Abstract

An extensive literature exists on how environmental conditions, especially temperature, impact animal body sizes. However, there remains considerable discrepancies, and misunderstanding, in the key definitions and concepts of body size used to describe observed impacts across studies. Size can be measured using continuous growth metrics, including von Bertalanffy growth coefficients, or static 'size' metrics, such as population-averaged length or mass, average size-at-(arbitrary)-age, size-at-maturity, adult size, asymptotic size, or the maximum observed size. Critically, these concepts of size are not equivalent, and temperature is likely to affect each in different ways. The use of these disparate size and growth metrics as response variables estimated across different biological scales (individual, population, or community) and empirical contexts (laboratory, field) has led to unnecessary confusion and apparent contradictions among practitioners. Here, we review nine common confusions associated with the measurement of 'size' in fish and other water-breathing ectotherms. We then highlight outstanding knowledge gaps on how temperature and global warming might affect different size metrics. Clarifying concepts, definitions, and applications of body size measures is important as it can help reconcile divergent findings, target future research, and improve our predictions about the warming impacts on wild populations.