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Domesticated and growth hormone (GH) transgenic salmon provide an interesting model to compare effects of selected versus engineered phenotypic change on relative fitness in an ecological context. Phenotype in domestication is altered via polygenic selection of traits over multiple generations, whereas in transgenesis is altered by a single locus in one generation. These established and emerging technologies both result in elevated growth rates in culture, and are associated with similar secondary effects such as increased foraging, decreased predator avoidance, and similar endocrine and gene expression profiles. As such, there is concern regarding ecological consequences should fish that have been genetically altered escape to natural ecosystems. To determine if the type of genetic change influences fitness components associated with ecological success outside of the culture environments they were produced for, we examined growth and survival of domesticated, transgenic, and wild-type coho salmon fry under different environmental conditions. In simple conditions (i. e. culture) with unlimited food, transgenic fish had the greatest growth, while in naturalized stream tanks (limited natural food, with or without predators) domesticated fish had greatest growth and survival of the three fish groups. As such, the largest growth in culture conditions may not translate to the greatest ecological effects in natural conditions, and shifts in phenotype over multiple rather than one loci may result in greater success in a wider range of conditions. These differences may arise from very different historical opportunities of transgenic and domesticated strains to select for multiple growth pathways or counter-select against negative secondary changes arising from elevated capacity for growth, with domesticated fish potentially obtaining or retaining adaptive responses to multiple environmental conditions not yet acquired in recently generated transgenic strains.

There is persistent commercial interest in the use of growth modified fishes for shortening production cycles and increasing overall food production, but there is concern over the potential impact that transgenic fishes might have if ever released into nature. To explore the ecological consequences of transgenic fish, we performed two experiments in which the early growth and survival of growth-hormone transgenic rainbow trout (Oncorhynchus mykiss) were assessed in naturalized stream mesocosms that either contained predators or were predator-free. We paid special attention to the survival bottleneck that occurs during the early life-history of salmonids, and conducted experiments at two age classes (first-feeding fry and 60 days post-first-feeding) that lie on either side of the bottleneck. In the late summer, the first-feeding transgenic trout could not match the growth potential of their wildtype siblings when reared in a hydrodynamically complex and oligotrophic environment, irrespective of predation pressure. Furthermore, overall survival of transgenic fry was lower than in wild-type (transgenic = 30% without predators, 8% with predators; wild-type = 81% without predators, 31% with predators). In the experiment with 60-day old fry, we explored the effects of the transgene in different genetic backgrounds (wild versus domesticated). We found no difference in overwinter survival but significantly higher growth by transgenic trout, irrespective of genetic background. We conclude that the high mortality of GH-transgenic trout during first-feeding reflects an inability to sustain the basic metabolic requirements necessary for life in complex, stream environments. However, when older, GH-transgenic fish display a competitive advantage over wild-type fry, and show greater growth and equal survival as wild-type. These results demonstrate how developmental age and time of year can influence the response of genotypes to environmental conditions. We therefore urge caution