Excessive phosphorus (P) levels in freshwater aquaculture effluents are a majorenvironmental problem in certain receiving water bodies. This study aimed to test anapproach alternating that alternating feeding P deficient and P sufficient diets andmeasure P loading from rainbow trout (Oncorhynchus mykiss) culture. Threeexperimental 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. Sixdifferent feeding regimes of P-sufficient diet continuously (P08), P-deficient dietcontinuously (P04), optimum dietary level of P (P06) continuously, one weekP-deficient/one week optimum level of P diet (P04/P06.1), 2 weeks P-deficient/2 weeksoptimum level of P diet (P04/P06.2) and 4 weeks P-deficient/ 4 weeks optimum level of Pdiet (P04/P06.4) were tested. Fish were fed twice daily to apparent satiation level 16weeks. Fish fed all alternating regimes showed growth rate (weight and length) comparableto those of continuous feeding with P08 and P06 diet. The feed conversion ratios (FCR) forall alternating regimes were comparable to that of the P08 and P06 continuous feedingregime. Neither the thermal unit growth coefficient (TGC) nor condition factor (K)significantly influenced by feeding regimes. Vertebrae P, ash and whole body ash contentdid not differ among regimes. Except fish fed continuous P04 diet, the ash and P contentin opercula and whole body total P content were not significantly different among eachother in a continuous feeding and alternating feeding schedule.
Fish fed all alternating regimes showed significantly lower P consumption than those fedcontinuously 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 continuouslywith P08 diet, in contrast P loading values were lower for all alternating feedingregimes. The study demonstrated that growth and tissue mineralization of fish maintainedon alternating feeding regimes with P04 and P06 diet were comparable to those continuouslyfed with diet of P08. These results demonstrate that it is possible to reduce P intake by34% 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 dietarylevels using practical ingredients can be adopted as a means of reducing P loading fromrainbow trout culture without compromising growth.