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Soluble phosphorus in salmon feed by Sissel Albrektsen, senior scientist Nofima


hosphorus (P) is an essential mineral that has to be added to salmon feed to achieve normal growth and skeletal development. P from marine ingredients, plant protein and crystalline P salts provide respectively 46, 30 and 24 percent of dietary P in Norwegian salmon feeds. Hence, fishmeal is an important P-source, even though the level of fishmeal in the feed has dropped significantly from 64 percent in 1990 to 20 percent in 2012 (Ytrestoyl et al., 2014). About 40 percent of P in the fishmeal originates from the bones, and is present as calcium (Ca)-phosphate salts in hydroxyapatite. P in hydroxyapatite has low solubility and is poorly available to salmon. The digestibility of P in different fishmeal reflects this and further shows great variation (20 – 60 percent), depending on the fish raw material and seasonal variations. No reliable direct measure of available P exists, and this makes it difficult to know how much available P is present in any given fish feed. In this article we will focus on the possibility of using a new method on soluble P as a measure of available P in ingredients and feeds, and what opportunity this gives to ensure better control of available P in the feeds. Industrial fish that traditionally has been used as raw material for fishmeal production, such as herring in the Nordic countries, is today used for consumption. As a consequence, an increasing amount of fishery byproducts is used as raw material for fishmeal production. The global by-product material used for fishmeal and fish-oil production has been rising by 1–2 percent year−1, and represented 25 percent of world production in 2010 (Shepherd and Jackson, 2013). In Norway, fishery byproducts accounted for 30 percent of the raw material in fishmeal in 2013 (Ytrestoyl et al., 2014). Fishery byproducts contribute with high content of total P from the fish bones, while at the same time, the percentage of P that is available for the salmon is actually reduced compared with a traditional fish meal produced without trimmings. Fig. 1 shows total P and soluble P in herring meal produced with different levels of trimmings from herring, and other fish meals produced from blue whiting and capelin and also from Antarctic krill. High inclusion levels of trimmings reduce the proportion of soluble P

Figure 1: Total P and soluble P in herring meal produced with 0, 41 and 62 % herring offal, in fish meal produced from blue whiting and capelin, and in Antarctic krill. Soluble P is also given as % of total P on top of each respective bars

from 61 to 35 percent of total P, which means that the level of soluble P is reduced despite an increase in total P. The analysis of total P tells little about the proportion of P that is available for the salmon, i.e. how much of P that is present as free phosphates, and thereby available for digestion. The amount of trimmings used in global fishmeal production varies greatly, from 0 to 100 percent, which increases the unpredictability with respect to how much of dietary P that is available. In some commercial smolt feeds, total P ranged from 0.7 to 1.77 percent, soluble P from 0.36 to 0.7 percent and the proportion of soluble P from 31 to 70 percent of total P. The plant ingredients used in fish feed today contain 60-80 percent phytic acid. Phytic acid contains P (phytate-P) but many fish species, including salmon, has no or little phytase activity and cannot utilise this P source. Phytic acid also acts as an anti-nutrient by inhibiting the absorption of available P and other minerals by forming poorly soluble mineral complex in the intestine. Nofima has analysed total P and soluble P in many plant proteins (Fig. 2), and the main finding is that most of the total P in plant proteins will be analysed as soluble P. The analytical method for soluble P apparently cannot distinguish between phytate-P and other soluble forms of P in plant proteins. Total P and the proportion of phytate P and other P-components are known and quite stable for most plant ingredients. By analysing soluble P in the plant ingredient, it is easy to correct for the proportion of P that is present as phytate-P, an approach that is also applied in the aquafeed industry today. Overall, the method will give much more reliable measures of available P in salmon feeds and feed ingredients compared to the current total P analysis. The P requirement is affected by a variety of biological and environmental factors, and it can vary with life stage and growth rate of fish, diet composition and temperature. When available P in the diet is low, the fish will regulate this by increasing the P uptake in the intestine, reduce the excretion of P in the kidney, and mobilise P from the skeleton to cover vital functions in other body compartments. Sustained demineralisation of the bones over a long period will weaken the skeleton and eventually cause deformity. In fast-growing Atlantic salmon fed 0.3, 0.5 and 0.7 percent soluble P in the diet

Figure 3: Digestibility of total P (A) and soluble P (B) in Atlantic salmon decrease with increased dietary P levels, and the utilization of dietary P is most effective in fish fed low dietary P levels

14 | INTERNATIONAL AQUAFEED | January-February 2015

Jan | Feb 2015 International Aquafeed magazine  

The January February 2015 edition of International Aquafeed magazine