Soybean meal for fish

Aquaculture is the fastest growing segment of livestock production worldwide, growing in large part because production is switching to more intensive, higher input systems. The main input is feed, which offers tremendous opportunities for aquafeed manufacturers and associated product suppliers. It also presents significant challenges to all sectors of the industry.

The protein content in commercial aquafeed, while differing for various species, is much higher than those for domestic animals, ranging from 30% to more than 50% protein by dry weight. Fish meal has traditionally been a major protein source in aquafeeds because of its protein quality and palatability. However, increasing demand, high cost and uncertain availability, finding an alternative protein source has become a major focus of research from the viewpoint of producing a stable supply of commercial aquafeeds at a reduced price. In addition, the need to formulate diets which minimize phosphorous (P) excretion for fish and consequent eutrofication of waters requires replacement of fish meal with low-P protein sources.

Soybean is considered to be one of the most suitable and economical candidates for replacing fish meal in commercial aquafeeds. It has been identified as having the best amino acid profile of all protein-rich plant feedstuffs for meeting the essential amino acid requirements of fish (Lovell, 1991). On the negative side, the sulfur containing amino acids methionine and cystine are generally considered to be most limiting in soybean products compared to the quantitative amino acid requirements of most fish species (NRC, 1993). It is however the presence of antinutritional factors (ANFs) that limits its use in crude and processed forms in aquafeeds.


Antinutritional factors

Raw or non-heated soybeans contain a number of heat-labile ANFs that inhibit some digestive processes of fish when replacing high dietary levels of fish meal with soybean meal (Refstie et al., 1998).

Proper heat treatment of the soybean is important to inactivate the antinutritional factors without affecting their nutritional quality to optimally be used in aquafeeds.

Phosphorus in soybean meal is primarily in its organic form as phytin phosphorus or phytate, which typically comprises approximately 67% of phosphorus in plant feedstuffs. This form of phosphorus is not readily available to monogastric animals including various fish species (NRC, 1993) because of their lack of phytase, the enzyme required to liberate phosphorus from phytate. Phytate also readily chelates di- and trivalent cations such as Ca, Mg, Fe, Zn and Cu at intestinal pH, thus reducing the availability of these minerals.

Adverse effect of phytate on zinc bioavailability has been demonstrated in fish (Liener, 1980). Thus, if soybean meal is to be used as a major protein source, supplementation of phosphorus, zinc, and possibly other trace minerals is advisable.

Increasing the availability of phosphorus from soybean products is desirable to restrict the amount of supplemental phosphorus required in diets and also limit the mount excreted by fish into the environment. Direct addition of the enzyme phytase to the diets of fish has been shown to considerably increase the availability of phosphorus (Lim & Akiyama, 1992; Vielma et al., 2000). Pre-treating soybeans with phytase (dephytinization) may be a another option.

Of the carbohydrate portion of soybeans, the non-starch polysaccharides and oligosaccharides (such as raffinose and stachyose) are not easily digested by fish and may hinder digestion. Their presence in the intestinal contents increases the osmotic pressure of the fluid and thereby restricts the absorption of water.

These indigestible oligosaccharides do not pose much problems in freshwater fish, which are constantly excreting water to maintain the osmotic pressure of their body fluids in a hypo-osmotic environment. In marine species, however, it is believed that the reduced absorption of water from the intestinal contents is a source of osmoregulatory stress when the fish are raised in seawater.

Research on non-starch polysaccharides in diets for Atlantic salmon (Refstie et al, 1999) attributed a trend of reduced digestibility of fat and protein to the possible effect of increased viscosity of intestinal contents on diffusion and mixing of digestive enzymes. However, this observation has never been reported in studies with freshwater fish. It may be that non-starch polysaccharides simply have the same effect as oligosaccharides on the water balance in fish raised in a marine environment (Elangovan, 2000).

Other antinutritional factors are the inhibitors of trypsin and chymotrypsin, enzymes involved in the digestion of proteins. Rainbow trout fed diets containing purified Kunitz soybean trypsin inhibitor had diminished growth, decreased intestinal trypsin activity, and an increase in fecal excretion of protein and lipids (Berg-Lae et al., 1989). Boonyaratpalin et al., (1998) attributed pancreatic atrophy found in seabass fed soaked raw full-fat SBM to the high trypsin inhibitor activity (14.64 mg/g).

