Excessive phosphorus (P) levels in freshwater aquaculture effluents are a major environmental problem in certain receiving water bodies. This study aimed to test an approach alternating that alternating feeding P deficient and P sufficient diets and measure P loading from rainbow trout (Oncorhynchus mykiss) culture. Three experimental practical diets consisting of P-deficient (0.4% P, P04), optimum level of P (0.6% P, P06) and P-sufficient as control diet (0.8% P, P08) were formulated. Six different feeding regimes of P-sufficient diet continuously (P08), P-deficient diet continuously (P04), optimum dietary level of P (P06) continuously, one week P-deficient/one week optimum level of P diet (P04/P06.1), 2 weeks P-deficient/2 weeks optimum level of P diet (P04/P06.2) and 4 weeks P-deficient/4 weeks optimum level of P diet (P04/P06.4) were tested. Fish were fed twice daily to apparent satiation level 16 weeks. Fish fed all alternating regimes showed growth rate (weight and length) comparable to those of continuous feeding with P08 and P06 diet. The feed conversion ratios (FCR) for all alternating regimes were comparable to that of the P08 and P06 continuous feeding regime. Neither the thermal unit growth coefficient (TGC) nor condition factor (K) significantly influenced by feeding regimes. Vertebrae P, ash and whole body ash content did not differ among regimes. Except fish fed continuous P04 diet, the ash and P content in opercula and whole body total P content were not significantly different among each other in a continuous feeding and alternating feeding schedule. Fish fed all alternating regimes showed significantly lower P consumption than those fed continuously fed with P08 and P06. Different feeding regimes had no effect P retention. Significantly higher P loading (solid and dissolved) was noted in fish fed continuously with P08 diet, in contrast P loading values were lower for all alternating feeding regimes. The study demonstrated that growth and tissue mineralization of fish maintained on alternating feeding regimes with P04 and P06 diet were comparable to those continuously fed with diet of P08. These results demonstrate that it is possible to reduce P intake by 34% and reduce P loading 52% by adopting alternating feeding regimes compared to P08 diet. This study provides evidence that alternating feeding of P deficient and optimum dietary levels using practical ingredients can be adopted as a means of reducing P loading from rainbow trout culture without compromising growth.
