The relationship between Protein Source and Nitrogen Metabolism in the Trout

Abstract

Various experiments connected with the use of novel proteins (SCP's) in fish diets are described. In the first series of experiments classical nitrogen balance techniques were used. From these, MFN was found to be 175 mg N/100g diet and EUN 12.37 mg N/100g fish/day. Using this data, true NPU's were determined at various levels of N intake ranging from 36 to 140 mg/100g fish/ day. True NPU decreased linearly as N intake increased, the rate varying only slightly with protein source. For herring meal (HM) these values were between 0.97 and 0.48, the others being - petroyeast (PY) 1.0 - 0.67; bacterial protein (MB) 0.71 - 0.48; algal protein (SA) 0.87 - 0.55; and fungal protein 0.79 - 0.54. Apparent NPU was practically constant regardless of N intake. A further experiment showed that the decrease in true NPU was not ameliorated by the use of high energy diets. Growth experiments incorporating NPU determinations by carcass analysis gave the following true digestibility (%), true NPU, BV and PER values:- PY 91.6, 0.42, 0.46, 2.01; MB 93.5, 0.37, 0.40, 1.62; SA 83.1, 0.32, 0.38, 1.33; HM 91.2, 0.38, 0.41, 1.91; soyabean (SY) 43.6, 0.18, 0.41; brewers yeast (BY) 79.9, 0.30, 0.38, 1.17; casein (CS) 98.7, 0.40, 0.41, 1.97. Dietary effects on growth rate, body composition and food intake were also noted. For carp, the equivalent values were:- PY 96.6, 0.47, 0.49, 2.08; MB 95.5, 0.49, 0.52, 2.54; SA 87.1, 0.36, 0.41, 1.15; HM 80.3, 0.64, 0.79, 2.82,; SY 83.7, 0.42, 0.5, 1.353 CS 93.0, 0.49, 0.52, 2.54. The results are discussed in relation to the amino composition of the diets. Food preference trials with demand feeders gave inconclusive results as trigger preference was often prevalent. Additional work on photoperiod and demand feeding rhythm showed that trout at 12°C exhibit peaks of feeding every 8 hours.