IPM in the South

Depending on where you live and whether you fish for sport or income, Asian carp can be a blessing or a curse. News stories in the past few months have covered some of the fear in the Great Lakes region about silver and bighead carp threatening the fishing industry. In some of the southern states, sport fishermen actually stock lakes with carp to clean the water of algea and to eat. Fishermen and enforcement agents along the Mississippi River are not among the southern crowd who takes pleasure in these alien fish, however. They can see the damage the fish have done along the Mississippi River, and they are first in line to the carp’s travel up the river to the Great Lakes.

There is a lot of information about Asian carp on the web already, so I’m not going to try to reiterate a lot of those details. I am, however, going to explain the situation for those who aren’t sure about why a few species of fish are causing such a ruckus in Michigan.

silver carp

The term “Asian carp” refers to four species of carp imported into the U.S. from Asia: grass carp, bighead carp, silver carp and black carp. The first three species are vegetarians and feed on grass or phytoplankton. Black carp feed on snails and mollusks. Grass carp were the first to be imported into Arkansas (in the 1960s), to clean the algae from ponds and lakes. Bighead and silver were then imported to control reeds and other weeds, some of them invasive themselves. When the zebra mussel began to invade waterways (brought into the U.S. through ballast water), the black carp was introduced to control them.

The threat imposed by Asian carp is similar to the threat imposed by most invasive aquatic creatures: it interrupts a link in the food chain. Like the terrestrial food chain, the aquatic food chain consists of a plant base (phytoplankton or algae, and aquatic plants), primary consumers (consume the plants), secondary consumers (consume the primary consumers or other secondary consumers), and tertiary or top-level consumers.

Healthy food webs have a balance of predators to prey. Species in the higher tropic levels don’t allow species in the lower trophic levels to become too numerous, and the abundance of food from lower level species keeps the higher level species nourished.

When species in one level are scarce, species on other levels become overly plentiful, sometimes to the point of becoming pests. When foxes and wolves are over-hunted, deer and rabbits look for extra habitats, sometimes in residential gardens. When salmon are overfished, secondary consumers like perch breed until the population out-consumes prey on lower levels. When exotic species on lower levels—with no predators at higher levels—reproduce faster and consume food faster than their native competitors, less lower level food is available for higher level predators, leaving higher level consumers to starve to death.

Black carp consume mussels and snails, and even though they consume zebra mussels, they consume native mussels and snails as well. Zebra mussels and the other three carp species feed on algae or aquatic plants, leaving less for native snails, worms and small vegetarian fish. Fewer primary consumers mean fewer secondary consumers such as perch, and top-level consumers such as walleye and salmon (particularly in the Great Lakes) become rare.

In addition to damaging the ecological balance of the lakes, competition by Asian carp could potentially devastate the Great Lakes’ $7 billion fishing industry. As low-valued carp increase, high-priced salmon and walleye diminish. Boating and water sports would be affected as well, since the silver carp literally jump at the sound of a boat motor and have injured several people.

Southern fishermen have already experienced some of the ill effects of the carps’ presence, but because the carp have been in the region for several years, most fish and wildlife specialists know that eradicating them is not an option. To facilitate management of the aquatic pest, Louisiana chefs came up with an option: serve them for dinner. “Silverfin” (silver carp) is now a specialty on many Louisiana menus.

Great Lakes fishermen have not yet become that resigned to the carps’ existence because of the high value of the fish that the carp threaten to displace. Below are several links that go into greater detail about the origin of Asian carp, why they were introduced and the scope of the economic situation in the Great Lakes region.

• Asian Carp (Duke University)
• USGS CERC Project: Ecological Impacts of Non-native Asian Carp
• National Park Service, Mississippi: Asian Carp Overview
• USDA Species Profile: Asian Carp
• Asian Carp Regional Coordinating Committee

Filed under: Invasive species Tagged: | Asian carp, bighead carp, black carp, carp in South, grass carp, Great Lakes, Louisiana fish, silver carp