Jouni Vielma and Santosh P. Lall
Chemical and physical characteristics of diets used for different fish species and for different stages of production vary. Instead of collecting empirical data under husbandry conditions, a more general physiological model is preferred. By formulating a simple excretion model, we studied model sensitivity for variations in diet composition, and energy and nutrient digestibility in 15 commercial diets. The paper emphasizes nitrogen and phosphorus excretion, since distinctive absorption, metabolism and excretion mechanisms may exist for compounds containing nitrogen and phosphorus. Carbon load reflects the overall feed efficiency, and is not discussed here.
Nitrogen is found in protein fraction of the diet, and nitrogen excretion is closely linked to the efficiency of protein utilization. The main source of protein in salmonid diets is fish meal, but potential alternate plant protein sources like soybean meals are now being evaluated. The quality of protein, mainly its digestibility and amino acid composition, and the ratio of digestible protein to digestible energy are the two significant variables affecting both nitrogen excretion and overall feed efficiency. Protein digestibility is around 90 % in the most commonly used protein sources. Energy digestibility reflects protein, fat and carbohydrate digestibilities, and varies more due to the variation in their contents in diets. Due to the development in feed manufacturing, content and digestibility of energy in diets has increased. Fish meal is also the main source of dietary phosphorus. There is large variation in phosphorus content (15-40 g kg-1) and digestibility coefficient (45-70 %) of fish meals. Plant products contain less phosphorus (5-15 g kg-1) of which most part is unavailable to monogastric animals like fish, since it is bound as insoluble salts (phytates). Despite the different functions in animal body, a model of similar structure can be used for simulating both nitrogen and phosphorus load in fish farming.
A model similar to the generalized energy budget was constructed. 15 commercial diets used for feeding adult Atlantic salmon and rainbow trout, and supplied by four manufacturers operating in Scandinavia, were selected. Total energy, protein and phosphorus content of the diets was recorded as labeled by the manufacturers. The range for the values were 21.4-24.9 MJ/kg, 400-470 g/kg and 8-11 g/kg, respectively. Nutrient and energy contents of diets were fairly uniform. All had balanced protein to energy-ratio, i.e., there were moderate in protein and high in fat content. After surveying the literature for empirical values used in the model, the excretion of nutrients was computed by 1) estimating the energy cost of growth (assumed to be 19 MJ digestible energy kg-1 growth), 2) computing nutrient intake on the basis of the energy cost of growth, and digestible energy and total nutrient content of diet, 3) estimating the amount of absorbed nutrient on the basis of nutrient intake and nutrient digestibility coefficient, and 4) computing the amount of dissolved excretion by using the amount of absorbed nutrient and body content of the nutrient (assumed to be 4 g P kg-1 and 30 g N kg-1). Digestibilities used in calculations were for energy, nitrogen and phosphorus 85 %, 85 % and 50 % respectively.
The calculated biological feed conversion ratio (no mortalities, no feed wastage) was 0.90-1.04. Due to relative uniform nutrient and energy contents, there were no large differences in calculated nutrient excretion rates. The average calculated phosphorus excretion was 5.1 g kg-1 (standard deviation 0.7 g kg-1). 10 % of the phosphorus was excreted in dissolved form. For nitrogen, the average calculated excretion was 37 g kg-1 (standard deviation 3 g kg-1) of which 73 % was excreted dissolved. In comparision, feeding Baltic herring, which is still practised to some extent in cage farming in Finland, results in four-fold phosphorus excretion due to its low energy and high nutrient contents. Decreasing assumed phosphorus digestibility to 40 % induced phosphorus deficiency in all but one diet. Clearly, in high-energy diets phosphorus content is very close being a limiting factor if digestibility can not be increased. Increasing energy cost of growth and decreasing energy digestibility by 10 % increased nutrient excretion by 38 %. To compute total nutrient load, feed wastage and biomass growth must be estimated. There are several growth models all of which need to be validated for specific husbandry conditions. To estimate feed wastage, a modeler must rely on empirical data.
It is concluded, that labeled nutrient contents of present commercial salmon diets reveal no large differences in their loading capasity. Ignoring the energetic estimation of nutrient intake may result in biased excretion values if diets different to standard commercial diets are used.