Parental care is an essential life-history component of reproduction in many species, which entails a suite of behavioural and physiological investments to enhance offspring survival. Male smallmouth bass provide parental care for developing offspring. During this stage males cease active foraging as they defend their brood. Experimental manipulation of cortisol levels (via injection) and food (via supplemental-feeding) in parental males was used to investigate the fitness consequences of parental care. Improving the nutritional condition of nest guarding males increased their reproductive success by reducing nest-abandonment. However, supplemental-feeding and cortisol had no effect on parental behaviours. Cortisol reduced plasma lymphocyte numbers, but increased neutrophil and monocyte concentrations. Supplemental-feeding improved the physiological condition of parental fish by reducing oxidative injury. Increasing the nutritional condition of parental fish can reduce the physiological cost of intensive parental activity and improve reproductive success, illustrating the importance of nutritional condition as a modulator of parental fitness.