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 μm or less; coarse, 250–425 μm) 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 μg g −1 in fish fed no bone‐meal to 2.3 and 4.5 μg 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.