Abstract.It is becoming increasingly clear that cultivated salmon will not perform the way we want them to unless we have a thorough understanding of how the rearing environment will affect their behaviour. This paper illustrates some of the behavioural patterns that can influence feeding and growth rates, and how these may differ between individual fish. A salmon's appetite is not simply a function of temperature: there are complex daily, seasonal and developmental appetite rhythms, which result in some fish becoming anorexic while others continue to feed. The developmental switches which trigger some fish to become anorexic (and so delaying smolting) appear to be irreversible, but individual differences in behaviour in the period leading up to the switch point influence whether fish smolt early or late. Thus more competitive and dominant fish are more likely to become S1 smolts, and these differences in dominance status become established within the first few weeks of feeding. They appear to have a physiological basis: fish with higher metabolic rates (irrespective of initial size) tend to be dominant, and so subsequently grow faster. However, the extent of these behavioural effects will depend on the rearing environment. The challenge is therefore to allow all fish to feed without intimidation, and to devise feeding schedules which take account of complex appetite rhythms — only then will we be working with the fish, rather than against the