Exploring metabolism–growth relationships within and across individual fish

Differences in growth rate and metabolism between individual animals can influence survival, reproduction and competitive social dynamics during vulnerable life stages. Acknowledging this, among-individual variability has received growing research attention. Less attention has been given to temporal variability within individuals, which can also be substantial and influence individual-level performance through time. Here, we used the estuarine fish Galaxias maculatus to understand among- and within-individual variabilities in metabolism, their relative contributions to growth and their thermal dependence. We collected five repeated measurements of metabolism (standard, routine and maximum) and mass across five months and 160 fish assigned to either 18℃ (typical summer temperature) or 23℃ (challenging summer temperature). Among-individual variability in metabolic parameters was generally low, generating low trait repeatability, and did not differ between temperature treatments. The percentage of fish achieving reproductive maturity was higher at 18℃ (98%) than 23℃ (62%). The growth rate was also higher at 18℃ and varied significantly among individuals. Individual growth rate was not correlated with standard or routine metabolic rates, but was positively correlated with maximum metabolic rate. We conclude that growth and metabolic rates are highly plastic over time, responding to a suite of influences including temperature, life stage and, in some circumstances, each other. This article is part of the theme issue 'Embracing variability in comparative physiology: why it matters and what to do with it'.