Coho salmon (Oncorhynchus kisutch) transgenic for growth hormone (GH) show substantially faster growth than wild-type (WT) fish. We fed GH-transgenic salmon either to satiation (1-year) (TF) or the same smaller ration of wild-type fish (2-years) (TR), resulting in groups matched for body size to WT salmon. The myotomes of TF and WT fish had the same number and size distribution of muscle fibres, indicating 2-fold higher rate of fibre recruitment in the GH-transgenics. Unexpectedly, calorie restriction was found to decrease the rate of fibre production in transgenics, resulting in a 21% increase in average fibre size and reduced costs of ionic homeostasis. Genes for myotube formation were down-regulated in TR relative to TF and WT fish. We suggest muscle fibre size optimisation allows the relocation of energy from maintenance to locomotion explaining the observation that calorie-restricted transgenics grow at the same rate as WT whilst exhibiting markedly higher foraging activity.