by the Alltech Mineral Management Technical Team, USA

 

Aquaculture is one of the fastest-growing food-producing sectors in the world. Nutrition, specifically trace minerals, is just one of several key aspects that must be considered in successful, profitable and sustainable farm management. Trace minerals are absorbed from the marine environment through gills or the body surface, but they seldom meet the total requirements for farmed aquatic species and must be provided in the diet through supplementation (Katya et al., 2017).

Trace minerals, which are required in milligram or microgram amounts, are essential elements required for normal life processes and cellular metabolism. They form components of body fluids, hormones and biological compounds such as haemoglobin. Of particular importance are copper (Cu), iron (Fe), manganese (Mn), zinc (Zn) and selenium (Se), which are associated with proteins within metalloenzymes responsible for catalytic function (Lall, 1989). Supplementing the right amount of trace minerals ultimately contributes to the maintenance of health, fertility, hatchability and immunity, as well as performance parameters, including growth rate, feed efficiency and flesh quality. 

Trace mineral requirements in aquaculture: What needs to be explored?

Although a delicate balance of trace mineral nutrition is required to maintain physiological processes (Lall, 1989), the exact trace mineral requirements for different aquaculture species are still being explored and are hence a subject of much debate. Excess mineral intake, either dietary or environmentally, can cause toxicity, while a deficiency can compromise immunity, therefore increasing susceptibility to disease.

Lall (2003) recommends the following ranges of dietary trace minerals in fish.

The perspective from the National Research Council (NRC) is that there is much to be explored on the knowledge of trace mineral requirements for farmed fish and shrimp. Their recommendations for dietary trace minerals in all livestock species have yet to differentiate between trace mineral sources and forms. This can influence dietary inclusion rates, as well as physiological factors like mineral bioavailability and absorption and interaction with other dietary feed components.

 

Form and level affect function: Feed ingredient sources and mineral interactions

An increase in the demand for aquafeed and limitations on the availability of fish meal has directed the industry to either reduce the amount of fish meal in diets or explore other alternatives, such as plant meal (Domínguez et al., 2020). This can present a range of nutritional challenges.

Plant meal often contains high amounts of phytic acid, which is a known antagonist. Phytic acid binds strongly to mineral ions, making them unavailable for absorption by aquatic species (Prabhu et al., 2016; Domínguez et al., 2020). In diets containing high levels of plant protein, mineral supplementation is necessary to improve growth and bone development, especially in carnivorous salmonid and marine fish.

Trace mineral composition and bioavailability can also differ markedly among aquatic feed ingredients and complete feeds, which can result in exceptionally high trace mineral levels. Excess dietary minerals can affect the absorption and bioavailability of other trace minerals within digesta. For example, excess dietary phosphorus can interfere with the absorption of zinc (Lall 2003).

One way that nutritionists are counteracting excess mineral levels and antagonistic interactions is through the incorporation of mineral programmes that feature organic sources of minerals, as these have shown to be more bioavailable and highly stable.

Absorption and bioavailability between organic and inorganic trace mineral forms

Different sources of trace elements assume different molecular forms and ligands, and subsequent compounds formed in the gastrointestinal tract of the aquatic species may prevent mineral absorption and metabolism (Watanabe et al., 1997). It is during these conditions within the gastrointestinal tract that, often, inorganic sources of trace minerals dissociate, and release charged mineral ions before reaching absorption sites in the small intestine.

These ions often form insoluble and indigestible compounds that are excreted as waste. Other formed compounds can compete with dietary components and other trace minerals for favourable binding sites on proteins responsible for absorption and metalloenzyme synthesis (Chanda et al., 2015).

There is increasing interest in the use of trace minerals chelated to organic amino acid ligands or, in the case of selenium, incorporated and enriched into microorganisms such as yeast (Prabhu et al., 2016). Chelates are produced through the reaction of mineral salts with an enzymatically prepared mixture of amino acids and peptides, resulting in a highly stable trace mineral source.

The chelated structure of these minerals, specifically Cu, Fe, Mn, and Zn, protects the elements from forming insoluble complexes within the digestive tract, facilitating transport across the intestinal mucosa (Katya et al., 2017).

Due to high levels of dissociation, a greater level of inorganic trace mineral is required to produce similar levels of growth, tissue mineralisation and enzymatic activity compared to organic sources (Katya et al., 2017). Using organic mineral sources with higher stability and bioavailability in feed can reduce the level of mineral supplementation without compromising the dietary requirements of the species (Watanabe et al., 1997).

Organic trace minerals: How mineral form influences function and productivity

Peptide-based organic mineral chelates such as Bioplex®[1] and organic selenium incorporated into yeast (Sel-Plex®[2]) have demonstrated higher bioavailability compared to their inorganic counterparts. This warrants the use of lower dietary inclusion rates via reduced wastage of unassimilated minerals. Supplemented either separately or in combination with other feed solutions, these technologies have demonstrated many immune and growth benefits in a variety of aquatic species:

Immunity, survivability and growth: In Pacific white shrimp, Reyes et al. (2018) found that Bioplex supplementation increased total haemocytes, plasma protein, phenoloxidase enzyme levels and tissue mineral concentration. Positive trends towards copper were observed in tissue, as well as improved FCR and average shrimp weight.

Sea bream supplemented with 100 mg/kg Bioplex iron achieved the highest respiratory burst activity compared to 200 mg/kg of inorganic iron sulphate over 12 weeks (Rigos et al., 2010).

Trials also showed that diets including Bioplex minerals and Sel-Plex can maintain normal heath status. Atlantic salmon smolts receiving 150 mg/kg of Bioplex zinc showed a 33.4 percent reduction in mortality compared to the control group (Gatica, 2004).

Growth and efficiency: When post-smolt Atlantic salmon were reared in conditions with suboptimal oxygen, supplementation of Bioplex zinc and Sel-Plex selenium-enriched yeast improved FCR and growth rate compared to those supplemented with inorganic trace minerals. By the end of the trial, salmon from this group also exhibited firmer filets (Kousoulaki et al., 2016).

In rainbow trout fingerlings receiving Bioplex and Sel-Plex at 66 percent of the level of the inorganic trace mineral group, FCR, weight gain, mortality and immunity (via lysozyme activity) were all improved (Staykov, 2005).

Red hybrid tilapia that were fed Aquate Defender™, a feed technology that incorporates Bioplex and Sel-Plex, showed improved growth and improved feed utilisation after being challenged with S. agalactiae and A. hydrophila bacterium (Arifin et al., 2019).

Fukada and Kitagima (2019) attributed lower levels of Bioplex and Sel-Plex to improved weight gain, FCR and final weight in yellowtail fingerlings when dietary fish meal was reduced. They concluded that these levels could reduce the amount of trace mineral excreted into the environment and contribute to more sustainable aquaculture production.

Summary

Organic trace minerals have several positive effects on the health and performance of several aquatic species, including increased disease resistance, growth and improved feed conversion (FCR). Such effects have significant benefits in aquaculture, including decreased incidence of disease, better production, increased fillet quality and increased water quality from less mineral wastage.

When substituting inorganic trace minerals for Bioplex and Sel-Plex, it can often be done at lower inclusion rates without compromising performance in fish and shrimp. Lower dietary trace minerals in an organic form can also reduce environmental mineral output, therefore contributing to sustainable aquaculture production systems. Hence, incorporation of an organic trace mineral programme can have a significant role in maximising production and profitability for fish and shrimp.

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