Effects of Protein, Lipid, Feeding Levels and Their Interaction ... 65Research shows that dietary protein is poorly utilized when there is
inadequate non-protein energy in the diet because good amount of protein is
metabolized to generate energy which can lead to excessive ammonia
excretion into the environment consequently affecting water quality (Phillips,
1972; Shyong et al., 1998). Dietary lipids can spare dietary protein from use as
energy and limit ammonia production through a process called protein sparing
action (Cho and Kaushik, 1990; Gaylord and Gatlin, 2000) and improving feed
efficiency and growth (Hillestad and Johnsen, 1994; Einen and Roem, 1997).
Providing adequate energy through dietary lipids can minimise the use of more
costly protein as an energy source (De Silva et al., 1991; Jayaram and
Beamish, 1992; Bazaz and Keshavanath 1993; Van der Meer et al., 1997;
Jantrarotai et al., 1998; Company et al., 1999; McGoogan and Gatlin III,
2000). Thus dietary non-protein sources such as lipids and carbohydrates have
the potential of reducing the amount of dietary protein in fish diets hence
minimizing the use of more costly protein as an energy source (De Silva et al.,
1991; Jayaram and Beamish, 1992; Bazaz and Keshavanath 1993; Van der
Meer et al., 1997; Jantrarotai et al., 1998; Company et al., 1999; McGoogan
and Gatlin III, 2000).
Although dietary lipids are important in the diets, the inclusion level could
be influenced by the crude protein level. In many cases, the efficiency of high
crude lipid level in the diet is noticed when crude protein level is low
compared to high crude protein level. This practice is successful in salmonid
culture where lipid inclusion level can be as high as 300g/kg or more in the
diet thus reducing the dietary protein level significantly (Helland and Grisdale-
Helland, 1998; Rasmussen et al., 2000; Torstensen et al., 2001).
In addition, since feeding regime affects nutrient requirements, knowledge
of optimum feeding regime is considered a prerequisite to the determination of
nutrient requirements (Tacon and Cowey, 1985; Talbot, 1985). Feeding
regimes (e.g., ration size, frequency, duration of a meal etc.) are also reported
to influence fish growth, feed conversion and body composition (Reinitz,
1983; Li and Lovell, 1992; Munsiri and Lovell, 1993; Arzel et al., 1998;
McGoogan and Gatlin III, 2000). Efficient use of feed is important in
achieving profitable aquaculture. In fish an inverse relationship between
optimal dietary protein level (as a percentage of the diet) and feeding regime
(ration size) has been suggested (Ogino, 1980; Tacon and Cowey, 1985).
There is very little known about the protein-sparing action by dietary
lipids in warm-water species (Page and Andrews, 1973), unlike in cold-water
species (Lee and Putnam, 1973; Takeda et al., 1975; Reintz et al., 1978;
Beamish and Medland, 1986; Tabacheck, 1986; Davies, 1989). In view of the