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Nutritional condition reflects food quality and quantity and influences growth rate. Therefore, the ability to measure nutritional condition would be valuable in estimating the potential for growth of captive and wild lobsters. To assess potential indices of nutritional condition, we examined changes in blood refractive index and ratio of weight to carapace length of juvenile rock lobsters to treatments of varying food supply. Blood refractive index was strongly correlated with blood protein concentrations and was a reliable proxy for the circulating protein that is metabolized and diluted during periods of low food and starvation. The moult cycle imposes constraints on blood refractive index, however, so it is essential that the moult stage of the animal be assessed concurrently. Ratios of weight to carapace length were slower to respond to reduced food intake than was blood refractive index but were independent of the moult …