Invasive Asian Carps in North America

Abstract

<em>Abstract</em>.—The Asian black carp <em>Mylopharyngodon piceus </em>is of interest to the aquaculture industry in the United States as a biological control for snails and mollusks. However, past experience in North America with other Asian carps has raised concern that black carp will establish wild populations and negatively affect native populations of fish and invertebrates, especially mollusks. The demand for black carp has led biologists to seek ways to allow their use while at the same time maintaining control over their distribution and reproduction. Physical containment and restrictions on importation, release, and stocking have mostly failed. Control of reproduction holds more promise. The induction of triploidy (having three sets of chromosomes), which can render an individual biologically sterile, is of particular interest. The main purpose of this study was to evaluate the efficiency of proposed testing procedures used to assure genetic triploidy in black carp prior to distribution by the state of Missouri, using black carp and grass carp <em>Ctenopharyngodon idella</em>. Our objectives were to (1) verify if the ploidy determination methodology (nuclear size) employed was 100% accurate, (2) determine growth and survival of juvenile black carp over extended periods of time under laboratory and pond conditions, and (3) histologically examine development and gametogenesis in gonads collected from triploid and diploid black and grass carps of different ages and stages of maturation. Comparison of erythrocyte nuclear size using the Coulter counter method versus the more accurate method of flow cytometry that measures DNA content indicated an error rate of 0.25% by the former method. Black carp grew and survived well in mid-Missouri ponds. Triploid grass carp males appeared to produce functional gametes, and some triploid black carp male testes had apparently normal spermatocytes within cysts. A few normally developing oocytes at previtellogenic and vitellogenic stages were observed in triploid grass carp females, and a few normal perinuclear oocytes could be identified in triploid black carp females. Currently, the standards of the U.S. Fish and Wildlife Service’s triploid grass carp voluntary inspection program are being followed by some states to manage triploid black carp. Our results indicate that although the percentage of diploid black carp that could pass through the currently proposed screening program is small, overall numbers of diploid black carp distributed in a state could be substantial depending on the number of triploids distributed. Furthermore, despite indications that triploid male black carp can be expected to be functionally sterile, reproductive studies may be warranted given the large wild populations of diploid grass carp, bighead carp <em>Hypophthalmichthys nobilis</em>, and silver carp <em>H. molitrix </em>in the Mississippi River basin system.