Predation, metabolic priming and early life-history rearing environment affect the swimming capabilities of growth hormone transgenic rainbow trout

The period of first feeding, when young salmonid fishes emerge from natal stream beds, is one fraught with predation risk. Experiments conducted in semi-natural stream mesocosms have shown that growth hormone transgenic salmonids are at greater risk of predation than their non-transgenic siblings, due partly to the higher metabolic demands associated with transgenesis, which force risky foraging behaviours. This raises questions as to whether there are differences in the swim-performance of transgenic and non-transgenic fishes surviving predation experiments. We tested this hypothesis in wild-origin rainbow trout (Oncorhynchus mykiss) that were reared from first feeding in semi-natural stream mesocosms characterized by complex hydrodynamics, the presence of predators and oligotrophic conditions. Using an open-flume raceway, we swam fish and measured their capacity for burst-swimming against a sustained flow. We found a significant genotype effect on burst-performance, with transgenic fish sustaining performance longer than their wild-type siblings, both in predator and predator-free stream segments. Importantly, this effect occurred before differences in growth were discernable. We also found that mesocosm-reared fish had greater burst-performance than fish reared in the controlled hatchery environment, despite the latter being unexposed to predators and having abundant food. Our results suggest a potential interaction between predation and metabolic priming, which leads to greater burst capacity in transgenic trout.