Wild cod don’t have to worry about the composition of their prey. With every herring, capelin or smaller fish that they swallow they are taking in a good share of protein packed full with vitamins and minerals, plus a quantum of fat containing polyunsaturated fatty acids , some of them of the Omega 3 type. And as if that weren’t enough, the organisms they feed on also contain carbohydrates, enzymes, hormones and other substances because every living organism is a veritable cocktail of various chemical substances. So as long as a cod manages to get hold of a fish regularly enough it will be getting a well-balanced diet and lack nothing.
The situation for fishes that live in aquaculture facilities is completely different. However much trouble farmers go to to offer their fishes acceptable “real-life” farm conditions this is not possible where feed is concerned. The ponds, net cages, raceways and tanks in which the fishes are kept are made of plastic or concrete and as such are artificial habitats that offer much too little natural feed – in fact, usually none at all. This means that feed has to be introduced into the system from the outside… and there has to be enough of it, too, because the intention is not only that the fish will survive but that they will grow quickly and in the desired quality. So they have to get everything they need to stay healthy. During the early years of aquaculture farmers often fed their fishes on cheap slaughter waste or chopped up feed fishes. But this wet feed did not prove very useful because it was difficult to store and dose and also polluted the water in no small measure.
During the search for alternatives the idea arose that it might make sense to dry the feed fish, grind them and form the resulting meal into small dry pellets in the shape of pills. This could then be fed to the fishes. And that was the birth of the efficient feeds that were to give a decisive boost to the development of modern aquaculture. But there was still a long way to go to reach this point. Dry feed, too, proved to have some serious drawbacks at first. It had to be heated in order to dry it and during this heating process, for example, some key ingredients were destroyed so that the fishmeal did not have the same biological value as that of the live fish. Some of the substances that fishmeal contains are not suited to storage and evaporate or decompose over the course of time. Others alter during the mixing and extruding process. Yet another problem at that time was caused by the relatively hard consistencies and the different flavour of the new feed which of course had a different feel in the fishes’ mouths and also tasted different from their usual soft feed.
Additives should complement, correct and improve
In order to put an end to these and other inadequacies more and more substances were added to the feed to adapt it as closely as possible to the fishes’ needs. In countless experiments scientists searched for suitable substances that would solve a particular problem of the feed, and numerous new formulae were tested and optimised. This was no easy task because the feed had to contain all the nutrients and other components in the exact quantities that the fish needed for healthy development and growth. Added to this is the fact that different fish species make different demands on their feed and that juveniles have to be fed different feed from adult fishes. Since the share of fishmeal and fish oil in feed has been constantly reduced through substitution by vegetable alternatives from agriculture, the feed formulae have become much more complicated and demanding with regard to nutritional and physiological aspects. Fishmeal contains high-quality fish protein in a highly digestible form which suits the needs of fishes in aquaculture perfectly. Vegetable proteins, on the other hand, often lack certain amino acids, which then have to be added to the feed later on. Some plants can even slow down digestive and metabolic processes (anti-nutritional factors) or they can also be toxic.
Every additive that is mixed into the feed – however tiny the quantity – thus has a specific function. Some of them counterbalance deficits of important substances or enhance nutritional value and performance Others “complete” the feed, improving its digestibility and flavour, it ability to float or its stability in water. Additives mainly serve the following purposes:
• Improvement of performance by increasing feed efficiency (optimisation of the FCR, better digestibility and promotion of growth, reduction of feed costs, reduction or environmental pollution)
• Health and wellbeing of the fish: support and improvement of fish health (vitamins, immunostimulants, means of protection against parasites, probiotics)
• Preservation and stabilisation: these additives are intended to prevent spoilage of sensitive or instable ingredients in the feed
• Food safety of the fish produced in aquaculture: these additives prevent microorganisms or toxins getting into the fish via the feed that could constitute a risk to the consumers who eat them
• Support of aquaculture production: improvement of the physical and chemical properties of feed, such as its durability in the water, its ability to float, its smell, colouring or flavour (feed attractants should, for example, encourage the fish to feed)
A lot of additives are based on their natural models
It’s not easy to find one’s way through the large number of additives. The choice ranges from simple minerals which are added to the feed as micronutrients, through vitamins, enzymes and pH-reducing acidifiers, to pigments and complex premixes in which several additives are mixed together in specific combinations so that they complement and intensify their effects. Some additives are already added to the feeds during their production by the manufacturer, others are mixed by the fish farmer himself directly prior to feeding. Additives produce effects in relatively small quantities and so their share in the feed usually constitutes only fractions of a per cent or is even measured in thousandths. Additives can be inorganic by nature (e.g. minerals) or they can be of organic origin. Some are gained from natural raw materials such as bacteria, yeasts or plants, others through purely chemical processes, i.e. they are produced synthetically. The latter group also includes “nature identical” additives that are produced in test tubes but whose molecular structure is absolutely identical to that of their natural models.
