Differences in growth rate and metabolism between individuals can determine competitive social dynamics as well as survival during vulnerable life stages. While this inter-individual variability has received growing attention, variability within individuals can also be substantial but has rarely been studied. Here, we used the ecologically and culturally important estuarine fish Galaxias maculatus to understand inter- and intra-individual variability in metabolism and determine their relative contributions to growth.

Specifically, we collected repeated measurements of metabolic and growth performance across 6 months to gauge inter- and intra-individual variability in both metrics for a wild-caught population of 160 G. maculatus. Groups of fish were fed to satiation and housed in tanks where each fish had a unique colored subcutaneous elastomer tag. Fish were initially measured for active, routine, and standard metabolic rates at 18℃ before being randomly assigned to an acclimation group at either 18℃ (benign summer temperature) or 23℃ (challenging summer temperature). We tracked each individual’s growth and collected a further five metabolic measurements per fish (~1 per month).

Our results showed negligible correlation between specific growth rate and both standard and routine metabolic rates. Inter- and intra-individual variability were similar for both standard and routine metabolic rates (mass-standardized), including between temperature treatments. These findings provide some of the strongest evidence that the metabolic phenotype of an individual can change across time, matching the extent of the variability within a population.