**4.5 Other functional feed additives**

Supplementing PP-based aquafeeds with additives can improve the digestibility of feed's nutrients. In this context, it has been reported that supplementation of a diet contained high levels of legumes including SPC and pea protein (32%) with phosphatidylcholine pronouncedly improved lipid digestibility in Atlantic salmon [96].

On the other hand, it has been confirmed that the replacement of FM with PP sources could reduce cholesterol content in aquafeeds that may disturb bile acids synthesis in fish and result in low digestibility of lipid [97]. In this regard, it has been reported that supplementing SBM-based diets with cholesterol remarkably improved growth in channel catfish [98], turbot (*Scophthalmus maximus*) [85] and rainbow trout [99].

As mentioned earlier legumes are deficient in taurine or its precursors (*i.e.* cysteine and methionine) and some aquatic animal species especially marine fish unable, or have low ability to synthesize taurine [100]. Taurine is the main component of bile acids and increase the bile-salt dependent lipase activity in fish [101]. It has been proved that supplementation of soy protein-based aquafeeds with taurine improved growth performance, lipid metabolism, palatability, digestibility and overall nutritional quality of feeds in marine fish species such as common dentex (*Dentex dentex*) [101] and European sea bass larvae [102] and juveniles [103, 104].

Moreover, it has been confirmed that replacement of FM with plant protein sources with high levels of ANF (*i.e.* saponins, oligosaccharides, fibers and high molecular weight proteins) disturb bile metabolism in fish and may adversely affect fish productivity [105]. Bile acids as an emulsifier enhance digestion and absorption of lipid and lipid soluble nutrients through emulsification of lipids and activation of bile salt dependent lipase [105]. It also facilitates the excretion of cholesterol and toxic metabolites. The ANF in PP sources may induce gut inflammation that reduce resorption of bile acids or they may bind with bile salts and trigger extra excretion of bile acids into gut [105]. Thus, supplementing legume protein-based diet with bile acids may improve their efficiency and alleviate their negative effects on fish performance. For example, supplementing SBM-based diet with 1.5% bovine bile salts significantly improved growth rate in rainbow trout [106].

#### **4.6 Nutritional programming and selective breeding**

In recent years some studies were carried out on early nutritional programming of fish for increasing the acceptance of their offspring to new ingredients in aquafeeds. For instance, it has been reported that substitution of dietary FM and FO

#### *Legumes, Sustainable Alternative Protein Sources for Aquafeeds DOI: http://dx.doi.org/10.5772/intechopen.99778*

with vegetal feedstuffs through nutritional programming in brooders elevated the acceptance of vegetal ingredients and PP-based diets in rainbow trout [107] and gilthead seabream [108] offspring. In this regard, it has been reported that early nutritional programming in Atlantic salmon with a plant-based aquafeed enhanced growth rate and feed efficiency for 24% and 23%, respectively compared to those fed a diet contained FM and FO and then challenged with a plant-based aquafeed [109].

Recently, a new strain of rainbow trout (ARS-KO) was created by the US department of Agriculture by selective breeding over the course of two decades and this strain can grow better when fed with soy protein-based diets and does not develop enteritis [110, 111]. More research are required to be carried out in these genetic engineering to these novel techniques be advantageous and applicable at commercial stage.