A well-known example of a nature identical additive is astaxanthin, a pigment that is nearly always produced synthetically for aquacultural purposes and which (together with other carotenoids) gives salmon flesh its red colouring. Astaxanthin is, however, not only a “colorant” as some people still think but has other important biological functions beyond that. For example, it has an anti-oxidative effect, it encourages the function and health of the nervous system, and it improves growth and fertility in salmon. Wild salmon living in their natural environment absorb carotenoids when they eat shrimps. Because this is hardly possible in aquaculture, however, an alternative had to be found – and was found in the synthetic, but nature identical, astaxanthin. How close the “copy” is to the natural substance can be seen in the fact that it fulfils all the biological functions within the organism exactly as the natural pigment does.
Vitamins are among the most important feed additives
A reason for using additives can also be found in production technology. In the past, steam was used during the feed production process. The resulting pellets were compact and once in the water they sank quickly. With the modern extrusion techniques which are today state-of-the-art, however, the feed expands as soon as it leaves the extruder so that the specific weight of the pellets decreases and they thus sink more slowly or are even suspended in the water. This gives the fishes more time to pick up the feed particles. Two further advantages of extrusion are the higher digestibility of the feed produced in this way and a longer durability span of the pellets in the water. Both these benefits are the result of thermal treatment of the feed raw material because through heating the nutrients are better solubilised and as from a temperature of about 70°C the contained starch agglutinates the pellets, which also has a stabilising effect. However, heating also contributes towards the destruction of vitamins. Because some vitamins also react sensitively to light and oxygen and thus cannot be stored for long the feed producers had to come up with an idea to get around this problem, too, because a feed that lacks vitamins or certain vitamins would not be of any use to aquaculture. Rainbow trout, for example, needs at least a dozen different vitamins to grow normally and remain healthy.
Today, ready-made “premixes”, which contain all the necessary vitamins in a well-balanced mixture are usually added to the feed in the correct dosage. They are also of course available separately in case only one particular vitamin is lacking in a feed. In both cases additives have the advantage that before mixing in the vitamins they can be made less sensitive to light, oxygen, moisture and other external influences by special pre-treatment (mostly coating). Without this, the vitamins in the additives would decompose if the pellets were not fed to the fishes immediately after their production.
Environmental impacts can change additives
All feeds lose quality over the course of time. Their quality is at its best directly after their production because, from then on, creeping changes occur that gradually become more and more noticeable. Firstly it is only digestibility and nutritional value that change but after a time the feed can become completely unusable. How long that takes depends on the storage conditions. In the tropics, where it is always hot with high humidity and intensive sunlight, feed will often spoil within two to three weeks. The protein-rich pellets are also a welcome substrate for all kinds of germs, particularly for fungi. The fat in the pellets oxidises and makes the feed rancid. The small pellet sizes are particularly susceptible to rancidity because their surface is relatively large compared to their volume and this furthers oxidation. Feed producers try to prevent these processes by adding stabilising additives but ultimately decomposition processes cannot be halted but only delayed.
In the meantime a whole industry has developed that is specialised in feed additives and is constantly searching for new, rewarding additives. In complicated experiments attempts are undertaken to find out which substances are particularly effective and what benefits they offer. Dimethylthetin (DMT), for example, which belongs to the additive group of feed attractants, is said to offer a whole bunch of positive effects. This white powder is mainly intended to encourage the fishes to eat and to ensure that they eat more often and thus absorb more nutrients. At the same time, however, it increases their desire to swim and has a stress-reducing effect. DMT stimulates the release of the hormone ecdyson in crustaceans. This hormone is largely responsible for controlling moulting. The crystalline substance TMAO (trimethylamine-N-oxide dihydrate), another feed attractant is, beyond this, also said to directly stimulate growth. It accelerates the cell division rate in the muscle tissue and thus contributes towards faster growth of the fillet. Apart from that, TMAO also reduces fat deposits in the cells, stabilises protein structures and has a positive influence on the organism’s osmotic stability.
