Fish
Silver Carp
Hypophthalmichthys molitrix · also called flying carp, Asian carp
On some rivers in the American Midwest, the danger is not what is in the water but what comes out of it. The sound of a boat motor sets off an explosion of heavy silver fish, launching themselves metres into the air. This is the silver carp, an Asian filter feeder that has taken over whole rivers.
Overview
Hypophthalmichthys molitrix is native to the large rivers of eastern Asia and was introduced to the United States in the 1970s to strain algae from aquaculture ponds and sewage lagoons. It escaped in floods and is now one of the most abundant fish in the Mississippi, Illinois, Missouri, and Ohio systems. It is a filter feeder, straining plankton through a sponge-like organ of fused gill rakers.
The flying carp
Startled by an outboard motor, silver carp leap up to 3 m out of the water. A fish of 5 to 10 kg striking a moving boat has broken bones and knocked people out.
Why it matters
By stripping the plankton it starves the larvae of every native fish and outcompetes native filter feeders like the paddlefish and bigmouth buffalo. An electric barrier near Chicago is the main thing keeping it out of the Great Lakes.
Status
Near Threatened and declining in its dammed native rivers, even as it booms as an invader across the central United States. Fisheries now market it as copi to encourage bulk removal.
Silver Carp: common questions
Why do silver carp jump out of the water?
It is a startle response to sound and vibration in the water, and low-frequency noise like an outboard motor is an especially strong trigger. A disturbed school of silver carp can erupt all at once, with dozens of fish leaping several metres into the air around and into a passing boat. Biologists think the reflex evolved as an escape from predators or from the bow waves of large animals. It makes the fish uniquely hazardous among invasive species, since anglers, water-skiers, and jet-skiers on infested rivers have been struck and seriously hurt by airborne fish, and it has become the image most people have of the Asian carp problem.
How did silver carp get into American rivers?
They were brought in deliberately. In the 1970s fish farmers and water-treatment operators in Arkansas and neighbouring states imported silver carp, along with the related bighead carp, to control algae in aquaculture ponds and in the lagoons used to treat municipal and farm wastewater, since the fish are efficient living filters. Some escaped almost immediately, and major flooding along the Mississippi and its tributaries in the 1980s and 1990s let large numbers move from ponds into the open river system, where they found ideal conditions and spread rapidly upstream and downstream.
Why are silver carp so damaging?
Because they compete at the very base of the food web. Silver carp feed by filtering plankton, and in the rivers they have invaded they do it so effectively that they strip the water of the microscopic plants and animals that everything else depends on. The larvae and young of native fish, from paddlefish to bass, feed on that same plankton and go hungry. Native filter feeders such as the bigmouth buffalo and the gizzard shad decline. Because silver carp grow fast, breed in huge numbers, and have no natural controls in North America, an invaded river can end up with its fish biomass dominated by a single low-value species.
Can you eat silver carp?
Yes. The flesh is white, firm, clean-tasting, and high in beneficial oils, and silver carp is a common and valued food fish across its native range in China. The obstacle in North America is a mesh of fine intramuscular bones and a public image problem. To get around both, a marketing effort has rebranded the fish as copi and promoted it ground, smoked, or in cakes and patties where the bones are not an issue. The aim is partly culinary and partly control, since a strong commercial fishery is one of the few tools that can remove silver carp from a river faster than they breed.