Page 24




by, Professor Brett Glencross, Institute of Aquaculture, University of Stirling

ietary energy intake is the factor that largely dictates the dietary concentrations required for all essential nutrients (Figure 1). The energy density of the feed has been shown to directly influence the amount of feed consumed across a wide range of species (Dumas et al., 2010; Glencross et al., 2008). With more energy dense diets, a reduction in the amount of food eaten is usually observed and this has direct implications on the concentration of essential nutrients required in any daily intake to satisfy daily nutrient demands and as such the animal’s growth demands. Because of this link between energy and intake it also has the clear effect of being a key factor in influencing the feed conversion ratio (feed/gain) of fish to which those diets are fed (Figure 2). The energetic content of the three macro-nutrient classes; protein (23.6 kJ/g), lipids (38.5 kJ/g) and carbohydrates (17.3 kJ/g) is the source of this dietary energy, although the capacity of different fish species to utilise each of these nutrients varies according to trophic level (Enes et al., 2009; Saravanan et al., 2012; Schrama et al., 2012; Glencross et al., 2014). In species like tilapia, dietary energy demands can be met through the metabolism of dietary protein and lipid intake, like carnivorous species, but also through the digestion and metabolism of starch (Bureau et al., 2002). However, in carnivorous species, like Asian seabass, starch is not only poorly digested, but it has also been shown that the animal poorly metabolises the energy from this macronutrient, preferring to rely on protein and lipid as energy sources (Glencross et al., 2014).

maintenance also includes that energy utilised through activity and heat loss. Being a poikilothermic animal, the energy demand by fish will directly reflect the temperature of its environment. However, both above and below critical thermal ranges there is a deterioration in the nature of

Figure 1. The relationship between energy demand, feed intake and dietary nutrient demands.

Defining Energy Demands

Dietary energy demand is generally assumed to be the sum of the requirements of a growing fish for its maintenance and growth energy needs (Figure 3). Energy demand for

Figure 2. The effect of diet energy density on feed intake and FCR in Asian seabass. Data derived from Glencross et al. 2014.

22 | July | August 2016 - International Aquafeed

JUL | AUG 2016 - International Aquafeed magazine