REQUIREMENT OF RAINBOW-TROUT FOR DIETARY PHOSPHORUS AND ITS RELATIONSHIP TO THE AMOUNT DISCHARGED IN HATCHERY EFFLUENTS

Fingerling rainbow trout Oncorhynchus mykiss with initial mean weights of 9 g (small fish) and 35 g (large fish) were fed diets adequate in all known nutrients except phosphorus (P). In two experiments, triplic ate lots of rainbow trout were fed basal diets containing either 0.14 or 0.41% non-phytin P, with or without graded levels of supplemental P . Deficiency of P reduced growth, feed efficiency (weight gained/weigh t fed), bone ash, and whole-body ash contents. The requirement for non -phytin P by small trout for maximum growth and feed efficiency was no t more than 0.41% of diet; the requirement by large trout was between 0.34 and 0.54% of diet. The requirement of non-phytin P for maximum bo ne ash development was about 0.51% of diet for small trout and more th an 0.54% for large trout. Whole-body phosphorus content of small trout suggested a requirement above 0.51% but not more than 0.61% non-phyti n phosphorus. Although an effect of size of trout on the requirement w as not clearly demonstrated, these results show that trout required mo re dietary P for bone mineralization than for weight gain. The minimum dietary requirement for non-phytin P for bone mineralization was prob ably between 0.54 and 0.61% of diet. Discharges of P into effluent wat er increased significantly as trout were fed increasing levels of P. W hen trout were fed 0.61% available P, approximately 67% of P consumed was retained, and discharges of soluble P in effluents were 2.0 g P/kg weight gain or 1.8 g P/kg feed fed.