Feeding Behaviour

Interactions between fish and their environment, each other and their own physiology mean that feeding behaviour, whether in captivity or in the wild, is complex - digestion of food is merely the end result of these factors. Environmental influences will include such things as temperature, light intensity, seasonality, activity cycles and so forth. These will all act on internal physiological systems, e.g. the endocrine system, through the animals' senses. As most aquaculture species are kept in a relatively confined area or volume, social interactions associated with obtaining food, maintaining space, etc. will also impact on feeding behaviour.

Fish farmers actively seek to exploit certain aspects of this behaviour to optimise and hopefully therefore to maximise their production. Two aspects are of prime interest, namely increasing appetite and therefore consumption of food and ensuring that dietary energy is maximised for growth and minimised for general daily expenditure (swimming, feeding). Whilst diet composition will influence these factors, so will both rearing environment and feed management techniques. A positive side effect to maximum food consumption is a minimisation of waste food, leaching and overall pollution.

Many fish farmers use feeding behaviour as an indicator of appetite - so-called feeding response. Increased, vigorous activity when feed is introduced to fish indicates hungry stock, the response diminishing as more and more fish become satiated, i.e their appetite demand is met. A poor or reduced initial response is usually associated with a particular health problem.

At the physiological level appetite appears to be regulated by the amount of stomach distension and the level of certain nutrients circulating in the blood. Stomach distension acts through the brain to reduce feeding response to the point of cessation. As food is digested and passed through the intestine, distension reduces and the brain triggers feeding behaviour by means of a search for more food. Absorbed nutrients from food circulate in blood and changes in levels of these modify feeding behaviour. In some species of fish, including salmonids, the time taken for the stomach to empty influences the return of appetite, which determines how many times fish will feed in a 24-hour period.

Fish metabolism has an influence on feeding behaviour as it appears that food intake by fish is such that it meets their energy requirements. Thus, if a diet has a low energy value fish will compensate by eating more (within the limits of how much can be packed into their stomachs). The physical limits of a fish stomach means that diets with different energy values, though nutritionally balanced, will produce the same amount of growth weight for weight. Addition of feed attractants to high energy diets which are then used in a sensible feed management policy improves growth on a weight -for-weight basis.