Abstract
Recent advances in gene technology have been applied to create fast-growing transgenic fish, which are of great commercial interest owing to their potential to shorten production cycles and increase food production. However, there is growing concern and speculation over the impact that escaped growth hormone (GH)-transgenic fish may have on the natural environment. To predict these risks it is crucial to obtain empirical data on the relative fitness of transgenic and non-transgenic fish under nature-like conditions. Using landscaped stream aquaria with live food and predators, we show that the predation mortality of newly hatched GH-transgenic coho salmon fry (Oncorhynchus kisutch) is much higher than in non-transgenic conspecifics, and that this difference is amplified when food abundance decreases. The growth rate of transgenic and non-transgenic fish is similar at high food levels, whereas transgenic fish grow more slowly than non-transgenic fish when food abundance is reduced. Our results suggest that the fitness of young GH-transgenic coho salmon in the wild will be determined by both predation pressure and food availability.