Which body size metrics should be used for assessing temperature impacts on fish growth and size?

An extensive literature and debate exist on how and why temperature impacts animal, and especially ectotherm, body sizes. However, there remains considerable discrepancies and misunderstanding in the key definitions and concepts of body size used to describe observed temperature impacts across studies. For fish and other animals that continue growing throughout life, body size can be defined as size-at-maturity, adult size, asymptotic size, maximum observed size, population-averaged length or mass, or average size-at-(arbitrary)-age. These concepts of size are not equivalent, and temperature is likely to affect each in different ways. Some disagreement about temperature impacts on fish body sizes might relate to the different body size and growth metrics used, especially when combined with different biological scales (individual, population, or community) and empirical contexts (laboratory, field). Here, we review six common confusions associated with the measurement of ‘size’ in fish and other water-breathing ectotherms and recommend consistent and accurate use of terms and methodology to ensure that studies of global warming impacts on fish sizes can be compared and interpreted unequivocally.