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January | February 2012

Feature title: Redefining mineral requirements: Why it’s necessary?

The International magazine for the aquaculture feed industry

The increasing development of aquafeed technologies embraces a new generation of feed ingredients and additives leading to changes in

the specification of diet formulations.

This necessitates a new understanding of mineral nutrition and the need to redefine trace element requirements in keeping with intensive production whilst promotion of fish health.

This short review gives a basic outline of the biological mechanisms involved in one of these trace elements, zinc and describes why it’s important to re-evaluate the mineral require-ments for salmonids.

Why is zinc important? ‘Micro-nutrients’ is a generic term for die-

tary components required in small quantities. Minerals such as copper, zinc, iron, manganese and selenium are all micronutrients although they are usually categorised as trace minerals, and are essential for the health of all animals, including fish.

In aquaculture these dietary essentials are often supplemented as part of a vitamin/mineral premix due to the inadequate supply obtained from many commercially used feed ingredients.

Zinc is the most abundant trace mineral found in fish. It is essential for growth, the development and maintenance of healthy bones, and over 300 proteins require zinc as either a structural of functional co-factor. These include approximately 20 metalloenzymes such as alkaline phosphatase (required for bone min-eralisation/formation), alcohol dehydrogenase (required for fructose metabolism) and carbonic anhydrase (required to aid the removal of CO2 from cell respiration).

Fish deficient of zinc shows growth retardation, cataracts, fin and skin erosion, increased mortality rates and taste dysfunc-tion resulting in reduced appetite and feed conversion.

How does the fish obtain zinc?Fish have two routes of zinc uptake, first from

the diet and second from the surrounding water. There is the potential for waterborne zinc to be absorbed in both the gut, from the swallowed external water, and also directly from the exter-nal aqueous environment via the gills.

Salmonids “drink” very little, especially when in freshwater; freshwater zinc levels are usu-ally less than 10µg/l and saltwater levels even lower. This is considered too low to make any significant contribution to the whole body zinc levels even though the gills affinity for zinc is extremely high.

However, even with this high affinity the uptake rate of zinc from the gill is three to four times lower than from the gut (Bury et al, 2003). The uptake mechanism in fish is described as high affinity low capacity in the gills and low affinity but high capacity in the gut.

This supports the theory that despite the high affinity for zinc in the gill, dietary uptake is the major contributor to the body zinc status.

Free zinc ions (i.e. not bound to other com-pounds) are potentially very toxic to many bio-logical processes, yet the incorporation of these free zinc ions in numerous proteins is vital for these very same biological processes to function.

Thankfully, from a toxicological stance, only a very small fraction of the total zinc in the environment is in this “free” state.

Unfortunately, from a nutritional stance, the majority of zinc in the environment is therefore unavailable. For the zinc to become available these compounds need processing in some way.

This processing occurs when the compound is digested, freeing the potentially toxic zinc ion, which can now cross the intestinal barrier; or breaking the large compounds down into their smaller components, which can cross the intestine and take the zinc with it. Once inside the organism any free zinc is usually bound to another compound generally termed a chap-erone, ready to be used or transferred around the body.

How and where is zinc used?Zinc is very highly regulated in all aspects of

the fish’s body: its uptake from the water or the diet; its excretion by the gills, the intestine, the urine or the integument; and also by its distribu-tion within the body.

This regulation means that even a dietary level of 1700mg/kg ZnSO4 is still non-toxic to the fish. The ability to regulate this appears to come from the intestine, it is thought that excess zinc is simply not absorbed and passes through the fish in its faeces, however, it hasn’t been proved that the high levels of zinc remaining in the faeces hasn’t been processed by the liver and excreted back into the faeces in the bile.

Either way, excess zinc in the diet does not seem to present a problem. Low dietary zinc levels are however more serious and the regula-tory mechanism more complex.

Every tissue of the fish can be broadly grouped into one of two categories; either functional or exchangeable. A functional zinc pool, such as the liver, fins, eyes, gills and skin are generally considered metabolically important. It is these tissues, which maintain their zinc concentration regardless of the dietary levels. Exchangeable pools seem to be less important metabolically but it is in these tissues (bone, muscle, intestine) we see fluctuations in zinc levels corresponding to the dietary levels.

When the dietary supply exceeds require-ment these tissues increase in zinc concentration and act as a storage facility and when the diet is deficient it is these pools that decrease quickly and allow the metabolically important tissues to maintain their zinc levels. Regardless of the ability of the fish to regulate zinc within its body the turnover of zinc is relatively fast (~1% per day). This means that in order to avoid deficiency a continual supply of dietary zinc is essential (Davies et al., 2010)

Dietary zinc requirementsResearch into mineral requirements, espe-

cially trace minerals such as zinc is well defined

Redefining mineral requirements:

by Dan Leeming PhD Research Student, Aquaculture and Fish Nutrition Research Group, University of Plymouth, Uk

Redefining mineral requirements:

by Dan Leeming PhD Research Student, Aquaculture and Fish Nutrition Research Group, University of Plymouth, Uk

Why is it necessary?

