Introduction
Phosphorus (P) is essential for systemic functioning and bone mineralisation. The recommended level of total P for farmed Atlantic salmon (Salmo salar) is 10.0 g/kg. The requirements of dietary P are, however, life stage dependent. Early freshwater stages require increased levels of dietary P (up to 11.4 g/kg total P) due to high metabolic demand and fast growth. The P demand in seawater is reduced. Still, within the seawater phase, periods of prolonged day light and elevated water temperatures result in a faster growth potentially increasing the P demand. The current study provides knowledge about the P requirement during the final grow-out phase in seawater as an attempt to increase sustainability.
Materials and Methods
Seawater stages of Atlantic salmon were fed mono-ammonium phosphate supplemented diets with different P levels, 6.1 g/kg (A), 8.0 g/kg (B), 8.7 g/kg (C), 9.5 g/kg (D), 10.4 g/kg (E), and 11.2 g/kg (F) total P on dry matter basis, from December (average weight (a.w.): 1.7 kg) to July (a.w.: 4.1 kg). Animals (4 net pens/ diet, 90 fish/ net pen) were reared under natural photoperiod and sampled in April (water temperature range: 4.6 - 8.4 °C) and in July (water temperature range: 6.7 - 13.3 °C). Analyses included growth performance (weight and length increase), deformities, vertebral centra strength, and bone mineralisation.
Results and Discussion
In April, the growth of animals in all diet groups was comparable (0.61 ± 0.05 mm/day). Yet, in diet A animals, the resistance of vertebral centra to deformation (elastic modulus, MPa/%) and bone ash content (%) were already reduced (0.36 ± 0.03 MPa/%, 26.39 ± 1.54 %) compared with the remaining diet groups (0.53 ± 0.04 MPa/%, 32.56 ± 1.19 %). Later in April-July period, the increased water temperatures and prolonged day light led to growth differences among groups. In July, diet A animals had a slower growth rate (0.77 ± 0.05 mm/day) compared with animals in group B, D, and E (1.01 ± 0.06 mm/day). Diet A animals’ elastic modulus and ash content was further reduced (0.23 ± 0.02 MPa/%, 21.17 ± 1.54 %) compared to all groups (0.50 ± 0.03 MPa/%, 30.89 ± 1.24 %). Diet B animals began to lag behind in elastic modulus and ash content (0.43 ± 0.03 MPa/%, 29.14 ± 1.14 %) relative to diet D, E, and F animals (0.52 ± 0.04, 31.42 ± 1.27 %). Still, on x-ray images, low-mineralised vertebrae were only observed in diet A animals (Fig. 1). To conclude, the increased growth altered the dietary P requirements from 8.0 g/kg total P during December-April period to 8.7 g/kg total P during April-July period. Both values represent a significant reduction compared to the value of total P recommended for commercial diets. Optimisation of dietary P levels can lower the P effluent from fish farms, aids in sensible use of limited inorganic phosphate resources, and can help the industry to economise feed production.