by Cláudia Figueiredo-Silva, Zinpro Corporation, USA,
& Stavros Chatzifotis HCMR, Gournes Heraklion, Greece


When moving towards precise and sustainable aquaculture nutrition, a re-evaluation of the nutritional needs of the fish is always required. By conducting a comparison of the trace mineral (TM) composition of alternative protein sources to fish meal, the researchers expose significant limitations amongst zinc (Zn), Selenium (Se), Iron (Fe) and other nutrients.

In addition to a lower content in TM, antinutritional factors such as phytic acid found in many plant meals that are currently used to replace fish meal (FM), are not digested by fish and have negative effects on availability of minerals (NRC, 2011), making TM needs more difficult to be met. In this respect, stability and the way different supplemental TM are absorbed affects their availability and ultimately, animal performance.

Metal-AA complexes (a mixture of a single specific metal complexed with different AA in a 1:1 ratio) are taken up by AA-transporters instead of common metal ion transporters, reducing the risk for transport saturation and improving absorption efficiency.

Another advantage of using metal-AA complexes, instead of metals in their inorganic form, is that they are more stable and minimally antagonised by other dietary ingredients like phytic acid.

Previous research conducted by Paripatananont and Lovell (1995) showed that zinc methionine complex was three-to-five times more bioavailable than inorganic zinc (ZnSO4), in meeting the growth requirements of channel catfish that were fed purified and practical diets containing phytic acid, respectively.

Metal AA-complexes supplemented at half the rate of inorganic sources proved to maintain or even improve growth performance of European sea bass (Dicentrarchus labrax) and Atlantic salmon (Salmo salar), respectively.

Moreover, partial or complete replacement at 0.5 times the rate of inorganic minerals with metal-AA complexes, reduced skin lesions in Atlantic salmon after infestation with Caligus and increased number of goblet cells in intestine and skin of European seabass. This indicates the existence of an enhanced barrier defense mechanisms against pathogens.

In the later study, conducted in collaboration with HCMR, different TM premixes sources and levels in 20 percent FM diets were evaluated, which reflected FM inclusion levels practiced in commercial diets.

In a follow up study at HCMR and co-funded by the AquaExcel EU programme, the researchers evaluated how, by adjusting inclusion of a complete metal-AA complex, premix contributes to a cost-effective reduction of FM from 20 to 10 percent in European sea bass diets.

Three diets with varying fishmeal levels

Quadruplicated groups of European sea bass with an initial body weight of 47g (35 fish per tank) were fed to apparent satiety for 12 weeks, one of three diets formulated to vary in their FM level (20 or 10 percent) and adjusted for their trace mineral content.

A control diet had FM at 20 percent (FM20) and was supplemented with 50 ppm Zn as Availa® Zn, 40 ppm Fe as Availa® Fe, 12 ppm Mn as Availa® Mn, 3 ppm Cu as Availa® Cu and 0.12 ppm Se as Availa® Se, (Zinpro Performance Minerals, Availa®Mins line).

Two additional diets were formulated to reduce FM level in control diet by 50 percent (10 percent FM) and supplemented with same premix at 1.5 x (diet FM10, 1.5x) or 2x (diet FM10, 2x) that used in the control diet.

The analysed TM composition of FM20 and FM10 diets is shown in Table 1. Apart from FM, main protein sources used were soybean protein concentrate (10 vs 13.5%), soybean meal (10 vs 13.5%), wheat gluten (8 vs 9.2%), rapeseed meal (7.5 vs 8.63%), corn gluten (7 vs 8.05%) and haemoglobin (5 vs 5.75%); values within brackets are given for FM20 vs FM10 diets.

Maintaining growth performance

By adjusting mineral premix in FM10 at 1.5 or 2x the level used in FM20, the researchers manage to maintain growth performance and even slightly increase whole body Zn content of European sea bass, although not statistically significant (Figure 1). Yet, specific growth rate (SGR) and feed conversion ratio (FCR) were superior when TM premix was doubled, compared with one-and-a-half-times that in FM20.

Results indicate that adjusting the dietary TM content of FM10 to similar levels of that in FM20 may not be enough to keep performance of European seabass. Increase of the TM inclusion level from one-and-a-half-times to double that used in FM20 improved SGR and FCR by three and five percent respectively, back to levels closer to that seen with FM20. This may be at least partly explained by the likely lower nutrient availability in FM10 as a result of its higher plant protein content and antinutritional factors compared to FM20 diet.

Recent study shows that performance and health of European sea bass fed 10 percent FM or FMK-based diets could be kept similar but required tested inorganic TM premix to be increased by 260–300 percent, corresponding to dietary levels of approximately 200–285 ppm of Zn, 260–320 ppm of Fe, 70–90 ppm of Mn and 0.8–1.0 ppm of Se.

Although results are promising as they confirm the possibility to reduce FM significantly in sea bass diets without negatively impacting performance, required Zn levels would surpass EU upper allowed levels in sea bass diets.

In this study, the researchers showed that supplementation with metal-AA complexes contributes towards a cost-effective reduction of FM from 20 to 10 percent, while respecting EU upper limits for TM supplementation and TM content in sea bass feeds.

The exception to this was that dietary Se content in the feed that in this study, like in Henry`s study could not be kept at or below 0.5 ppm. Ingredient contribution to dietary Se content in aquafeeds, mainly contribution of FM and other marine ingredients, makes practically impossible to keep Se levels in diets within EU allowed levels.

Overall, the findings of this study indicated that metal-AA complexes allowed FM to be reduced from 20 to 10 percent without significantly affecting growth or FCR while respecting EU upper limits for TM in sea bass diets.

Moreover, this strategy resulted in 8.5 percent savings with feed cost turning out in a more sustainable a cost-efficient solution to the industry.

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