Why do some fish grow faster than others?

All animals must acquire food to grow, but there is a vast diversity in how different species and even different individuals approach and achieve this task. Individuals within a species appear to fall along a bold-shy continuum, whereby some fish acquire food aggressively and with seemingly high risk, while others appear more submissive and opportunistic. Greater food consumption generally results in faster growth, but only if the energy acquired through food is more than enough to compensate for heightened metabolism associated with a more active lifestyle. Fast-growing phenotypes also tend to have elevated baseline metabolism – at least when food is plentiful – which may be linked with gut morphology and digestive efficiency. The net energy gained from a meal (as calculated from the specific dynamic action (SDA) coefficient) is optimised with larger meal sizes, but the digestion of large meals can erode the aerobic metabolic scope available for other critical activities such as predator avoidance, perhaps at an interindividual level. Thus, complex interactions between an individual's genes and environment are likely to regulate the growth phenotype. This review compiles available knowledge to shed light on the question: Why do some fish grow faster than others? We discuss the elaborate interrelationships between behaviour, physiology and the gut microbiome with a goal to better understand what drives interindividual differences in growth performance.