Diet Composition

Most people will be familiar with the main components of food from reading the cornflake box at breakfast time. Diets comprise three major constituents in the form of proteins , fats and carbohydrates plus three or four minor ingredients in the shape of vitamins, minerals, water and, for older salmonids in particular, pigments.

Just as nutritional requirements change as animals age, so the composition of fish diets alters as the fish goes through its various developmental stages. Naturally those involved in fish feed formulation need to know the nutritional requirements of a particular stock species and the means by which these can best be met to give maximum growth in a cost-effective manner. An obvious starting point is to look at what the stock would eat in the wild at various life cycle stages, to ascertain nutritional profiles and attempt to replicate them. This is indeed what happens - most proprietary fish diets are based on piscine raw materials.

Within modern aquaculture, a stocks' nutritional needs can be met using different types of diet depending on the stock being cultured and what life cycle stage is involved. For herbivorous and omnivorous species (such as cyprinids) under extensive or semi-intensive culture, a natural food supply is utilised with, in the latter type, some supplemental fertilisation of holding units to encourage plant and plankton growth. A variety of diet types can be utilised for carnivorous species ranging from wet (minced/diced raw fish) through moist (basic fish meal and additives mixed on site with freshwater) to the well-known modern pelleted diets. For some species a live diet is required during larval development, as in the feeding of Artemia to larval halibut and cod.

Whatever diet is used, it must be nutritionally balanced to meet the energy budget of the stock so as to optimise growth and profit. Total energy value of diets will vary with the proportion of its various ingredients. As fish regulate their energy intake to match their energy needs, they in effect regulate the absolute amount of proteins that they ingest. Thus the balance of ingredients in a diet may be more important than definitive levels of one particular nutritional component. Consequently diets deemed to be 'low energy' ones may result in as much growth as a 'high energy' one, provided it is nutritionally balanced.