with least protein also had the lowest feed and protein efficiencies. While satiation feeding resulted in similar weight gain as feeding at a fixed rate, the efficiency of feed and protein
utilization was improved with satiation feeding. There were no clear trends in body indices or ammonia production of fish subjected to the various regimes. In conclusion, higher dietary energy
levels did not improve growth, but did lower nitrogenous waste production of red drum. Increasing dietary protein in conjunction with increasing energy levels also improved growth and
efficiency of feed utilization. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Ammonia; Diet; Energy; Protein; Red drum; Sciaenops ocellatus
1. Introduction
In the past, formulation of diets in intensive aquaculture has often been aimed at meeting requirements for maximum growth, while more recently, formulations are being
designed to meet other desired production goals. One such goal is to increase the sustainability of various aquacultural enterprises by reducing the environmental impacts
they may have on surrounding ecosystems. Aquaculture wastes are largely dietary in
Ž .
origin with estimates of 52–95 of feed nitrogen ending up as waste Wu, 1995 . Therefore, diet manipulations which result in less waste production are an attractive
approach to this predicament. One particular aspect of deleterious waste production now being addressed is formulation of diets to minimize ammonia production by cultured
species.
While high dietary protein levels are known to result in rapid growth rates for many species of fish, inclusion of levels beyond those needed for growth frequently result in
Ž .
high levels of ammonia production Campbell, 1991 . Reduction of ammonia production is understandably desirable as it represents a loss of costly amino acid nitrogen from
protein synthesis and is a source of detrimental water quality at even moderately high levels. One practice that has been employed in aquaculture to limit ammonia production
is restricted feeding which, in turn, generally limits growth of the species being cultured. Therefore, dietary manipulations which would reduce ammonia production but sustain
rapid growth of the fish are particularly appealing.
High dietary energy has been found to have a protein-sparing effect, resulting in lower ammonia production, in several fish species. These higher energy levels generally
come in the form of increased dietary lipid as lipid is an energy-dense nutrient and readily metabolized by fish. This protein-sparing effect and reduced ammonia produc-
tion have been reported in several fish species fed high-energy diets containing lipid as a
Ž major energy source Johnsen et al., 1991; Jayaram and Beamish, 1992; Vergara et al.,
. 1996 .
Ž .
As some fish have been demonstrated to eat to meet an energy need NRC, 1993 , it is important to consider that with feeding of high-energy diets, consumption may be
reduced, which may contribute to a decrease in ammonia production. If feed intake is severely restricted, protein intake may be too low to support rapid growth rates.
Therefore, increasing the percentage of dietary protein in conjunction with an increase in energy may help to provide amino acids at levels needed for maximal growth even if
Ž .
Ž .
feeding activity is reduced Cho and Bureau, 1997 . Cho et al. 1994 explained that the
most beneficial approach to increasing nutrient density of diets generally involves excluding ingredients with low protein and energy contents which are often poorly
digested. This strategy was evaluated with red drum and observed to have positive Ž
. effects on water quality of a closed recirculating system Jirsa et al., 1997 .
A previous experiment in our laboratory demonstrated that high dietary energy levels resulted in lower ammonia production by red drum but compromised weight gain and
Ž .
body composition McGoogan and Gatlin, 1999 . Additionally, high dietary energy seemed to have a more beneficial effect at a reduced feeding rate, resulting in favorable
growth rates and reduced nitrogenous waste production. Increasing dietary crude protein Ž
. CP provided greater weight gain but with higher ammonia excretion. Taking these
findings into consideration, the present study was conducted using the protein level Ž
. 45 CP previously determined to support most rapid weight gain and graded energy
levels to determine if an energy level could be ascertained which would provide a reduction in ammonia production without adversely affecting weight gain or body
composition. Because higher dietary energy levels may result in reduced intake, a second experiment was conducted to determine the effects of increasing dietary protein
levels in conjunction with energy while reducing feeding rates to allow adequate dietary protein and energy intake. Although protein and energy are known to have profound
effects on growth and body composition of fish, much less is known about the effects of these manipulations on plasma metabolites and enzyme activities associated with
ammonia production, and therefore, these aspects were also studied.
2. Materials and methods
