On a world-wide scale it is estimated that fed aquaculture supplies 125 MMT or 70 % of the total aquatic production. It is expected that this production, as well as the proportion of total aquaculture production, will increase with approximately 2 % per year until 2030 (OECD/FAO, 2021). The associated annual production of commercial aqua feed is estimated at more than 51 MMT and fish meal production is estimated at 5.4 MMT (OECD/FAO, 2021). The supply of fishmeal - an ingredient considered essential in many aqua diets - is limited, leading to high prices of the fish meal and consequently the feed produced with it. Alternative sources of protein (amino acids) have been considered and tried in a large range of diets. These include animal co-products and plant proteins; principal among the latter is soy since it is the main plant protein being produced and its nutritive value in terms of amino acid profile is most advantageous for most aquatic species.
On a global scale, production of soybeans has increased steadily for more than 100 years reaching close to 370 MMT/yr. (OECD-FAO, 2021). Soybeans are crushed and converted in many ways leading to many different types of soy products that can play an important role in replacing part - or all - of the fish meal in aqua diets.
The main soy co-product is soybean meal (SBM) with an estimated production of more than 265 MMT globally. SBM has been – and is - extensively used in fish feed. In omnivorous fish, accounting for more than 70 % of the global aquaculture volume (Nailor et al., 2021), plant proteins - most notably SBM – have largely replaced fish or animal proteins. In certain species such as carp, catfish and tilapia, SBM may represent as much as 60 % of the total formula (Gatlin, 2013) thus covering almost all of the protein requirements. Soy products have also successfully been included in diets of carnivorous fish where they have been able to replace variable proportions of fish and animal proteins. This includes specific diets for trout as well as various marine fish diets (Powell et al., 2016; Kotzamanis et al., 2020; Velez-Calabria et al., 2021.
Such relatively high levels of SBM are possible, primarily due to SBM’s relatively high levels of crude protein and amino acids but also because of its high protein (amino acid) digestibility and its adequate palatability. Among all plant proteins, soy products are recognized for having one of the most balanced amino acid profiles, although their content in sulfur-containing amino acids methionine and cystine may be limiting compared to the established, quantitative amino acid requirements of most fish species (NRC, 1993). Variable amino acid supplementation of soy-containing diets is needed and high-level replacement of FM by SBM (up to 90 %) has been successfully demonstrated in trout diets supplemented with valine, lysine, methionine and taurine without apparent decrease in growth or intestinal health status. (Velez-Calabria et al., 2021). Similar responses with additional improvements in carcass quality have been obtained with supplemental taurine in specific marine species (Kotzamanis et al., 2020). As a matter of fact, in many species the replacement of fish or animal proteins by soy proteins is hampered by an inadequate understanding of the digestible amino acid requirements for these species and age groups within species.
In the case of properly crushed SBM, amino acid digestibility is generally high although considerably differences among species have been shown. In fish and shrimp the digestibility of the soy protein fraction has been found to be high and generally close to 90 %, (e.g. Gatlin, 2013; Brezas and Hardy 2020; Kumar et al., 2020; Galkanda‐Arachchige et al., 2020). Much of the variation in digestibility of SBM is related to quality and especially the degree of heat treatment.
Following a correction for the specific amino acids, higher levels of inclusion of soy products in fish and shrimp diets are limited by the presence and possible effects of the residual anti-nutritional factors (ANF). This is especially true for fry and juvenile fish which are exceptionally sensitive to ANFs. These components – present to variable degrees in soy product – have been associated with intestinal inflammations (enteritis) in commercial fish species leading to reduced absorptive capacity, increased mucus secretion, hyperpermeability, and leucocyte infiltration. Directly or indirectly, they have also been related to pro-inflammatory cytokine genes expression and changes in the composition and population of gut microbiota through the supply of nutrients that preferentially support or depress the presence of specific gut bacteria (Kumar et al., 2020).
Partial removal of the heat labile ANF is achieved through toasting or expansion/extrusion. Heat resistant ANFs require alternative methods of treatment such as extraction (as in the case of SPC or ISP). An alternative, efficient and economical method for ANF reduction has proven to be the fermentation of SBM; alone or in combination with enzymatic extraction (Mukherjee et al., 2016; Zhang et al., 2021). These methods, most notably the solid-state fermentation of SBM (FSBM), are rapidly gaining interest allowing the inclusion – or increase in inclusion – of soy in aqua diets. This is especially important for carnivorous fish and shrimp where inclusion of FSBM allowed successful replacement of fish meal and improved performance. Production of FSBM may vary considerably by type of inoculum used and conditions of fermentation. However, these differences in production have not shown consistent, demonstratable differences in performance.
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