Supplemental citric acid and particle size of fish bone-meal influence the availability of minerals in rainbow trout Oncorhynchus mykiss (Walbaum)

Juvenile rainbow trout Oncorhynchus mykiss (Walbaum) were fed six low-phosphorus (P) diets supplemented with two different sizes of ground fish bone-meals (fine, 68 mu m or less; coarse, 250-425 mu m) and a coarse bone-meal diet containing four levels of citric acid (0, 4, 8 or 16 g kg(-1) diet) to investigate the effects of pH and bone particle size on P bioavailability. The basal diet provided 3.4 g P kg(-1) and bone-meal increased P contents to 5.4-6.0 g P kg(-1). Coarse bone-meal diets supplemented with 0, 4, 8 or 16 g kg(-1) of citric acid had pH values of 6.0, 5.7, 5.4 and 5.0, respectively. Weight gain and whole-body water, protein and lipid contents were not influenced by bone-meal supplementation. Supplementing the basal diet with both coarse and fine bone-meal significantly increased whole-body ash content. Fish fed no bone-meal were hypophosphataemic compared with fish fed with either fine or coarse bone-meals. Phosphorus in fine bone-meal had higher availability than P in coarse bone-meal. Bone-meal supplementation significantly decreased whole-body manganese content from 8.9 mu g g(-1) in fish fed no bone-meal to 2.3 and 4.5 mu g g(-1) in fish fed with fine and coarse bone-meals, respectively. The concentration of magnesium increased but zinc concentration was not affected by bone-meal supplements. Citric acid increased whole-body ash content but the influence of citric acid on the body P content was not significant (P = 0.07). Dietary acidification by citric acid significantly increased whole-body iron in a linear fashion. The bioavailability of dietary P can be improved by fine grinding the bone in fish meals.