Resource availability and quality determine the distribution and community structure of animals in ecosystems. However, the balance between energy supply and energy consumption of species has received little attention because of the difficulty of estimating these factors. We measured the carrying capacity of a stream ecosystem in relation to the metabolic demands of Banded Kokopu (Galaxias fasciatus), a drift-feeding fish, to investigate the proportion of their energy budget met by the supply of their primary prey, drifting invertebrates. We sampled drifting invertebrates from multiple pools in 2 freshwater streams once a month over 1 y and measured their energy content. We used the energy content of individual invertebrates to estimate the total energy available in the form of drifting invertebrates in each pool. We measured fish O₂ consumption rate (mg O₂ kg¹ h¹) and converted it to metabolic demand (cal/h). Drifting aquatic invertebrates alone appear to be insufficient to meet Banded Kokopu energetic requirements in most seasons because the fish could obtain up to only 60% of their total energy expenditure from this food source. This result indicates that Banded Kokopu must access other sources of energy, such as benthic food items or terrestrial invertebrate inputs, especially during episodic events, and highlights the potential importance of terrestrial-aquatic exchange and other energetic subsidies. Our results suggest that food supply is likely to limit assemblages of drift-feeding fish and plays a key role in determining the outcome of intraspecific interactions, growth, movement, and the distribution of individuals within streams.