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By Becky Beyers
Carp are the original bad boys of Minnesota’s waterways.
Although the common carp is Minnesota’s first and arguably most damaging invasive species, it has attracted little attention from researchers who until recently viewed it (like many invasives) as a lost cause. The common carp dominates fish communities in shallow lakes, wetlands and rivers across most of the upper Midwest as well as vast regions of Australia, Argentina and South Africa. It may be world’s worst invasive and damaging fish.
Through its habit of rooting in the bottom for food, this species literally turns aquatic food webs upside down, turning water in many lakes (including thousands of lakes in Minnesota) pea green to brown and removing beneficial vegetation. Because carp have been with us for more than 100 years, many people do not even know what their lakes should look like. Peter Sorensen, professor in the Department of Fisheries, Wildlife and Conservation Biology, is one of the world’s top carp experts and along with his research team is finding solutions to controlling this species that may provide lessons for controlling other invasives.
The common carp was imported to the United States in the 1870s by the predecessor of the U.S. Fish and Wildlife Service, at the request of European immigrants who longed for the fish from home. The agency raised adults in Washington, D.C., and then stocked young carp across the country for nearly three decades before eventually realizing that they had inadvertently created one of the continent’s worst environmental catastrophes.
Interestingly, the carp’s origins as an invader can be traced back to the Romans who discovered it in the Danube River basin—which Sorensen notes is similar to Minnesota’s environment—and spread it throughout Europe. Later, it was domesticated by the Catholic Church as a Friday fish choice. Even today, carp is prized by European aquaculture and in the United Kingdom it is a most prized game fish and is always carefully returned to waters after being caught. However, it is generally not invasive there, which gave Sorensen and his associate, Przemek Bajer, a clue as to what might be done in Minnesota: the interactions between carp biology and our environment.
Sorensen and Bajer’s approach is new and differs from that of fisheries biologists, waterfowl managers and watershed engineers (carp are often the leading cause of water-quality declines), who have desperately tried to reduce the number of adult common carp using poisons, water-drawdowns, barriers and commercial fishing. Generally, these nontargeted and expensive efforts have met with little sustainable success, which is not surprising given that a single female carp can carry several million eggs so carp populations can recover extremely quickly if any elude removal. Given that, the Sorensen lab has simultaneously targeted the origins of young carp in watersheds while removing adult carp. This has turned out to be surprisingly tractable because Bajer has discovered that young originate from only a few locations and can be controlled by native predatory fish.
Striking parallels may be found between the common carp and their nasty cousins, the Asian carp. Both are large, long-lived, fecund, produce many eggs and are highly mobile—and highly invasive and damaging. One species of Asian carp, the silver, is making headlines because it can grow to 40 pounds and when startled can jump as high as 10 feet out of the water, damaging boats and harming their occupants. Worse, Asian carp gobble up plankton in waterways, eliminating food for other, more desirable fish and disrupting food chains. Like common carp, Asian carp were purposefully introduced—in their case it was to clean sewage and aquaculture ponds—and now are moving up the Mississippi River and its tributaries and threatening to enter the Great Lakes. Officially, they haven’t established themselves in Minnesota’s section of the Mississippi, but Sorensen says that’s likely only a matter of time and is starting studies on this species as well.
The one thing nobody has wanted to do, Sorensen says, is study the carp. Research funding has been hard to come by, perhaps because “people hate spending money on things they don’t like.” But he’s been down this road before. His work on the invasive sea lamprey—and ways to control that species—showed him that with a deep understanding of a species comes an understanding of how to control it. Every species has its weaknesses; sometimes they are not obvious. After visiting Australia six years ago, where the common carp is considered a noxious species of national concern and the subject of 40-year management plans, he decided to take on the Minnesota problem.
Funding from many local watershed groups and the state lottery now supports Sorensen’s lab’s work in several Twin Cities watersheds and has led to a wealth of new information about how common carp eat, reproduce, spend their winters and interact with other species. A neurobiologist by training, Sorensen pays attention to details. The DNR also provides in-kind support. Experimental management plans that rely on integrated tools also are now in place in several lakes and appear to have great promise. Basic advances in fish biology have gone hand-in-hand.
Sorensen has great respect for the carp, which he believes to be invasive because of several impressive adaptations. “The carp have a very sophisticated feeding system, and it is very efficient—capable of discerning and selecting complex flavors released by tiny organisms found deep in the mud,” he says. Carp also hear and smell better than other fish, and they grow very fast. Their pheromone system is one of the most sophisticated of the fishes and while that’s normally an asset, it can be used against them. The carp also appears to be relatively intelligent and easily able to recognize and remember specific locales. They then use these abilities, together with excellent swimming strength, to find and exploit very specific locations to spawn in ways that few native fish can. Further, because they live to be old (more than 50), their life history allows for them to tolerate and exploit variable and often degraded habitats that other fish cannot.
Studying the carp’s winter habits has also yielded some clues, and success, in eliminating adult common carp from two lakes in Chanhassen, Minn. “They move around differently from native fish,” Sorensen says. In winter, common carp gather very tightly in schools, which provides an opportunity to selectively net large portions of the population.
Last winter’s netting in Chanhassen netted more than 90 percent of the fish in one lake in one afternoon. The trick, then, is to prevent new populations from producing young, but this now appears solvable by managing native fish populations because, given the chance, bluegills and a few other fishes eat vast numbers of carp eggs if they are abundant, Sorensen says. At Lake Keller, in Ramsey County, an experiment directed by Bajer this spring showed that the strategy can work: native fish ate the carp eggs when present, while a control group that lacked these native fish saw normal numbers of carp hatch.
Multiple, integrated approaches likely are the key to controlling populations of carp and other invasive aquatic species, Sorensen says. “Rebalancing native communities in our overfished lakes is an excellent first step that naturally carries other benefits too.” His team’s future work is now seeking to employ pheromonal attractants in combination with radiotagged fish tracked by remote sensors to monitor carp populations in highly precise manners while their abundance can be measured using DNA.
Much of this work is being developed in the laboratory and appears to demonstrate that given a chance, creative modern science has a role helping Minnesota face what otherwise appear to be insoluble invasive-species problems.
