Soybeans and the future of aquaculture

by Melissa Alexander
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Research is expanding the potential for utilizing soybeans as an alternative protein source for aquafeeds

by Steven Hart and Paul Brown

Demand for formulated fish feeds continues to grow, reflecting the global expansion of aquaculture production.

Feed costs are one of the highest annual variable expenses associated with intensive production of fish and crustaceans. The most commonly used crude protein source fed to most intensively reared fish and crustaceans has been fishmeal.

The global supply of fishmeal has plateaued to between 6.5 million and 7 million tonnes. Because of the high demand and limited supply, fishmeal prices tend to be high. In 2004, the cost of fishmeal was typically around U.S.$600 per tonne.

This limited supply and high cost has lead to an interest in identifying alternative protein sources for use in the aquaculture industry.

ALTERNATIVE PROTEIN SOURCES
Fish and shellfish require relatively high dietary concentrations of protein to achieve maximal growth, so fishmeal alternatives with high crude protein levels need to be utilized.

Protein feedstuffs originate from two basic sources: plants or animals. Both categories have associated benefits and disadvantages.

Many available rendered animal protein sources have variable nutritional quality and may not be wellutilized by fish. Also, supply of rendered animal products is less than total fishmeal supply.

Alternative plant sources are more readily available, and prices for those products tend to be lower.

Soybean meal has probably received more attention as a protein alternative to fishmeal than any another ingredient in aquaculture feeds. The global harvest of soybeans is larger than any other oilseed crop (189.5 million tonnes in 2003).

More importantly, approximately 65% of the harvested crop is used for oil extraction, about 123 million tonnes annually. The defatted cake is ground and toasted, resulting in soybean meal.

The next largest global supply of cakes from oil extraction is rapeseed at 17.7 million tonnes and cottonseed at 12.4 million tonnes.

The cost of soybean meal is typically a quarter to half the price of fishmeal, so it has a strong economic advantage. Another benefit of soybean meal is the high protein content: 44 to 49%, depending on processing. Many other processed plant feed ingredients contain 32 to 38% protein.

Finally, the essential amino acid content of soybean meal is generally good for aquatic animals, with the exception of methionine and possibly lysine and threonine. However, the addition of feed-grade amino acid supplements can overcome these deficiencies relatively inexpensively.

Soybean meal has already been incorporated into commercial aquaculture diets. It is the primary protein source supplied in channel catfish grow-out diets.

While the potential benefits of using soybean meal are undeniable, certain problems still need to be overcome before it can be fully utilized throughout the aquaculture industry. In trout and salmon diets, diminished growth responses have been reported to occur when soybean meal is incorporated at around 15-25% of the diet, even when the diets are formulated to meet essential amino acid requirements.

Several compounds are present in soybeans that may be potentially inhibiting a higher incorporation of soy in the diet. Many of these compounds have been labeled as anti-nutritional factors because they negatively impact nutrient digestion and absorption in the gastrointestinal tract.

Lectins, oligosaccharides, saponins and trypsin inhibitors are compounds present in soybeans that have been identified as having negative effects on fish. Soy isoflavones (also called phytoestrogens) have also been implicated as a potential problem by interfering with reproduction.

Conversely, soy isoflavones may provide potential benefits by acting as antioxidants. Asian communities that consume higher amounts of soy have a lower rate of heart disease than communities that consume less soy (i.e., the U.S.).

It has been theorized that the antioxidant power of soy isoflavones may be contributing to this rate. If soy isoflavones can accumulate in fish muscle, the potential for this benefit transferring to human health exists. This antioxidant accumulation may also help extend the shelf life of seafood that was reared on a high-soy diet.

SOY-IN-AQUACULTURE
In 2002, the United Soybean Board (USB) and seven American universities, led by Purdue University, initiated the Soy-in-Aquaculture Managed Research Program (SIA). The goal of the USB was quite simple: to increase the amount of soybeans used in diets fed to aquatic organisms.

To accomplish this, they initially wanted to target well-established intensive aquaculture industries that were underutilizing soybeans in their diets. The salmonid industry was an obvious choice, but in order to achieve their goal, they recognized certain barriers needed to be overcome.

The focus of the first two-year SIA program was to systematically evaluate the anti-nutritional and estrogenic effects of soy and determine which ones, if any, were limiting the use of soy in salmonid diets.

SIA researchers found that certain soy compounds exert anti-nutritional effects on salmonids, while others do not. Lectins and trypsin inhibitors were identified as being of possible concern.

However, saponins and soy isoflavones were found to have potential benefits by increasing immune response in fish and increasing shelf life, respectively. SIA researchers also evaluated the effects of different processing conditions during extrusion of aquafeeds on production of rainbow trout.

Sensory tests were conducted to determine if feeding high soybean meal diets impacted the quality of fillets. Untrained panelists were asked to detect differences in color and taste of fillets from fish fed high soybean meal diets and fishmeal control.

The panelists were able to detect differences in color (fish fed soybean meal had whiter fillets) but were not able to detect differences in flavor.

Finally, an economic analysis was conducted to determine the level of soybean meal inclusion that would provide the greatest financial benefit. Under current nutritional and economical conditions, the optimal amount of soybean meal inclusion was determined to be 25% for rainbow trout and 15% for Atlantic salmon.

Currently, SIA projects are focusing on developing high-soy practical salmonid diets and comparing them to traditional fishmeal controls. Also, a marine shrimp component has been added that is far more extensive in scope than the initial salmonid project. This project will consist of incorporating high levels of soybean meal into shrimp feeds and measuring the effects on production.

THE FUTURE
Soy-based research will continue to be a key component in the development of aquaculture nutrition. Future projects will continue to evaluate the effects of high-soy diets on crustaceans and fish. Additional work needs to be conducted on eliminating anti-nutritional factors through traditional breeding of soybeans, transgenic modifications and optimized processing and extrusion techniques.

Steven Hart is the administrative coordinator for the Soy-in-Aquaculture Research Program and Dr. Paul Brown is the director of the Soyin-Aquaculture Research Program, Professor of Fisheries and Aquatic Sciences, Department of Forestry and Natural Resources, Purdue University, West Lafayette, Indiana, U.S. For more information, contact them at: pb@purdue.edu or sdhart@purdue.edu.

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