The use of grass carp has been limited in many areas because of concern about possible adverse effects from their establishment in large river systems and associated estuaries. In response to this problem, researchers developed a technique to create sterile offspring from normal parents by altering the genetic material during fertilization. This technique causes the newly fertilized egg to form one extra set of chromosomes (triploid condition), resulting in a fish that is normal in all respects except that it cannot reproduce. Red blood cells of triploid fish contain an extra set of chromosomes in the nucleus and, consequently, are larger than those of normal, fertile fish. A blood sample from each fish is examined before it is shipped from the hatchery to ensure that only sterile, triploid fish are sent to the distributor for sale. In some states, this test may be repeated on a sample of fish upon delivery.

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