The effects of dietary protein and energy on ingestion and growth were determined for nutrient ranges that correspond to primary foods in freshwaters: algae, aquatic macrophytes and organic detritus. Sixteen diets containing four levels of metabolizable energy (ME) (3.1, 6.7, 10.5, 14.1 kJ/g) and four levels of protein (3.0, 13.2, 23.2, 33.4 mg/kJ ME) were each fed ad libitum to four replicate groups of juvenile Tilapia aurea for 42 d. Protein, energy, and protein-energy interaction affected both ingestion and growth (two-way ANOVA, all P < 0.01). Increased ingestion largely compensated for lower energy levels within each protein level. Growth was proportional to diet protein content, and ingestion did not compensate for protein limitation. A second-order polynomial for growth as a function of diet protein content and energy assimilation rate fitted by linear regression accounts for 91% of variation in growth, and provides a model for comparison of the relative importance of protein and energy as nutritional constraints for animals feeding on invertebrate prey, algae, aquatic macrophytes, and organic detritus. Protein appears to be the primary constraint to food value of macrophytes, and detritus, and we predict from our results that consumers of these materials will increase growth most by feeding selectively on the most protein-rich material available, as has been observed. In contrast, growth of animals feeding on algae will be increased most by increased ingestion. Omnivory is interpreted as a compromise strategy in which protein from scarce animal prey is complemented by energy from abundant primary foods.
