Effect of ration level on nitrogen excretion, nitrogen retention and efficiency of nitrogen utilization for growth in largemouth bass (Micropterus salmoides)

Largemouth bass (Micropterus salmoides) with an average weight of 45.5 g were used to study ration level effects on nitrogen excretion, nitrogen retention, and gross efficiency of utilization of nitrogen for growth. Bass were starved 3 to 4 days and then each bass was placed into an aquarium containing a known volume of water. One day after the fishes were placed in aquaria, nitrogen excretion rates were determined; this rate is the maintenance nitrogen excretion. Each fish was then fed one or more shiners (Notropis cornutus); fish were fed only once. Nitrogen excretion measurements were made daily until the rates were similar to maintenance rates. The nitrogen excretion rates for each day after feeding which were above the maintenance nitrogen excretion were combined and reflect the total nitrogen excretion for a given ration level. All ration levels were converted to nitrogen consumption in mg and nitrogen absorption was calculated from subtracting the average faecal nitrogen from nitrogen consumption. From data on nitrogen consumption, nitrogen absorption, nitrogen excretion, and faecal nitrogen, calculations were made for nitrogen retention and gross efficiency of utilization of nitrogen for growth. As ration level and nitrogen absorption increased, nitrogen excretion increased and is described by the equation, Y= 8.56+0.40 X, where Y is total nitrogen excretion and X is nitrogen absorption. Nitrogen retention also increased with nitrogen absorption and is described by the equation, Y=–8.57+0.60 X, where Y is nitrogen retention and X is nitrogen absorption. Efficiency increases rapidly above maintenance, but levels off at higher ration levels and approaches an asymptote of 60%.