Soybean Meal in Aquaculture

There are 4 major markets for aquatic plants and animals, all of which are experiencing increasing demand. The food aquaculture market is the only one for which data have been compiled and the focus of this review. Demand for new food aquaculture production over the next 30-40 years is between 60 and 120 million metric tons (mmt), which is similar in size to the global poultry and swine industries. Increases in aquaculture production began in the late 1980’s as commercial harvest was declared at maximum sustainable yield. Since 1990, most of the increases in aquaculture production have been with fish, mollusks and crustaceans, in the southern hemisphere, primarily Asia, and in developing countries. Production is almost equally divided between fresh- and saltwater. Of the approximately 30,000 species of fish, commercial harvest records are maintained for 1,173. Currently, 358 are cultured and of those, 54 have been fed soybeans. Indeed, soybeans have become the focus of nutritional research with new culture species, yet the evaluations have been on a gross level, lacking specificity. If detailed studies are not conducted, processors will not be able to provide the desired form of soy for the aquaculture markets and biotechnology groups will not have sufficient information for their work. There are numerous markets that should be aggressively pursued ranging from existing industries that underutilize soy to emerging industries. The emerging markets are those for which a significant commercial harvest formally existed, yet supply diminished.

Initial diets for new aquaculture species typically contain high levels of fish meal and fish oil, which are flavorful ingredients when fed to aquatic animals. Additionally, fish meal is a high protein ingredient with a good quality balance of essential amino acids (EAA) and fish oil contains n-3 fatty acids, required by many aquatic animals. The aquatic medium does not contain a high percentage of carbohydrates available as calories so carbohydrate content is of lesser importance. Given this generalization, it is not surprising that most aquatic animals grow maximally when fed relatively high levels of crude protein and lipid, and that essential amino acid and fatty acid concentrations in the diet are high priority considerations when formulating diets. However, fish meal and oil supplies are insufficient to realize growth in aquaculture production over the next 40 years.

Global fish meal supplies are approximately 6.8 mmt, of which approximately 1.7 mmt or 25% is used in diets for aquatic animals. Most fish meal is made from sardines, herring, anchovy and menhaden, which are small saltwater forage fishes experiencing similar declines in populations as other species of commercially harvested fish. If we assume aquaculture grows the least amount predicted (60 million metric tons, mmt), and that we can restrict fish meal usage to 10% of the diet, then aquaculture will demand 6 mmt of fish meal in the next 40 years. The global supply is only 6.8 mmt and that supply will most likely not increase. Fish meal is also subject to wide market fluctuations.

Fish meal contains higher crude protein concentrations than solvent-extracted soybean meal, but soybean meal prices are more stable than fish meal prices. During recent El Nino years, fish were scarce off the coast of South America as the fishing season began and prices increased significantly. As fish moved closer to the coast and harvest increased, prices decreased. However, during the time prices were high, they were approaching double their baseline price. Availability alone can be viewed as the primary deterrent to use of fish meal in the future; price of fish meal and resulting fish feed is a contributing factor. Diminishing use of fish meal and increasing use of soybean meal appears to improve the effluent characteristics of aquaculture operations (Cain and Garling 1995, Riche and Brown 1999). This will be an increasingly important consideration in aquaculture as it is in other animal production industries.

Aquaculture currently demands approximately 36% of the global fish oil supply or 0.4 mmt. As with fish meal, this is a finite resource and increases in demand will strip supply if growth projections for aquaculture are realized. Replacing fish oil will most likely be a more significant challenge than replacing fish meal.

Dietary development for aquaculture has, in general, followed a consistent pattern. The first evaluations of new species, often unpublished, use those diets that are available. Most initial diet choices were high fish meal diets formulated for trout or salmon. These diets are palatable by most species of fish. As research studies are conducted, diets are gradually modified to more closely meet the nutritional requirements of the target species.

The initial studies with a new species are evaluation of optimal dietary crude protein concentration, optimal protein to nonprotein energy ratio, optimal ratio of carbohydrate to lipid, then evaluation of commercial feedstuffs. As dietary development for other animals progressed, essential amino acid requirements were recognized as important dietary formulation parameters. However, of those species currently cultured we have estimates of all ten EAA requirements for approximately 9 species. Lysine and methionine requirements, the two most limiting EAA in most feed formulations for most species, exist for another 8-10 species. Despite this lack of knowledge, various forms of soybean meal have been evaluated in a variety of aquatic animals.

