Feeding fish is one of the more pleasurable daily tasks. Kids absolutely love to help and fish quickly learn to greet the hands that feed them. Fish appear to cope with an amazing variety of feeding regimes. Many fish keepers just throw in a handful once a day and just make sure that it is all eaten with a minute or so. Many go away for a fortnights holiday and don’t feed at all over the period. Fish are nothing if not adaptable.
It is important to recognise that you are not just feeding the fish. Fish food is also indirectly the source of food for the aquaponic bacteria that are necessary for a successful biofilter. In addition, it is also the ultimate source of all the plant nutrients required in an aquaponic system. When you are feeding your fish, you are in fact feeding three separate life support systems. The plants, the filter bacteria and the fish themselves. For this reason, feed quality and quantity issues play a vital role in designing all aquatic systems.
Design Parameter for Filtration Systems
The amount of fish food given each day is, in fact, the basis that I prefer to use when calculating the size and type of filtration system, the fish stocking rates, aeration techniques, fish production capabilities and aquaponic grow bed design. It is a much more relevant figure than, for instance, the pond volume that is so often used in these type of calculations. Some of the more enlightened koi filter manufacturers now give guidelines concerning how much fish feed their filters will purify. Over the year’s scientists have come up with a range of guidelines concerning how much oxygen is required, how much ammonia is produced and how much suspended solids are produced, all expressed in terms of kg. of feed given.
Optimising fish feeding for maximising fish growth however requires very careful consideration. Maybe you are not driven to maximising fish growth and are perfectly happy to just maintain healthy fish. Most koi dealers feed at comparatively low ration sizes to minimise water pollution and maximisie the number of fish they can display in a tank. Their prime objective is to offer as big a choice of fish to the customer as possible.
There are many people however who are interested in optimising feed rates. They may be operating an aquaponic system and want to maximise the production of fish for the table. Others may enjoy buying small koi in the autumn and growing them on over the winter period. Others may simply strive to produce fish as big as possible. The following advice is primarily aimed at these fish keepers.
Fish Feed Quality
Many fish keeper have their favourite food that they have arrived at over many years of trial and error. Each food will have a fundamental influence on not only fish appetite and growth but also the quantity and quality of waste products that are produced. Some foods are much less polluting than others. Some foods are much more nutritious than others. The popular ‘pond sticks’ contain comparatively low levels of nutrients so fish need to eat more of them to obtain adequate levels of nutrients. The following points are applicable to fish in general and form the basis of my recommendations.
- Use an expanded, floating pellet. As well as the obvious point that uneaten food is easy to detect an expanded pellet also produces less waste for the filter to deal with.
- A protein level of at least 40 %. Young fish in particular benefit from a slightly higher protein level as do carnivorous fish such as trout.
- An oil content of 8 – 12%. There is ample evidence to show that oil can provide much of the energy requirements of fish and consequently saves the more expensive protein for body building purposes.
- The inclusion of astaxanthin and /or spirulina if colour intensity is important as it is for most ornamental fish such as goldfish and koi.
- Manufactured by one of the larger commercial feed producers as they have the facilities to ensure professional quality control standards are maintained
- A general vitamin and mineral supplement included. Although vitamin requirements for fish is not an exact science it is a fact that the shelf life of these essential elements is limited.
How much food to give.
Fish manufacturers and fish farmers calculate how much food to give as a percentage of the total fish bodyweight per day. More commonly expressed as % BWD. For example, one hundred fish averaging 100g. body weight would require 400g of fish food per day if the recommended ration size is 4%BWD. In contrast pond keepers invariably never use this method. The general recommendation is to give as much food as will be consumed within a few minutes. This is a simple and practical method but it rarely achieves optimal growth as feed frequency is rarely considered. Optimal %BWD will vary significantly according to many factors of which the following are the most important.
- Water temperature. The lower the temperature the lower the %BWD
- Fish species.
