Size-based competition explains large inter- and intra-individual variation in both intake and growth

Size-based intraspecific competition can lead to high variation in both feed intake and growth. However, isolating the effects of competition on fish physiology and growth from other environmental factors is challenging. To address this, we analysed a longitudinal dataset of individual-level feed intake and growth measurements of 543 fish. We used relative body size as a proxy for competitive ability and integrated it into a bioenergetic model to estimate body mass-dependent scaling of feed intake and energetic costs of living. Including relative body size effects in the model improved the model fit and replicated the observed size variation. Our results demonstrate that size-dependent competitive interactions can be an important driver of size-at-age variation even under controlled conditions. The findings also suggest that suppressed growth in smaller fish was more strongly associated with increased energetic costs of living than with a reduction in feed intake. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.