Grass carp (Ctenopharyngodon idella), have been introduced throughout the Midwest and South of the US to clear ponds choked with weeds. However, the release of fertile fish is illegal and so is the purchase or trade in Ohio and surrounding states. For the very same reason they are used as biological control agents grass carp pose a number of threats. They eat soft-stem vegetation - the kinds of plants that dominate coastal marshes which are prime breeding grounds for game fish and act as filters that clear the water.
Hatcheries are breeding carp, but shock their eggs with drastic changes in water pressure. The shock results in a triploid animal and the third set of chromosomes inhibits the production of viable gametes and causes individuals to be reproductively sterile These sterile fish can then be responsibly used for biological control of invasive aquatic plants, but the release of fertile fish could result in growing populations that could devastate the Great Lakes ecosystems. Although the pressure shock method of creating triploid grass carp is nearly 100 % effective, a small proportion of diploid individuals are produced and those have already been found in in Lake Erie's western basin.
Current methods of determining ploidy of an individual require the use of rather expensive laboratory equipment and cannot be performed under field conditions. Researchers have now developed a fast, inexpensive technique to distinguish diploid and triploid grass carp under field conditions using a compound microscope. The shape of the nuclei in red blood cells of carps looks different depending upon whether the fish has a pair or three or more sets of chromosomes. The proportion of abnormally shaped nuclei grows with the increasing number of chromosomes.
The colleagues smear a drop of fish blood on a slide, let it dry and fix it with methanol. They then stain the slide to bring out the nucleus, and, after rinsing and drying, view it under a standard microscope. Sterile triploid grass carp have a noticeably larger proportion of red blood cells with abnormal nuclei.
The study also included a blinded test to confirm the diagnostic reliability of the visual assessment of ploidy. By viewing blood-smear slides, 14 of 15 staff and interns of the Natural Resources department of Cleveland Metroparks correctly identified the fish; only a single intern incorrectly identified a single fish's reproductive potential.
This test has the potential to be very helpful. The results are pretty convincing, but I would like to see it field-tested by management agencies.