Chapter 1: Introduction
Bigheaded carps of the family Cyprinidae, the largest family of freshwater fishes (Nelson 2006), have long been introduced beyond their native ranges in Asia, a practice that continues today. Although carps have been introduced for several centuries, the widespread introduction of the genus Hypophthalmichthys, the bigheaded carps, is a relatively recent phenomenon. All three recognized species of Hypophthalmichthys—H. nobilis, in North America referred to as bighead carp; silver carp H. molitrix; and largescale silver carp H. harmandi—are native to fresh waters of eastern Asia. Largescale silver carp have been introduced elsewhere in west-central Asia as a hybrid with silver carp but are not known to have been brought to North America. Both bighead and silver carps have been introduced to many countries, including the United States, for use in aquaculture production of food fishes and biological control of plankton in aquaculture ponds, reservoirs, and sewage treatment lagoons.
Bighead and silver carps were first imported into the United States in the early 1970s. Soon after, both species were used in research projects and stocked into wastewater treatment lagoons and aquaculture ponds in several states without regard to their potential negative effects on native species and ecosystem functioning. Bighead and silver carps escaped confinement and are now well established with reproducing populations in much of the Mississippi River basin. The introduced range of both carps in the United States continues to grow. Based on the climate where these fishes are native, bighead and silver carps might eventually be found in many of the flowing waters of the United States, much of Mexico, and parts of Canada.
Escape of bighead and silver carps during evaluation as plankton biological control organisms for commercial aquaculture ponds and sewage treatment facilities has left a legacy that could affect native fish populations within the Mississippi River basin for decades to come. Populations of these carps in parts of the Mississippi River basin appear to be increasing exponentially (Chick and Pegg 2001). If food resources become limiting, bighead and silver carps may compete with native planktivorous fishes, like gizzard shad Dorosoma cepedianum, bigmouth buffalo Ictiobus cyprinellus, and paddlefish Polyodon spathula. In addition to continuing to expand their distributional ranges farther in the Mississippi River basin by natural dispersal, the spread of bighead and silver carps could be aided by transportation of fishes caught for live bait, by live haulers, the live food fish industry, and by those practicing prayer animal releases (a cultural practice by those who believe that freeing captive animals into the wild is virtuous; Severinghaus and Chi 1999).