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Tuesday, August 16, 2016

16/08/2016: Fueling the fire within - digestible energy demands by fish

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

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

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). 

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

Figure 3: dietary energy intake and growth energy flows 
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).

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 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 this relationship between temperature and energy demand. This also influences the efficiency with which dietary energy is utilised (Glencross and Bermudes, 2011).

Read the full article HERE.

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