FEATURE following seawater transfer, the fish developed 30, 15 and 0 percent deformity in the lower jaw bone concomitant with increased mineral content of the spinal bones within a feeding period of only 12 weeks. In another smolt trial with similar size fish, reduced mineralisation was found in fish fed 0.5 percent soluble P in the diet compared to fish fed 0.7 percent soluble P, while no sign of external deformity was observed. This illustrates the risk of feeding slightly sub-optimal P (0.5 perFigure 2: Total P and soluble P in plant protein ingredients. Soluble P is also given as % cent soluble P in the diet), it is cost effective of total P on top of each respective bars and environmentally friendly, but may occasionally affect fish welfare due to variation in the fish material, growth not only distinguish between the P in hydroxyapatite and other P-forms, rate, feed or the environmental conditions which is not easy to control. but can also be used to distinguish between P from inorganic salts with The digestibility of P depends on dietary P level, irrespective of different solubility, which is the main criterion for P absorption. Monomeasuring total P or soluble P in the feeds. In salmon fed diets with Ca-P salt was found to contain about 65 percent soluble P, while monodifferent P contents, the digestibility of total P varied between 30 and Na-P salt contained more, about 94 percent soluble P, demonstrating 50 percent (Fig. 3a), while the digestibility of soluble P in the same that the mono Na-P salt contain higher levels of available P than mono feeds were higher and varied between 60 and 80 percent (Fig. 3b). Ca-P despite similar levels of total P. Inorganic mono-salts of P will also Other trials have shown that the digestibility of soluble P in the feed be more soluble than di-salts of P and this will contribute to different P can be as high as 90 percent in salmon, indicating a very efficient diges- digestibility and thus different availability of P from the feed. Nofima has tion of soluble P. The amount of dietary P that is retained (deposited) conducted experiments that indicate that feed that contain 0.7 percent in the body is about 30 percent, which means that about 70 percent soluble P provide adequate P in salmon at the smolt stage, while a higher of dietary P will be released to the environment. As this is a major dietary P content of 0.8 percent soluble P is required in Atlantic salmon global concern, dietary P is usually added close to or slightly below the fry during early start feeding. More research is needed to understand requirement in order to obtain maximum P utilisation and minimum P the potential for using dietary soluble P when analysing commercial high load to the environment, as well as to keep the cost as low as possible. plant protein diets with variable phytate levels, although it is possible to This is a reasonable strategy, but requires better feed control with correct for this. The soluble P method has been developed and validated available P than is realistic to achieve today by analysing dietary total P. by Nofima and found to have high accuracy, resembling the analytical The new method distinguishes between insoluble and soluble P, method for total P (Hovde, 2013). References available on request which is to be seen as indigestible P and digestible P, respectively. It does
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January-February 2015 | INTERNATIONAL AQUAFEED | 15
The January February 2015 edition of International Aquafeed magazine