Effects of low phosphorus ingredient combinations on weight gain, apparent digestibility coefficients, non-fecal phosphorus excretion, phosphorus retention and loading of large rainbow trout (Oncorhynchus mykiss)

The objective of the present study is to develop an eco-friendly aquatic feed to reduce phosphorus (P) loading, in large rainbow trout. Gain in fish mass, nutrient digestibility, non-fecal P excretion, P retention and loading were compared among triplicate groups of fish reared with a P-sufficient (commercial feed), a previously-developed P-deficient (Aquaculture Collaborative Research and Development Program, ACRDP diet) and an experimental diet formulated with low level of domestic fish meal (herring meal, 150 g/kg diet) and low-P ingredients: Hamlet soy concentrate (110 g/kg diet), corn gluten meal (250 g/kg diet), feather meal (120 g/kg diet), blood meal (100 g/kg diet) and whey powder (50 g/kg diet). The total P content was 10.7 g/kg, 3.7 g/kg and 4.7 g/kg, for the P-sufficient, P-deficient and experimental diet, respectively. Fish (initial average mass 314.7 ± 5.9 g) were fed in triplicate tanks per diet for 60 days. At the end of the trial, gain in fish mass and thermal growth coefficient (TGC) were comparable among fish from different treatments. With the exception of the P-deficient diet (higher FCR), feed conversion ratio (FCR) was not (P>0.05) influenced by diets. Coefficient of total tract apparent digestibility (CTTAD) of P and P retention were higher (P<0.05) in fish fed the experimental diet than commercial feed. The maximum non-fecal soluble P accumulation curve and concentration were different (P<0.05) and mainly affected by dietary P. The fecal P content was found to be lower (P<0.05) in rainbow trout fed the experimental diet (10.6 mg/g dry fecal matter) than commercial feed (24.3 mg/g dry fecal matter). Non-fecal P excretion was higher (P<0.05) in fish fed commercial feed (6.22 g soluble P/kg dry diet) than experimental diet (2.20 g soluble P/kg dry diet). Moreover, estimated P loading was lower (P<0.05) for the experimental diet than the commercial feed (3.98 kg/ton of fish produced versus 11.07 kg/ton, respectively). The present study demonstrates that for large rainbow trout, low P loading diet can be formulated through an adequate and optimal combination of diverse and practical low-P protein ingredients.