gains (Pisenti et al., 1999). In 1950, a commercial bird took 84 days to reach a market weight of 1.8 kilogram. By 1988, this market weight was reached by only 43 days (Pisenti et al., 1999) on about half the amount of feed (Lacy, 2000).
Scientific breeding, combined with better nutrition and veterinary care, clearly has produced breeds of animals that are remarkably productive, although sometimes strikingly different in habits and appearance from those farmed early in the twentieth century. The practice has also led to a loss of many breeds of livestock and fowl, and a decline in genetic diversity within the breeds that survive. For example, it has been estimated that there were several hundred specialty lines of chicken in North America at the beginning of the last century, whereas the number of commercial hybrid strains now available through suppliers is fewer than 10 (North and Bell, 1990).
Aquatic animals, including finfish and shellfish, now are farmed, and specific breeds that have been selected for growth and other traits are established now in the largest industrial sectors of aquaculture, such as channel catfish, rainbow trout, and Atlantic salmon. The growth and quality of such animals are also amenable to genetic engineering through modern biotechnology. Genetically engineered or highly selected aquatic species present special problems in terms of confinement, as the features that might make them attractive commercially might pose risks to the genetic base of their wild relatives with which they can interbreed (Hallerman and Kapuscinski, 1992b).
Insects also have been domesticated for farming. The two best-known examples are the honeybee and silkworm; considerable genetic gains in productivity have provided strains of these insects far removed from the ancestral species from which they were derived. Attempts to develop strains of honeybee with improved resistance to pathogens and silkworms that produce proteins other than silk are on the horizon. Insects, like fish, are especially difficult to confine so that “escapes” are almost inevitable. In addition, insects, including ones that can be engineered transgenically, are likely to continue to be used as part of biocontrol programs for pest insects and invasive plant species and, as such, might be intentionally released into the environment. There will almost certainly be attempts to replace or to infiltrate native populations with insects that have been engineered in such a manner that they are less of a pest or unable to transmit pathogens (Hoy, 2000). Private-sector companies already have begun to farm recombinant proteins (antibodies, cytokines, enzymes, and bioactive peptides) from insect larvae. Whereas the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) regulates the release of insects for pest management, it is unclear which agency is responsible for protecting against accidental release of insects from mass rearing factories. Horizontal gene transfer, disruption of ecosystems, and native species extinctions are among the potential hazards that arise from permanent releases of transgenic arthropods into the environment (Hoy, 2000).