Table 1 depicts the species in which any form of soybean meal has been used in a diet as a source of crude protein and EAA. Many of these published reports are not formal evaluations of soy, simply an initial diet developed for the target species that contains soy or digestibility values developed using a form of soybean as a test feedstuff. The full-fat soybean meal designation includes raw, heated and roasted soybeans. The list is impressive and the number of recent publications (1998-present) far outweighs those prior to 1996. In fact, an article written in 1999 and published in 2000 identified 17 species in which formal evaluations of soybeans had been conducted (Twibell and Brown 2000). That number has grown to 54 species. Despite the number of evaluations, the specificity in the research is generally lacking. For example, there have been very few definitive studies on trypsin inhibitors from soybeans and their effects on fish.

Twibell and Brown (2000) summarized the available data on use of soy in fish and argued that virtually all fish could tolerate a minimum of 10-15% soybean meal in diets, but that several of the more carnivorous species could not tolerate more than 20% soybean meal. The salmonids (trout, salmon and char) are one of the most sensitive species to soy in diets, tolerating no more than 25-30%, with some species tolerating no more than 15%. This is part of the reason there have been a higher number of soybean evaluations in salmonids than other species. However, it is unclear why we experience this limitation.

With some of the new aquaculture species, soybean incorporation into the diet can be relatively high. For example, once the critical EAA were established for the hybrid striped bass, Brown et al. (1997a) were able to incorporate up to 40% solvent-extracted soybean meal into commercial diets. A subsequent series of experiments identified mineral supplementation as limiting higher soybean use in hybrid striped bass (Kasper and Brown 2000), and, once that was evaluated, soybean use could be increased to 45-50% of the diet. Most of the evaluations with fish and crustaceans have not had the luxury of quantified EAA requirements of the target species. Thus, experimental formulations are most often on a crude protein basis. This is indeed a crude method of evaluating commercial feedstuffs and is probably yielding results that are suspicious. The hybrid striped bass is a strict carnivore. Thus, following the model created by the myriad of evaluations with salmonids, we would assume that hybrids would not tolerate high levels of soybean meal. This has not been the case. The evaluations with hybrids were one of the few in which soybean meal was substituted as a source of EAA rather than crude protein. Thus, one of the most serious problems with use of soy products in diets fed to fish is a basic lack of understanding of the EAA requirements of the target species and scientists who substitute soy products simply on a crude protein basis. However, this is not the sole problem.

The lack of specificity in experiments with soy led to numerous speculations on the cause of the limited use in diets. Most focused on trypsin inhibitor activity, others on saponins, and more recently on allergenic proteins. However, the number of detailed studies examining any of these antinutritional factors is 6 (El-Sayed et al. 2000, Bureau et al. 1998, Refstie et al. 1998, Escaffre et al. 1997, Olli et al. 1994, Wilson and Poe 1985). Given that there have been over 200 evaluations of soy in diets fed to fish and crustaceans, this lack of detailed analysis and evaluation is both surprising and appalling. In general, the most common method used to date for evaluating soybeans in diets fed to aquatic animals is to include them at increasing levels and sometimes maintain isonitrogenous diets, other times to simply include them at some level, then speculate on the cause of any problems. The research needed to expand use of soy products in aquaculture are detailed studies examining the various components of soy we consider antinutritional factors. Those data will guide us to appropriate processing methods to remove those components that are legitimately causing problems or to biotechnological solutions that will result in a modified soybean for the aquaculture market. The misconceptions about soy use are already resulting in evaluations to replace soybeans with lower quality feedstuffs (Cruz-Suarez et al. 2001, Sudaryono et al. 1999, Fasakin and Balogun 1998, Zaki and El-Ebiary 1997). Some of the misconceptions are also limiting evaluations of positive components in soybeans such as the phytoestrogens (Ko et al. 1999). In some aquatic species, the female grows faster than the male and is less aggressive. We might be able to efficiently feed some species of fish and achieve the desired feminization with soybeans.

In the 20th century, extensive production of livestock diminished and intensive production became the dominant method of growing animals. This change in production method resulted in significant new markets for soybeans. The last major food item we still hunt and gather from wild populations are those animals that live underwater. However, we cannot harvest more from the world’s bodies of water. If we want to consume fish and shellfish in the future, the percentage grown in aquaculture must increase. The projected needs from aquaculture are similar to the current global production of hogs and poultry. These new aquaculture industries need feedstuffs and they need to know how much of each feedstuff they can use. In 1999, soybeans had been fed to 17 species, by 2001, this number increased to 54. There are over 350 aquatic species currently cultured and there are estimates that up to 1000 species are under evaluation as new aquaculture species. There is a great deal of research required in the short term to participate in this new global industry.

Literature Cited


Paul B. Brown, Purdue University, Department of Forestry and Natural Resources, 1159 Forestry Building, West Lafayette, IN 47907-1159, and Keith Smith, Keith Smith and Associates, 15 Winchester Road, Farmington, MO 63640