confinement and enter natural ecosystems (USDA, 1995). Induction of triploidy reduces the likelihood that an introduced species would establish self-sustaining populations, because such animals are theoretically sterile. For example, the triploid Suminoe oyster (Crassostrea ariakensis) is being assessed for oyster production in the Chesapeake Bay, where diseases complicate restoration of the native Eastern oyster (C. virginica). Should triploidy prove an effective means for reproductive confinement, culture of sterile Suminoe oysters could support the recovery of the declining Chesapeake oyster production industry.
Another benefit of producing sterile mollusks is in maintaining product quality throughout the year. The meat quality of oysters is high just before they spawn, but low after spawning. The product quality of reproductively sterile, triploid oysters remains high year-round. Hence, triploid stocks of Pacific oyster (Crassostrea gigas) provide a tangible benefit to aquaculturists, and now make up almost half of commercial production in the Pacific Northwest.
Unfortunately, repeatable induction of 100 percent triploidy on a commercial scale poses a considerable technical challenge. Non-triploid larvae within batches of larvae easily can go undetected if their frequency is low (USDA, 1995). Should triploidy be desired for purposes of maintaining product quality and the species is indigenous to an area, no harm is posed. If, on the other hand, triploidy is to be utilized for reproductive confinement purposes, the presence of reproductively fertile individuals—even in low numbers—might establish progeny and a self-sustaining population. There also are indications that a small percentage of triploid oysters can progress to a “mosaic” state, with diploid cells arising within the background of triploid cells, leading to the possibility that they could produce viable gametes (Calvo et al., 2001; Zhou, 2002).
Triploidy often has been used to reduce the likelihood that introduced finfish species would establish self-sustaining populations. Use of all-female triploid stocks has been suggested as a means of achieving reproductive confinement of transgenic fishes, including Atlantic salmon (the leading candidate for commercialization). As with mollusks, however, repeatable induction of 100 percent triploidy poses a considerable technical challenge, and commercial net pen operations produce hundreds of thousands of salmon, with many escaping (Hallerman and Kapuscinski, 1992b; Carr et al., 1997; Fiske and Lund, 1999; Volpe et al., 2000).
Another technology used on finfish is to farm monosex fish stocks (Beardmore et al., 2001), which are preferred by producers either because one gender grows faster or larger than the other (e.g., males in catfish and tilapia, females in rainbow trout), or because certain species (e.g., tilapia) attain sexual maturity before reaching harvest size. Monosex populations have been established in several ways, but most reliably through hormone-induced gender reversal. All-male fry can be produced by direct administration of testosterone in feed, or all-females by administration of estrogens. Monosex stocks also can be