.

“Do we want another sea lamprey program, which costs $20-30 million a year to control them? That’s where we’re heading if we don’t act soon,” says Mr. Mandrak.

This is the first part of a series on grass carp in the Great Lakes. If you would like to learn more about Asian carp and the work being done to manage and understand them, visit or See our website at for further coverage.