This study investigated whether two farmed rainbow trout families that differed in two production traits, size at age (SA) and condition factor (CF), also differed in aspects of their physiological energetics and cardiorespiratory performance. Five groups of a family with large SA (LSAF) and six groups of a family with smaller SA (SSAF) were reared in tank respirometers in freshwater at 14 °C for 84 days. The LSAF grew from a mean (± SD) mass of 182 ± 6 g to 449 ± 24 g, a significantly higher mass gain than the SSAF, which grew from 77 ± 4 g to 307 ± 22 g. The LSAF had a higher lifetime specific growth rate (SGR) but, contrary to expectations, exhibited a lower SGR than the SSAF when compared over a mean mass interval of approximately 180 g to 300 g. This was a result of lower daily rates of feed intake coupled with higher metabolic rates in the LSAF during daylight feeding hours, this latter apparently due to increased spontaneous activity. Thus, a higher lifetime SGR in the LSAF presumably reflected rapid growth at earlier life stages, and a large familial SA may bring a tendency to increased aggressive behavioural interactions as fish approach marketable size. Instantaneous fluxes of O2, CO2 and waste nitrogen in the tank respirometers immediately after feeding revealed that lipid fuelled over 50% of metabolism, protein approximately 40% and carbohydrates less than 10% in the families. When, however, feed had been withheld for 24 h, protein fuelled less than 20% of metabolism and carbohydrate increased to over 20%. The LSAF exhibited higher critical swimming speeds, maximum metabolic rates and aerobic metabolic scopes than the SSAF, indicating that selecting broodstock for large SA does not necessarily compromise functional integrity. The SSAF had a more rounded ventricular morphology than the LSAF, and also a higher CF. These results are consistent with other literature reports whereby familial CF in farmed trout is an indicator of ventricular morphology and cardiorespiratory performance.