- Food quality. The higher the nutrient content the lower the % BWD
- Individual fish size. Smaller fish are able to consume higher %BWD than mature fish.
Fortunately feed manufacturers publish tables of %BWD for various water temperatures and fish sizes for each of their formulations. It is important to note that these are intended as a general recommendation only and fish farmers will adapt these to suit their own particular systems. When kept at 25 C I calculated my own feed rates for mirror carp in accordance with the following Table.
|Mean Fish Weight.g.||% BWD||Food Conversion Rate. FCR.|
|0.01 – 0.04||15||1.1|
|0.04 – 0.1||12||1.2|
|0.1 – 1.0||11||1.2|
|1.0 – 3.0||10||1.2|
|3.0 – 9.0||8||1.3|
|9 – 15||7||1.4|
|15 – 40||6||1.5|
|40 – 75||5||1.6|
|75 – 90||4||1.6|
|90 – 200||3.5||1.6|
|200 – 400||3||1.7|
|400 – 10000||2.5||1.8|
Table 1 : Feeding Chart for Mirror Carp at 25 °C.
It is essential to know the total weight of fish to use these tables effectively. Mean weight is easy to check by simply weighing a sample of fish. It is important however, to have an accurate estimate of fish numbers. I usually resort to manually counting the fish in a tank every so often and recording any mortality that occurs on a daily or weekly basis. I find that this rather tedious method is worth the effort needed.
Feed Conversion Rate. (FCR)
One way of assessing a fish’s performance is to measure food conversion rate , or FCR for short. This is relatively simple to measure and represents the amount of food needed to be given to gain the same set amount of growth. So, if 100kg fish gained 14 kg in one week after consuming 28 kg of feed (4%BWD for 7 days) the FCR would be
= (114 – 100)/ divided by (4*7)
In other words, for every kg.of fish food given the fish will increase in weight by 0.5kg. This is vital information for the fish farmer as feed costs represent a high percentage of their production costs. It is also an excellent indication of whether or not the amount of food given is correct. Again, FCR will vary according to similar parameters as %BWD but probably not to the same degree. Most fish farmers have an accurate idea of the overall FCR for their own farm. This is useful information when comparing one diet with another.
How often to Feed Your Fish?
The anatomy of carp tells us that they do not have a stomach as such. They have evolved to feed on a more ‘little and often’ basis. Trout, on the other hand, do have a distinct stomach. They are capable of taking larger prey at less frequent intervals. This should reflect the number of feeds per day that should ideally be given.
The consumption of oxygen and the production of fish ammonia is closely aligned with the feed that is given. It is beneficial to the filter bacteria, as well as the fish, if these demands are smoothed out over the course of a day. Spikes of ammonia can be avoided if the feed rate is slow and steady.
Fish Feeding Methods
It is simply not practical for most people to provide the optimal number of feeds for maximum growth by hand. Some form of automatic feeder is required. I have always preferred a clockwork feeder that relies on a continuously moving belt to dispense the feed. An exact amount can be weighed and gently trickled in over an 18-hour period. The disadvantage is they need to be set up every day at the same time. They can also be quite messy due to the high humidity levels usually present. They are also quite expensive. A hopper style feeder with a controller that allows around 8 feeds per day should be sufficient.
Although I recommend that when feeding pond fish a commercial pellet should form the basis of any diet it can be very useful to occasionally supplement this rather boring diet with a few occasional treats. One of the great advantages of an aquaponic system is that it is ideal to also produce additional plants that can be added to the daily diet of the fish. Duck weed in particular is extremely fast growing and is loved by most fish. Of course, various leafy greens can also be readily snapped up.
Another really top-notch culinary delight are the worms that thrive in the grow beds. Many amateur fish breeders swear by these to get their fish into the peak of breeding condition. Other co-inhabitants of an aquaponic systems that would be welcomed by many fish include water snails and various insect larvae. Everyone is a winner if these extra titbits can be recycled into healthy fish. They all help to make an aquaponic system so inherently sustainable.