28 | InternatIonal AquAFeed | January-February 2012

FEATURE

January-February 2012 | InternatIonal AquAFeed | 29

IAF12.01.indd 28 10/01/2012 15:57

for many higher organisms, but for fish only the commercially valuable species have received sig-nificant attention. Numerous studies have been carried out on rainbow trout, Atlantic salmon and Channel catfish; these species have well defined and frequently cited requirement levels.

These requirement levels tend to be cal-culated using purified (non-realistic) diets and inorganic forms of the minerals. In reality, aqua-culture diets contain anti-nutritional factors (ANFs); these are components of the feed that inhibit the uptake or utilization of another part of the feed. When concerned with mineral digest-ibility and availability two of the main ANFs are tricalcium phosphate and phytate (phytic acid). Tricalcium phosphate is found in the bone tissue of animals and phytate in many plant proteins.

These ANF’s bind to minerals such as zinc and effectively render them unavailable to the fish. An example of the effect of these ANFs can be seen in rainbow trout. Rainbow trout have a requirement of 15-30mg Zn/kg diet (Ogino and Yang, 1987). This was calculated using a purified egg albumin diet, with no ANFs, and using an inorganic zinc sulphate. When a practical diet containing fishmeal was used an additional 40mg Zn/kg diet (probably bringing the total dietary zinc level closer to 80-100mg Zn/kg diet) was required to maintain normal growth.

Similarly, Atlantic salmon fed a fishmeal diet containing 65mg Zn/kg could not maintain their normal zinc status (Lorentzen and Maage, 1999).

The higher the bone content of the fishmeal the more zinc needs to be added, espe-cially when using an inorganic zinc salt. The replacement of fishmeal with plant protein may exacerbate this effect. Rainbow trout fed a soyabean meal based diet required 150mg Zn/kg to achieve optimal growth. The increased use of sustainable fishmeals, often from trim-mings high in bone content, and plant protein sources high in phytate may mean that a set of minimum requirement levels for fish fed more realistic diets will be of more practical use to the industry.

The development of more advanced feed supplements such as proteinate sources of min-erals may reduce the effect of ANFs on mineral availability. Mineral proteinates bind the mineral within their structure, ‘protecting’ the mineral from the ANFs.

This relationship between the protein and the mineral is complex. The mineral has to be bound tight enough not to be released in the gut where it would be a free mineral ion, susceptible to the ANFs, just like an inorganic salt, but the mineral still needs to be available to the animal once it has been taken into the cells.

If the correct protein/mineral complex is used the level of the mineral used in the diet can be reduced by as much as 70 percent (Paripatananont and Lovell, 1995; channel catfish with zinc methionine).

If research into the type of protein/mineral

complex is carried out for each species the effi-ciency of mineral supplementation can be hugely improved. It would also reduce the problems (lower availability and excessive mineral excretion) associated with higher levels of mineral inclusion, which is required when using more sustainable animal and plant based protein sources. ■

References:

Bury, NR, Walker, PA, Glover, CN, 2003. Nutritive metal uptake in teleost fish. J Exp Biol. 206, 11-23.

Davies, SJ, Rider, S, Lundebye, A-K, 2010. Selenium and zinc nutrition of farmed fish: new perspective in feed formulation to optimise health and production. In: Bury, NR, and Handy, RD (Eds) surface chemistry, bioavalability and metal homeostasis aquatic organisms: An integrated approach. SEB, London, pp. 159-181.

Lorentzen, M, Maage, A, 1999. Trace element status of juvenile Atlantic salmon Salmo salar L fed a fish-meal based diet with or without supplementation of zinc, iron, manganese and copper from first feeding. Aquac. Nutr. 5, 163-171.

Ogino, C, Yang, G.Y, 1978. Mineral requirements in fish.4. Requirement of rainbow-trout for dietary zinc. Bulletin of the Japanese Society of Scientific Fisheries. 44, 1015-1018.

Paripatananont, T, Lovell, RT, 1995. Responses of Channel Catfish Fed Organic and Inorganic Sources of Zinc to Edwardsiella ictaluri Challenge. Journal of Aquatic Animal Health. 7, 147-154.-

28 | InternatIonal AquAFeed | January-February 2012 January-February 2012 | InternatIonal AquAFeed | 29

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Volume 15 I s sue 1 2 012

the international magazine for the aquaculture feed industry

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On-farm feed management practices– for three Indian major carp species in Andhra Pradesh, India

Oxygenation in aquaculture

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