Aquatic Invaders Threaten the Great Lakes

If you live near the Great Lakes, there is a good chance you’ve heard of the fish menace known as the Asian Carp. In the past several years, tens of millions of dollars have been spent to keep these invaders out of our waterways. So, what exactly are Asian Carp? How did they get here? And what makes them so harmful to our native fish?

What are Asian Carp?

The name “Asian carp” refers broadly to four separate species of fish, all hailing from Asia: the bighead carp (Hypophthalmichthys nobilis), black carp (Mylopharyngodon piceus), grass carp (Ctenopharyngodon idella) and silver carp (Hypophthalmichthys molitrix). All four species pose a significant threat to native fish, but their effects on ecosystems differ.

Silver carp and bighead carp are the two most damaging species, since they feed on aquatic plankton — tiny organisms which form the base of the aquatic food chain. Grass carp (perhaps unsurprisingly) feed on aquatic plants, while black carps feed on aquatic snails. All four species can grow very large in size, up to four feet in length and weighing up to 100 pounds! They can reach this massive size because they are voracious feeders. In fact, both bighead and silver carps lack true stomachs, so they must eat more often and consume larger portions than other native fish species to survive.

How did Asian Carp invade the Great Lakes?

The story of Asian Carp in the US spans several decades. In the 1970s and 1980s, Asian Carp were used in aquafarming in the US, where they were bred in a captive environment. Typically, these fish were used to control algal growth and improve water quality, thereby supporting the growth of other aquatic species being bred as food. By the mid-1970s, the first Asian Carp began to escape from stock ponds and facilities and make their way into natural waterways. Within a few years, populations of bighead, grass, and silver carp began to migrate north up the Mississippi River, providing access to the Great Lakes. By the late 1980s, sterile grass carp had entered the Great Lakes. In 1996, electric barriers were constructed in the Chicago Sanitary and Ship Canal (CSSC), which connects the Mississippi River to Lake Michigan. The goal of this project was to prevent bighead and silver carp from entering the Great Lakes.

Despite its initial promise, concerning signs that some Asian Carp had made it through eventually emerged. In 2010, a bighead carp was caught only six miles from Lake Michigan. A 2012 study detected environmental DNA or eDNA, genetic material that is expelled into the environment, from Asian Carp in Lake Erie. The next year, eDNA was found in Lake Michigan, near Green Bay, Wisconsin. By 2014, eDNA from either silver or bighead carp had been found in the Kalamazoo River in Michigan, indicating that Asian Carp had invaded waterways upstream of the Great Lakes. Today, there are sporadic indications of Asian Carp living in the Great Lakes, but no signs of a large, established population. This year, an additional electric barrier is set to be constructed in the CSSC, to help catch any fish that make it past the original boundaries.

What makes Asian carp a harmful invasive?

The fact that Asian Carp are big eaters causes big problems. Grass carp can drastically reduce the number of aquatic weeds used for native fish habitats. Additionally, their waste can introduce nutrients into the water, causing algal growth and decreased oxygen levels. Black carp feast on mussels and snails, some of which are threatened or endangered species. Decreasing the supply of these organisms can also negatively impact mammals, such as otters and muskrats, that depend on them for food. Similarly, by consuming vast quantities of plankton, silver carp and bighead carp can harm the mollusks and native fish species that feed on them. This could in turn affect native waterfowl that rely on these fish species for sustenance. Furthermore, Asian Carp are explosive breeders and can quickly fill up a lake, crowding out native fish and using up resources. They are notorious for their ability to leap out of the water when disturbed by watercraft, or even at random, which can pose a risk of injury to humans using invaded waterways for recreation. The invasion of Asian carp would also create substantial economic loss for fishermen, for whom the Great Lakes generate billions annually.

Where populations of the fish have exploded, local communities have adapted. Though it is not considered to be especially high quality, some fishermen are catching Asian Carp for food. Businesses have emerged that offer paying customers the chance to hunt Asian Carp by bow or shotgun. Additionally, official Asian Carp control programs are available for hire by federal agencies. It is important to remember that while humans can adapt our way of life to deal with these fish, native wildlife doesn’t always have this same ability, and that it’s up to us to protect the larger ecosystem. Since their unintentional introduction in the mid-20th century, Asian Carp have gradually spread throughout much of the waterways in the Midwest, and now pose a grave danger to the Great Lakes. In January, an 800+ million-dollar project called the Brandon Road Ecosystem Project was discussed between Michigan and Illinois to control the fish, which will include more barriers and deterrents to keep the fish out. There are uncertainties regarding the extent of problems the invaders will have on the species native to the lakes. For now, our best bet for management is continued surveys, monitoring, and hunting programs to keep our lakes and streams natural and healthy for the years to come.