Effects of dietary phosphorus and calcium level on growth and skeletal development in rainbow trout (Oncorhynchus mykiss) fry

The effects of dietary levels of phosphorus (P) and calcium (Ca) on skeletal development and mineral deposition in rainbow trout (Oncorhynchus mykiss) fry were studied. Six semi-purified diets were formulated with graded levels of P and Ca. The basal diet A contained only P supplied by casein at 0.5% of dry matter. Other diets B, C, D and E were supplemented with 0.4, 0.8, 1.2 and 1.6% P supplied as a 1:1 mixture of NaH₂PO₄/KH₂PO₄ resulting in 0.8, 1.2, 1.6 and 2.2% total P, respectively. These five diets were supplemented with 1% Ca supplied as CaCO₃ whereas another diet F, supplemented with 0.8% P, was Ca-free. Each diet was distributed to 3 replicate tanks of 600 swim-up fry (initial mean weight: 0.1g) at a water temperature of 17°C over a 12-week growth trial. Fish were hand-fed 6 times a day to visual satiety. There was no significant effect of dietary P (0 to 1.6%) or Ca (0 or 1%) supplementation on growth (final mean weight: 4.0±1.2g). Survival of fish fed with diet E containing a high level of P was significantly lower compared to other groups (10% vs. 65%, respectively). Fish fed with diet A displayed lower whole-body ash and P content compared to other groups (1.98 vs. 2.49% and 0.35 vs. 0.46% wet weight, respectively). Phosphorus retention decreased with increasing dietary P level (from 100% in group A to about 20% in group E). The quantitative image analysis of 28-day-old fry double stained with Alcian blue and Alizarin red S revealed that individuals from group A were less ossified compared to individuals from other dietary groups. Quantitative analysis of skeletal formation using cumulative counts of endochondral and dermal structures revealed a significant delay in ossification of endochondral structures at day 11 in fish fed with mineral rich or poor diets (diets A, E and F). However, at day 28, no significant difference in ontogeny of endochondral or dermal structures was found between the trout fed with the control diet and those fed with a high P diet (E) or a low Ca diet (F), suggesting recovery is possible in rainbow trout. In conclusion, we found that both dietary deficiency and excess of P are detrimental to rainbow trout fry development: excess P affects survival, while P deficiency affects bone calcification. Calcium deficiency appears to exert some delay in ossification processes without affecting final bone mineralization.