The proportion of fish meal use in fish feeds is predicted to decrease, because the amount of fish meal from wild fish is limited to 5-7 million tons a year (Chamberlain, 2011), while demand for fish feed resources is expected to continue growing rapidly (Tacon and Metian, 2008). Plants represent a highly abundant source of protein for use in food and feed. Typically, the world production of soybeans in 2010 was 258 million tons (Soystats, 2011), while that of rapeseeds was 61 million tons (Agricomodityprices, 2011). Plant protein sources with low degree of processing, such as defatted soybean meal, defatted rapeseed cake, and lupin kernel meal, have been widely used in fish feeds. Energy- and nutrient dense diets for carnivorous fish, however, have limited formulation space for this type plant protein sources. This is both due the presence of anti-nutritional factors found in these ingredients (Francis et al., 2001), and their high content of indigestible carbohydrates (Knudsen, 1997). Plant protein concentrates generally contain less of these factors. Protein concentrates from rapeseed (Thiessen et al., 2004), lupin (Glencross et al., 2011), potato (Refstie and Tiekstra, 2003), pea (Øverland et al., 2009), and soybeans (Kaushik et al., 1995; Storebakken et al., 1998a; 2000b) have shown promising results for use in salmonid diets. Thus, the use of plant protein concentrates has gained increasing interests by the feed industries.
Plant protein concentrates are produced by different methods. Air classification is commonly used to produce pea protein concentrate. This involves fine grinding, and separation of fractions high in starch and protein based on different settling points in an air stream due to different densities (Schutyser and van der Goot, 2011). Soy, lupin and rapeseed protein concentrates are often produced by extraction, either with hot water or in combination with ethanol. The defatted and de-hulled seed is ground prior to extraction off soluble, indigestible sugars and non-starch polysaccharides (Karnofsky, 1980). One fortunate effect of this process, is that the components in the soybean causing enteritis in the distal intestine of salmonids is extracted along with the carbohydrates (van den Ingh et al., 1991; van den Ingh et al., 1996), improving the usefulness of this ingredient in fish feed. The plant protein products with highest concentration are produced by precipitation of the proteins from an aqueous solution. One example is soy protein isolate, produced by iso-electric focusing or filtration of soy proteins (Alibhai et al., 2006).
Plant protein concentrates, however, have several limitations for direct use in fish feeds. All plants are deficient in essential amino acids, when compared to the requirements of fish.
Typically, the first limiting amino acid in soy protein is methionine, while lysine is the second limiting. For most other plant proteins lysine is the first limiting amino acids (NRC, 2011).
Salmonids and other carnivorous fish species can efficiently utilize crystalline amino acids (Espe et al., 2006). The frequent or even continuous feeding is preferred to minimize the difference in absorption between peptide-bound and crystalline EAA (Yamada et al., 1981; Cowey and Walton, 1988; Kaushik and Seiliez, 2010). Thus, amino acid deficiencies can be overcome by supplementing the diets with essential amino acids. Other essential nutrients may also become deficient when plant proteins account for the majority of protein in fish feeds. One example is taurine, a sulphur-containing derivative from methionine, can be also provided by fish meal, does not exist in plant-derived ingredients. Several fish species have lacking ability to synthesize taurine (Goto et al., 2003; Takagi et al., 2008; 2011) and recent findings show that rainbow trout benefits from dietary taurine supplement when given a diet with high proportion of plant proteins (Gaylord et al., 2006). Thus, taurine should be supplemented jointly with essential amino acids to diets with high content of plant proteins.
One important reason for using fish meal is that it is a feeding stimulant (Kousoulaki et al., 2009). Several plants contain bitter and detractive components such as alkaloids in lupin (Serrano et al., 2011) and soyasaponins in soy (Bureau et al., 1998). Some of these may be not always completely removed during the processing of the concentrate. Other marine products have strong attractant effects to fish. One of these is the krill, and several experiments have demonstrated increased feed intake and growth rates by using krill meal or krill hydrolysates, both in diets based on fish meal and in feeds with high concentration of plant ingredients (Oikawa and March, 1997).
All seeds contain phytic acid. This anti-nutrient cannot be removed by air classification, and it may even be concentrated by extraction to produce plant protein concentrates. Phytic acid has high concentration of phosphorous, which is not available to monogastric animals. It also chelates di- and trivalent cations in the intestine, making these unavailable for absorption (Storebakken et al., 2000a). Experiments have shown that phytic acid in soy protein concentrate can result in incomplete mineralization of hard tissues in salmonids (Storebakken et al., 1998a)
and fish meal based diets supplemented with phytic acid may introduce spinal deformities (Helland et al., 2006). Phytic acid can be hydrolyzed by including phytase in the feed if fed to warm water fish or coldwater fish at temperatures exceeding 10-15oC (Vielma et al., 1998;
Carter and Sajjadi, 2011). At lower temperatures, the effect of dietary phytase is minimal. Thus, incubation of plant concentrates with phytase (Storebakken et al., 1998a; Vielma et al., 2002;
Denstadli et al., 2007) should be considered before using including them in the diets.
Nitrogen is the limiting nutrient for algal growth is seawater, while phosphorous limits growth in freshwater. The main nitrogen pollutants from fish farming are water soluble ammonia from deamination of amino acids and urea from catabolism of nucleic acids, and particulate loss of faeces. Faecal loss is the main source of pollution with phosphorous. Uneaten feed may also represent a significant source of pollution, but it can be largely eliminated by the use of feeds with high technical quality (Sørensen et al., 2010; Aas et al., 2011), and by correct feeding (Storebakken and Austreng, 1987). In order to minimize the impact of fish farming on the environment, it is of high importance to simultaneously minimize pollution from water soluble, metabolic loss, from faeces, and from uneaten feed.
All feeds are in practice mixtures, while feed ingredient research has largely focused on single ingredients. The use of mixture design can be helpful to determine if the synergetic effects which can increase the performance or desirability of feed may become significant when mixing dietary ingredients. Mixture models also facilitate the determination of optimal mixtures or feed formulations, based on given response criteria. Such designs have been widely used in the chemical (Akalin et al., 2010; Lin et al., 2010), pharmaceutical (Mahdhi et al., 2010; Malzert-Freon et al., 2010) and food industries (Karaman et al., 2011) to optimize processes or formulations. Only few studies using mixture models to optimize fish and shrimp feed have been reported (Ruohonen et al., 2003; 2007; Forster et al., 2010; Draganovic et al., 2011).