outcomes of introgression of a transgene might differ among receiving populations. Coho salmon (Oncorhynchus kisutch) expressing a growth hormone construct exhibited extraordinary growth (Devlin et al., 1994), underwent parr-smolt transformation approximately six months before nontransgenic siblings, and some males matured at just two years of age (Devlin et al., 1995b). However, swimming performance of transgenics was poor (Farrell et al., 1997), perhaps because of a developmental delay or from disruption of locomotor muscles or associated support systems, such as the respiratory, circulatory, or nervous systems. Some growth-enhanced fish exhibited abnormalities of opercular (gill cover) morphology that might disrupt respiration and contribute to poor swimming performance. In competitive feeding trials, Devlin et al. (1999) showed that GH transgenesis increases the ability to compete for food, suggesting that transgenic fish might compete successfully with native fish in the wild. Devlin et al. (2001) noted that the greatest response to expression of the transgene was in Coho hybrids of a wild and domesticated strain; hence, the effects of an introduced growth hormone gene might differ among stocks.

In a study posing implications for introgression of transgenes into wild populations, Devlin et al. (2001) examined the fitness effects of expression of a GH construct in both wild and selectively bred commercial rainbow trout (O. mykiss) strains. Transgenic wild-strain rainbow trout retained the slender body morphology of the wild-type strain, but their final size at maturity was much larger than that of their nontransgenic ancestors. Both domestic and wild-strain trout exhibited reduced viability; in the domestic strain, all transgenic individuals died before sexual maturation. The tradeoff of size (and likely mating success) and decreased viability parallels the case modeled by Muir and Howard (1999), and suggests that the viability of a receiving population might be compromised. Devlin et al. (2001) noted that the greatest response to expression of the transgene was in hybrids of a wild and domesticated strain; hence, the effects of an introduced growth hormone gene might differ among stocks. The importance of genetic background on expression of growth hormone was demonstrated also by Siewerdt et al. (2000a,b) and Parks et al. (2000a,b). While indicative that risk issues must be regarded with seriousness, the growing collection of empirical risk assessment studies of transgenic salmonids does not yet provide a body of data useful for parameterizing a model useful for predicting the likelihood that transgenes would become permanently introgressed into wild or feral salmon populations.

However, many of the same physiologic and behavioral differences seen in GE salmon can be induced by using growth hormone implants (Johnsson et al., 1999). As such, implanted fish can model the effects of the transgene and allow the fish to be safely tested in native habitats—an experiment that would be hazardous with GE fish. Working with brown trout (Salmo trutta), Johnsson et al. (1999) showed that survival of GH-implanted trout did not differ from that of