Feed additives can improve a fish’s health status
An application area of additives that is of economic significance to aquaculture is the improvement of certain of the fishes’ performance parameters, particularly their growth and survival rate. In the past, antibiotics often used to be added to feed in aquaculture, too, because they have these very effects, i.e. they accelerate growth and encourage good health. Due to growing concerns among the public this routine use of antibiotics in fish feed is illegal almost throughout the world. At the same rate as antibiotics disappeared from feed scientists began looking for alternative substances that would have similar effects but would pose less of a risk. The substances they came up with included derivatives of organic acids such as potassium diformate, a crystalline salt of formic acid. If added to the feed in low dosage potassium diformate improves, for example, feed utilisation in salmon and encourages growth in tilapia.
A fast growing group are “medical” additives that are used to prevent diseases or are used as disease therapy. Here, too, the range extends from relatively simple substances with broad-spectrum effects which are mostly organically based and thus have a high natural availability to complicated and very complex mixtures that have very specific effects. Examples of simple additives with broad-spectrum effects are L- and DL-selenomethionine. The whitish powder is said to “optimise” the fishes’ immune systems, improve the body tissue’s anti-oxidative power and prevent neoplasm and cancer. The selenium additive apparently also improves the mobility of the sperm cells in spawners. In contrast, a premix of ciprofloxacine and berberine which is used specifically to fight bacterial infections in eel has a highly specific effect.
Additives can also include certain vaccines if they are administered orally with the feed. Vaccines – which are mostly either part of a pathogen or a weakened pathogen – stimulate the protective system of the body to build up an immune defence so that it creates antibodies. The immune system “stores” this reaction and can call it up quickly if this type of germ threatens the organism in reality. Vaccine additives are not to be found in every pellet, however, because they are only mixed into the fish feed for the short duration of immunisation. To prevent the vaccines from being washed out of the feed or modified and being made inactive by digestive enzymes in the intestine they are protected using a special technique (e.g. bioencapsulation, coating). Oral vaccination is easy and causes little stress but it cannot be used against all diseases. Apart from that it mostly only enables immunity over a limited period of time. There also exist special additives for fighting external parasites like salmon lice which can be mixed into the feed. Giving it with the feed is much less harmful to the environment than the immersion therapies which are also used but does not work so quickly and entails a certain toxic risk for the fishes. Among the particularly effective and therefore frequently used substances in feed for delousing fishes are emamectin benzoate and some avermectines.
Trend towards natural substances
As is the case in a lot of areas of life there has been an unmistakable trend for some years now in the direction of using natural substances in the feed additive segment, too. Scientists are trying to replace synthetically produced additives with substances that are isolated from yeast cells, for example, or taken from certain plants. Natural additives often have similar if not identical effects to those of purely chemical products which makes careful testing essential prior to usage. Experiments to compare natural and synthetic antioxidants that were carried out by Fiskeriforskning in Norway showed, however, that natural additives are almost as effective as chemical products and might possibly be able to replace controversial synthetic additives such as ethoxyquine (EMQ).
“Probiotic” effects that are mainly based on lactic acid bacteria have recently become increasingly popular in aquaculture. They are said to have a biological balancing and correcting effect. At the moment probiotic bacteria are mainly only used in aquaculture for improving water quality (they are said to “balance” the bacteria population in the water and suppress the development of pathogen species) but they are also already available as additives that are mixed into the feed to strengthen the fishes’ immune systems, and to improve feed utilisation and growth. The use of probiotics in aquaculture is a relatively new and controversial concept. Alleged benefits for human, farm animal and fish nutrition are often still faced by a lack of any scientific proof of these kinds of positive effects to date. Irrespective of such disputes a lot of research teams are working all over the world on selecting even more effective probiotic bacteria strains for aquaculture.
The growing number of possible additives in fish feed makes it increasingly difficult even for experts to keep pace with developments and maintain an overview to enable appropriate evaluation of the risks connected to them. Institutes like the Danish DHI Centre for Environment and Toxicology have specialised (among other work areas) in evaluating scientific publications and carrying out their own eco-toxicological studies to enable realistic evaluation of the benefits and risks of additives. Upon the basis of their findings, it is then possible to make expert assessments, for example, or prepare applications for the approval of new additives.