About Carp / An Invasive Fish

First introduced into the United States as a potentially important food fish, the carp was stocked throughout much of the country in the 1880s and 1890s. The species spread rapidly, so that by early this century a fish still highly esteemed throughout much of Europe was considered a pest in the United States, a reputation that, with few exceptions, it retains to this day.

Carp are primarily bottom (benthic) feeders, eating a variety of animal and plant life (although it is not uncommon to see carp feeding at the surface on hot, stagnant summer days). Overabundant carp can severely deplete lake bottom food sources needed by species such as yellow perch, bluegills, and channel catfish.

Rooting activities of foraging carp stir up sediment and increase water turbidity. Turbid water makes it difficult for sightfeeding game fish such as walleye, sauger, and northern pike to see their food, as well as an angler's bait. Carp impede growth and establishment of submerged aquatic plants by directly uprooting plants, and hindering growth as turbid water blocks sunlight needed for photosynthesis.

Elimination of submerged plants can diminish water quality, because submerged plants reduce windcaused turbidity and help cycle nutrients. Submerged plants also tie up nutrients, which helps control algae blooms in nutrient rich waters. Besides aiding water quality, fish food organisms live on or near aquatic plants. Some fish species, like northern pike and perch, need aquatic plants for spawning. Underwater plants also provide cover for small fish to escape from predators.

How Carp Dominate Waters?

In many nutrient-rich reservoirs, carp may at first reproduce successfully, resulting in one or two strong year classes in succession. Their long lifespan (up to 15 years or more) and potential to reach a large size (up to 20 pounds or more) allow fish from strong year classes to grow steadily, rapidly at first, but more slowly later on as they become large and exceed natural limits of their food supply.

Eventually the reservoir is full of large, old carp whose reproductive success is poor, because the reservoir's carrying capacity for carp is reached. Growth and survival rates of young-of-the-year are low. Although few young carp are added to the population, the damage was done when the strong year classes were produced.

The Old Carp Syndrome may persist for years until a die off reduces the dominant year classes. If the die off is sudden, the entire process can start over. This cycle explains why biologists have often removed large numbers of carp from waters, only to see survivors reproduce successfully and recreate the Old Carp Syndrome 10 years later.

With fewer old carp, food for younger carp may be abundant. They can grow rapidly, and quickly reach a size too large for predation. This rapid growth permits a carp to live most of its 15 or more years without risk of predation. The carp's high fecundity (reproductive capacity) up to 2 million eggs in a 20-pound female and its adaptability to spawn in different situations gives it an edge over many native or preferred species.

In addition, poor landuse practices in a watershed can result in excessive sediment input and nutrient enrichment in a lake, conditions which often favor rough fish over other species. Good land use practices become especially important in reservoirs draining large watersheds, a common situation on the Great Plains.

Because carp can alter their own aquatic habitats to the detriment of other species, and because they can quickly outgrow predation, fishery managers must often intervene in carp-dominated waters to reduce their numbers. Contracts with commercial fishermen for harvesting them have often been used, but economic returns of harvesting and marketing carp are marginal.

In ponds and small reservoirs (less than 300 acres), a common approach to restoring a desirable fish community is complete eradication of the fish community with rotenone. Rotenone, a natural fish toxicant, biochemically inhibits oxygen use by fish. After the eradications, the waters are typically stocked with more desirable species.

Midsized reservoirs with carp create special problems because eradication is usually too expensive. Other methods must be developed to manipulate a fish community, improve water quality, and rehabilitate the sport fishery. For this reason, the North Dakota Game and Fish Department has begun a three-year project on Bowman-Haley Reservoir to evaluate methods for controlling undesirable species in "problem" midsized reservoirs.

The intent of the project is to develop an effective approach for sustaining a quality fishery, and incorporate the approach into a long-term management plan for this and similar-sized reservoirs with carp problems.

Bowman-Haley Reservoir is a 1,750 acre impoundment on the North Fork of the Grand River located 20 miles south of Bowman.

Despite repeated stockings of game fish, fishing success has gradually declined as rough fish thrived. In 1993, the fish community in Bowman-Haley was dominated by an estimated 1.5 million pounds of carp, nearly all of which weighed at least five pounds and were at least 12 years old.

Submerged aquatic plants were nearly absent throughout the reservoir despite large areas of shallow water apparently suitable for them. Without aquatic plants, the reservoir became turbid when wind riled sediments below shallow water. When the wind remained sufficiently calm for the lake to clear, high nutrient levels in the clear water quickly gave rise to blooms of bluegreen algae.

Such blooms act much like sediment caused turbidity, preventing light from penetrating the water and hindering growth of submerged plants. Fishing, boating, and other recreational uses of the reservoir were suffering. Predators such as northern pike and walleye have had little effect in reducing the multitudes of large carp, and disappointed boaters and anglers increasingly voiced their concerns.

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