It is becoming increasingly popular to carry out supplementary feeding of coarse fish; and a further and more recent development is the addition of other natural or synthetic components to activate the fish’s innate immunity. This practice can be a very helpful management tool but unfortunately if carried out incorrectly by the well meaning but experienced amateur it can add to water pollution, have an anti-nutritional effect on your fish and actually cause immune system suppression.

So to make the correct management decisions it is important to have at least a basic understanding of the feeding mechanisms of fish, their dietary requirements and the associated nutritional factors including immuno-stimulants.

Before undertaking a supplementary feeding strategy the question must be asked - Why ?

If your fish are displaying normal growth rates what will be the benefit of supplementary feeding ? To answer this you perhaps need to understand if your fishery is depleted of food, this can often happen in intensively stocked stillwater fisheries but is unlikely to occur in less heavily stocked waters.

If there is sufficient natural food but the fish are not growing as well as you might like you need to ascertain exactly how well they are growing, this can be done by analysing scale age against fish length and comparing it with known growth rates. It is often the case in mixed fisheries that one species is doing well whilst another species might display relatively poor growth. In this case the simple answer might be to simply reduce the population of the species that is out competing the other species. It can be seen that like so many scenarios that fishery managers are faced with there is not an easy and obvious answer to feed or not to feed. It is recommended that professional advice is sought to help get these answers.

Lets assume that the evidence suggests that supplementary feeding is a good idea. What do we do next? Well the timing of the supplementary feeding is critical; and that timing is temperature dependant. Fish are cold blooded and as such all their bodily functions are triggered by changes in temperature, and feeding is no exception.

As temperatures gradually fall towards 10° C so does the fish’s appetite diminish and conversely as temperatures increase towards 10° C and above so their appetite increases. With this knowledge it is clear that there is very little to be gained in supplementary feeding fish in the colder winter climate as the food will remain largely uneaten, as fish only require enough nutrition to sustain their essential life functions.

So whilst there is evidence to suggest that feeding fish in the winter can be beneficial this is largely made inappropriate in the UK climate because of low water temperature. So the general advise is to begin feeding in spring as temperatures move from about 8° C towards 10° C and above.

Having decided when to feed perhaps one of the most controversial areas is deciding what to feed. There is some evidence to suggest that nutrition such as trout pellets might have an immune suppression function, this is thought to be the result of higher amounts (amounts greater than required by carp species) of dietary lipids (oils or fats) that can effect the function of cells. Whilst there is no direct evidence to confirm that trout pallets actually harm carp and other cyprinid species, it is known that the salmonid and cyprinid nutritional requirement is significantly different.

Most pelleted growing diets for trout are much higher in protein and oil than that required by cyprinid fish. There is anecdotal evidence to suggest that as well as any immune suppressing function that to much oil may have that the build up of lipids may adversely affect liver and other bodily functions. The higher levels of protein may simply amount to a waste of money and an increase in water pollution. If the carp and associated other cyprinids cannot utilise the high levels of protein it simply may remain undigested and be passed out into the water where it adds to the organic composition and may have an impact on water quality. It is possible to purchase specially formulated diets that are blended to suit the nutritional profile of carp, because they are lower in protein and lipid than trout pellets they should be less expensive however because the UK market is relatively small there is a premium to pay unless you order in large quantities. It is recommended that if you wish to supplementary feed that you feed the correct diet.

The next problem is how much to feed ? There is no correct amount, no right or wrong when it comes to quantity what is important is that all the feed is eaten. It is prudent to start slowly as temperatures rise and to check every few days in the areas where you are feeding to make sure that there is not a layer of uneaten feed forming.

Having started to supplementary feed you may wish to consider adding immuno-stimulants to the ration. This can be a very good idea but as always it is possible to get it wrong and do more harm than good. It should be remembered that the idea of boosting the fish’s immune system is to enable it to help itself to fight disease, so this is a preventative measure; once the fish are sick it is too late.

Whilst it is difficult to overdose when giving immuno-stimulants with supplementary feed in a wild fishery, it is possible to continue the dosage for to long thus decreasing the effectiveness of the procedure.

As water temperatures increase above 12° C the Cyprinid metabolism increases and they begin to eat and digest more food; the immune system begins to work more effectively, however as spring towards summer their immune system will be fully functional and it is now time to stop any feeding with immuno-stimulant additives.