Allergic effects of soy components in fish appear to be highly species specific. Soy isoflavones have been shown to cause increased plasma concentrations of sex hormones in immature sturgeon. However, this effect has never been reported in any species of bony fish. Likewise, salmonid species exhibit allergic reactions to full-fat or fat-extracted soybean meals. Soy components, other than protein, apparently cause morphological changes in the mucosa of the distal intestine. This “allergic” symptom is more pronounced in Atlantic salmon.

It is known that the lectin, soybean agglutinin (SBA), binds to the surface carbohydrates in epithelial cells in mammalian intestinal tracts resulting in deleterious side-effects. Buttle et al., (2001) concluded from a study on rainbow trout that SBA binds in vivo to the intestinal epithelium of fish and has a contributory role in pathological changes associated with aquafeeds containing high levels of soybean proteins. Boonyaratpalin et al. (1998) attributed a decrease in absorptive area of the anterior intestine to the presence of lectin when seabass were fed soaked raw full-fat SBM.

Antigens present in soybean products may stimulate the non-specific defense mechanisms of trout. Susceptibility to furuncolosis, caused by Aeromonas salmonicida ssp. salmonocida, is higher in Atlantic salmon fed diets containing soybean meal compared to fish fed diets containing fish meal or soy protein concentrate, presumably because the soybean meal diet caused distal enteritis, and the pathogen is presumed to infect fish via the gut (Krogdahl et al., 2000).

The replacement of fish meal by soybean meal often changes the composition of flavour components and feeding attractants in die diets (Refstie et al., 1998). Some specific types of carbohydrates in soybean meal can impart a “beany” taste, and may influence its palatability. Especially when fish are adapted to fish meal-based diets, switching to diets with high inclusion levels of soy may reduce feed consumption and growth temporarily. Even moderate reductions in feed consumption may severely effect growth in salmon in a given period.

Poor palatability may however be overcome by including feeding attractants.

Saponins are triterpene or steroid glycosides found in certain plants and marine animals, such as starfish, and can be feeding deterrents for some aquatic animals. Fishes are sensitive to certain water-borne saponins since they may cause damage to gills. Soya saponins can contribute an undesirable taste to aquafeeds and may alter intestinal functions or cause intestinal damage. The presence of soyasapponins in soy products is highly dependant on the mode of preparation (Bureau et al., 1998).

Finally, the ability of salmonids and other anadromous species to adapt to changes in salinity is dependant on the capacity to modulate osmoregulatory functions of the gills, kidneys, as well as the intestine.

Corresponding with this, carrier-mediated absorption of nutrients, particularly by transporters coupled to sodium gradients, are responsive to changes in salinity.

Results from a study by Nordum et al. (2000) on Atlantic salmon and rainbow trout indicated that SBM caused a decreased carrier-mediated transport and increased permeability of distal intestinal epithelium for amino acids, and the capacity of this region to absorb was diminished.


Soy products

Soy products relevant to aquaculture feeds can be divided into four categories: soybean meals, soy protein products, oil and lecithin. The proximate composition of some these soybean products is presented in Table 1.

Soybean meals (SBMs)

De-oiled soybean meals – A considerable amount of fish meal in the diet can be replaced with SBM in omnivorous freshwater fish, such as carp, tilapia, and catfish. Similar studies with rainbow trout, yellowtail, seabass, Japanese flounder and red drum have revealed that SBM is also a viable source of protein to carnivorous fish. Red drum and Japanese flounder can effectively grow on diets containing almost equal amounts of fish meal and soybean meal without adversary effects (Chou et al., 2004).

However, the level of soybean meal (SBM) in certain salmonid diets must generally be limited since moderate to high levels of incorporation lead to a reduction in growth performance, partly attributable to a reduction of feed intake (especially juvenile diets). Young Chinook salmon are very sensitive to inclusion of soybean meal in their diet, whereas other salmonids, such as rainbow trout, coho salmon and Atlantic salmon appear to be more tolerant (Bureau et al., 1998). In diets for rainbow trout and seabass juveniles, solvent extracted SBM replaced up to 40% of fish meal and up to 100% for pond-raised channel catfish, without significantly lowering production performance parameters (Boonyaratpalin et al., 1998).