The problem of what additives and how to add them is another vexing question, the range of additives which include both synthetic and biological substances perform different stimulating functions and one additive can often be more effective that another.

The addition of additives to fish feed to stimulate the immune system is becoming very fashionable, these additives are called immuno-stimulants and when used in the correct circumstances can be a very useful management tool for the fishery manager. The technology has been transferred from the commercial world of aquaculture and the keeping of valuable ornamental fish such as koi carp. Indeed it is the link with koi carp and its communalities with our coarse fish that has helped fishery managers to understand the benefits of supplementary feeding and on occasions the addition of immuno-stimulants.
Fish are cold blooded and as such their biological systems are temperature dependent. At low temperatures around 10ºC the immune system in fish such as our coarse fish species is virtually in active. The metabolism is so slow at these temperatures that there is effectively very little immune system response when the fish is challenged by a pathogen or environmental stressor.
In a very similar vane to administrating antibiotics without good reason, giving fish immuno- stimulants at the wrong time of the year, without good cause, and for prolonged periods can lead to them becoming ineffective.
Extended addition of immuno-stimulants to fish feed will lead to the fish adapting to certain of the stimulants being fed. If the course of the immuno-stimulants is continued, the body adapts and the immuno-stimulants become ineffective and thus a waste of money. So immuno-stimulants should only be fed in very controlled circumstances and for a limited time.
The immune system is the body's best defence mechanism against disease. The ancestors of our present fish fauna developed in a relatively stable environment over millions of years. In this stable environment, there was never any need to develop a highly effective system against environmental changes that cause stress in fish. Because of this, the immune system is, unfortunately, very sensitive to environmental changes.
The best method to ensure a healthy immune system is to provide a stress-free environment and quality nutrition (See Info Sheet ?). Fish, like humans have evolved to successfully cope with everyday living in their specialised environment. Therefore it is abnormal for fish to become sick to such an extent that they cannot recover by using their own inbuilt defence systems.
As a generalisation, and as a rule generalisations should be avoided, the best time to feed immuno-stimulants to your fish is during the early spring as water temperatures begin to rise above 10ºC and their metabolism starts to speed up. This is in when a fish is at it most vulnerable as some of the pathogens that can effect fish are very simple organisms by comparison and they become fully functional much quicker, many fish parasites have this ability, as do bacteria and viruses.

To enable a greater understanding of these feed additives that can have such a beneficial effect on the well being of our valuable fish a short description of the principal types is given below:-
IOFLAVONOIDS: Rooibos (Aspalathus linearis) is a leguminous shrub indigenous to the north-western Cape in South Africa. Its leaves and stems are used for the manufacture of rooibos tea that is rapidly gaining in popularity as a health beverage for humans. It contains certain flavonoid glycosides and flavones that act as natural antioxidants, i.e. scavengers of active oxygen species that adversely effect fish health.
PROBIOTICS: The fish gut’s first line of defence against disease - are health promoting bacterial supplements, which improve the intestinal microbalance by selectively suppressing the harmful bacteria in the gut of fish. They include the families of Lactobacillus and Bifidus bacteria. The role of these bacteria is to inhibit undesirable micro-organisms that may cause disease or harm to the fish. Supplementing the diet with probiotic bacteria therefore offer new dietary alternatives for the stabilisation of the intestinal microflora, especially in the following cases:
After antibiotic treatment, during conditions of stress, during abrupt diet change, also in young fish. These bacteria also produce a natural antibiotic-like substance called bactericine, which helps to eliminate unwanted pathogens.
PREBIOTICS refer to a group of natural sugars that are resistant to digestion by fish but can exclusively be utilised by specific friendly beneficial bacteria. For example prebiotics might benefit the fish’s health by selectively stimulating the growth or activity of friendly bacteria, causing them to outgrow their pathogenic competitors in the fish’s gut.
IMMUNE-STIMULANTS stimulate the macrophage immuno-force of the fish to eliminate unwanted pathogens in their bloodstream. Furthermore it also relieves allergies that may occur due to depressed functionality of the immune system. By supplementing the fish’s diet with these immune stimulants, such as natural mannans and glucans, fish may be better-prepared to meet a disease or other challenges to its well-being.