At fish meal replacement levels greater than 50%, fish fed SBM-containing diets exhibited a progressive decline in growth rate which was accompanied by a corresponding depression in non-specific immune capacity and exacerbation of pathological changes in the distal intestine (Burrels et al., 1999). Current levels in trout diets are no more than 20% soybean meal in post-juvenile diets (grow-out diets). It is estimated that at these levels of soybean meal in salmon and trout grow-out feeds, by 2010, 391 000 mt will be used annually in salmon an trout diets alone.

Full-fat soybean meal (FFSBM) has been evaluated and incorporated into diets of warm-water and omnivorous species to a more limited extent than coldwater and carnivorous species, primarily due to the more restrictive lipid levels typically included in the diets of the former species.

Adequate heat treatment of FFSBM is required to inactivate the heat-labile antinutritional factors known to be present in raw soybeans. The additional heat associated with extrusion of full-fat soybean meal lowers the level of tripsin inhibitor activity and likely lowers the levels of other heat-sensitive ANFs. However, at present, their use in commercial aquafeeds is minimal, possibly due to the effect of its high oil level on the extrusion process and hence, on gelatinization.

The viable levels of inclusion of SBMs in aquafeeds are generally influenced by the nutritional physiology and feeding nature of aquatic animals viz. herbivorous, omnivorous, carnivorous, detrivorous etc. Guidelines according to Hertrampf & Piedad-Pascual (2000) are summarized in Table 2.

Soya proteins

Soya proteins are valuable protein-rich products derived as by-products from soybean oil extraction. Three forms of soya proteins derived from de-hulled soybeans, find application in aquafeeds – soya protein concentrate, soya protein isolate and soya protein hydrolysate.

Soya protein concentrate (SPC) – SPC is produced through aqueous ethanol or methanol extraction of defatted soy flakes by which carbohydrates, phytic acid, oligosaccharides are removed. As the alcohol denatures the protein, this product has low protein solubility. (Gatlin, 2002). It has low trypsin inhibitor levels and is more acceptable or supports better growth than soy flour or soybean meal for salmonids and other fishes.

SPC may replace 50% of the dietary LT-FM without adverse effects on growth and nutrient retention of rainbow trout (Mambrini et al., 1999). Its cost generally limits use in most aquafeed formulations. Possible exceptions might be its inclusion in diets designed for specific, time restricted life stages such as the larval stages of some fish species. If soy protein concentrate were to replace 40% of fish meal in salmon and trout diets by 2010, the potential market would be approximately be 780 000 mt.

Soya protein isolate (SPI) – Soy-protein isolate (SPI) is produced by aqueous solubilization and isoelectric precipitation of protein from soy flakes. Water soluble carbohydrates are dissolved while the undissolved constituents are separated by centrifuges or filters after which the extract is acidified to pH 4.5 and the proteins are precipitated. After neutralizing the pasty precipitate, spray-drying the material yields a fine soya protein isolate which is readily soluble in water. SPI appears to depress feed intake in a manner similar to soybean meal in Chinook salmon despite containing very low levels of ANF (Bureau et al., 1998).

Soya protein hydrolysate (SPH) – enzymatic hydrolysis of either soya protein concentrate or soya protein isolate renders soya protein hydrolysate whereby the insoluble protein is converted into soluble protein. The use of SPH is limited in aquafeeds.

Soy protein concentrates and soy isolate mainly used in diets for fry, or for speciality diets custom made for growers willing to pay higher feed prices to obtain low fish meal diets that do not contain rendered products. The price of SPC is higher than high quality fish meal and fish hydrolyzates in South Africa and North America. In Europe, the price of SPC is positioned to be more competitive with LT fish meal. The high price of these products limits the potential of these products as economical protein sources for aquafeeds.


Soy oil

Soy oil contains high levels of mono- and di-unsaturated fatty acids, but is deficient in some of the specific highly unsaturated fatty acids required by many species of marine fish, and high levels of inclusion will likely need to be accompanied by supplementation of essential fatty acid containing sources.

Marine fish have a requirement for highly unsaturated fatty acids (HUFA), since they cannot synthesize these fatty acids from C18 precursors in significant amounts (Regost et al., 2003) - soybean oil contains only about 8% linolenic acid (C18:3, n-3). These HUFAs are long-chain omega-3 fatty acids, especially eicosapentanoic acid (EPA, C20:5 n-3) and docosahexanoic acid (DHA, C22:6, n-3). Nevertheless it has been shown that it is possible to totally replace fish oil by soybean oil in salmonid aquafeeds without affecting growth.

Residual fish oil in fish meal often contributes the essential fatty acids in aquafeeds.

Soy oil substitution is known to modify muscle fatty acid composition. The effect of soy oil replacement on fatty acid composition of fillets are likely to be an elevation of linoleic acid (C18:2, n-6) if soybean oil is used. In rainbow trout broodstock, it has also been shown that both egg and milt fatty acid composition is affected by dietary vegetable oils without affecting reproductive performance (Regost et al., 2003). The current trend is toward replacing up to 50% of added fish oil with plant oil in grow-out feeds for trout and salmon.

Parallel with the concern of the declining ability of fish meal to meet the growing demands of the animal feed industry, especially for aquatic animals, the availability of marine oil is also declining. (Forster, 2002) As soy protein products worldwide have increased dramatically over the past four decades, so has the availability of soybean oil. Soybean oil is a leading candidate to replace a portion of fish oil in salmon diets, especially in the United Kingdom. However, canola oil is a leading competitor, and price dictates use patterns.


Soy lecithin

Soy lecithin’s bio-surfactant property makes it suitable for use as a agent that enhances the emulsification of dietary lipids in the intestine (Hertrampf & Piedad-Pscual, 2000; Forster, 2002). Soybean lecithin is also the most common source of phospholipids (PL, such as phosphatidylcholine and -inositol) in aquafeeds.

Phospholipids are the major constituents most membranes and have a regulatory function inside the cell, within the plasma membrane as well as outside the cell. Phospholipids play a vital role in the acclimatization of fish to different environmental temperatures, in the migration of fish from freshwater to seawater and vice versa and the regulatory mechanisms of marine fish.

The beneficial effect of dietary phospholipids on growth and survival of larval and juvenile stages of many species of marine fish and shrimp, including various species of penaied shrimp has be well documented (reviewed by Teshima, 1997; Coutteau et al., 1997) - despite the de novo synthesizing ability demonstrated in crustaceans.

Research has proven that fish and crustaceans that received soya lecithin supplemented diets at the larval and juvenile stages have shown better production performance and survival, better maturation and spawning and reduced incidences of malformation of some body parts like the jaws. Gong et al. (2002) reported that in the juvenile shrimp Litopenaeus vannamei, growth was enhanced with supplementation of dietary soy lecithin at levels of 3%.

Soybean lecithin supplementation to diets of red tail prawn Penaeus penicillatus (Chen & Jenn, 1991), common carp, Cyprinus carpio larvae (Geurden et al., 1998), rainbow trout Oncorhynchus mykiss (Poston, 1990), Atlantic salmon Salmo salar (Refstie, et al. 2000.) showed significant improvement on growth and survival.

Interestingly soy lecithin it has a discrepant EFA profile (> 50% 18:2n-6) compared to PL sources in the natural diet of marine organisms which are rich in n-3 highly unsaturated fatty acids (HUFA, e.g. copepod polar lipids). Coutteau et al., (2000) however demonstrated that the supplementation of PL purified from soybean lecithin superiorly improved growth and reduced sensitivity to osmotic stress, compared to PL purified from marine fish roe.

The lecithin requirement of fish and crustaceans depends on the total fat content and fatty acid profile of the feed, the age and developmental stage of the animal and the water temperature in which the animal is cultured (coldwater fish need more phospholipids than warm water fish).

References

Author: Lourens de Wet
University of Stellenbosch