One of the first, and most exciting, choices you will make is deciding what species of aquaponics fish you wish to keep. In theory this is almost limitless as it is possible to control just about every aspect of their environment. Diet, water quality and temperature can all be carefully controlled. Plants don’t really mind what fish you select. Fish ammonia is always the primary excretory product and it is this that the biofilters will convert into the necessary plant nutrients.
Only a relatively small proportion of your aquaponic system is actually devoted to the fish. Fish tanks typically represent just 20-30% of the space occupied by the growing plants. With fish stocking densities over 25 kg/m3 this can be even smaller. If you have limited space then select fish that mature at a small size. Stands to reason really! Indoor systems tend to be small and need to be attractive. A species of tropical fish would be ideal. Goldfish would also be more appropriate than koi carp.
Do you want fish for supper?
You probably already have an answer. The ability to continuously feed your family with fresh organic fish straight from your own garden is very attractive to many. On the other hand, fish have real beauty and personality. Are you prepared to dispatch them once they have reached a portion size? How would you feel about something that you have been gently nurturing for many months? Fish keeping can be very addictive and it is all too easy to become very attached to your fish. They quickly learn to come and greet you every morning as you prepare to give them their breakfast first thing every morning. Somehow it seems easier to ‘do the necessary’ the more fish you have.
We are not fish keepers, rather keepers of water. Although aquaponic systems demand only a fraction of the water that traditional farming requires it is still an essential ingredient. It’s quality will have a major role to play in our selection of the best fish for aquaponics. For most, the only practical water supply is what comes out of the tap. Rainwater is a possibility but frequently has its own problems of inconsistency, pollution and lack of buffering capacity.
Water supply companies provide free and comprehensive analytical data relating to water quality. Use this information to match the needs of the fish to the water quality you have available. Pay regard to pH, hardness and alkalinity. Most water in the UK is hard with pH7-8. It would not be sensible to select fish that lived in the soft, acidic waters of tropical rainforests.
A big word of warning is due. Water utility companies invariably use chlorine to kill off bacteria and protect public health. Chlorine is deadly to all living creatures and would devastate an aquaponic system in no time at all. Fortunately, chlorine will naturally escape from water if left to stand for 24 hours. It can also be simply removed using a small cartridge filter containing activated carbon. The ability to carry our a slow 5-10% daily water change can greatly improve the health of the fish.
Do you want, or need, to control water temperature?
Fish are poikilothermic and their metabolism is directly related to the water temperature. Each species has an optimum temperature for maximum growth. They will also have a lower temperature when they will not eat, let alone grow. Go below this and some species will die. They will also have an upper lethal temperature as well.
Water temperature has an overriding effect on the plants, bio filter and fish. It is often advisable, and sometimes essential, to provide some kind of heating. Cooling is far more expensive and fortunately rarely necessary.
How big do you want to grow your fish?
If you are intending to eat your fish then there will be a minimum portion size. For most species this will be around 400 – 600g. Flexibility over this is really useful as individual fish will grow at vastly different rates. Fish farms have a need to regularly grade and segregate different size classes. Small aquaponic systems rarely enjoy this luxury. although it is perfectly possible to grow big fish install ponds it is not always desirable.
The finished fish size will also have a bearing on how best to manage you fish stock. The design of any aquaculture system should ideally be based upon the total amount of fish feed that can be fed. The same amount of food can be given to a large number of small fish or a small number of big fish.
If the cost of individual fish is high then it may well be better to harvest less but bigger fish. Obviously you would need to pay regard to the longer time needed to reach marketable size.
Smaller fish eat proportionately more food than bigger fish. They also tend to convert it into growth more efficiently. A 10g carp could easily eat 8% of its own weight in food each day and have a feed conversion rate (FCR) of 1.2. Compare this to a 1kg carp that would eat just 2% body weight per day (BWD) and have a FCR closer to 1.8. The annual production of smaller fish would be considerably higher than for the larger fish. However, this plan would require a higher level of stock control and a higher investment in fish stock. I would not recommend this route for beginners. Keep the number of fish down and the marketable size up.
Sourcing the best fish for aquaponics
At the end of the day you can only grow what is available to buy. A regular supply of disease-free fingerlings, at a fair price, is critical. Small live fish are able to be sent all over the world but not it is a stressful process. Disease-free status is of paramount importance. Biosecurity procedures have, quite rightly, been tightened significantly over the last few years. Lack of attention to this aspect can be devastating and can lead to a ban of all live fish movements. Very often the only way forward for a fish farm is to cull the entire stock, disinfect everything and start again!
Ways to protect yourself from this scenario include;
- buy only from registered fish farms
- select fish that you can breed yourself
- produce fish you can eat yourself and do not need to be moved to another site.
How important is profitability to you? If it is then remember;
- Live fish nearly always command a high price
- Ornamental fish invariably command higher prices than table fish
- Small fish convert fish food more efficiently than large ones.
- Small fish usually, but not always, command higher prices per kg. than larger ones. Exceptions include specimen carp for restocking and certain individual koi carp.
If you live in England or wales I would strongly recommend that you register any outdoor aquaponics system with the Fish Health Inspectorate. This is quick and easy to do using Form RW1 and is free. This will allow you to buy fish from a registered farm in the knowledge that it has a biosecurity measures plan (BMP) in place that satisfies the Centre for Environment, Fisheries and Aquaculture Science (Cefas).
Registration will ensure that any outbreak of fish disease will be properly measured and controlled. Notifiable diseases can, and do, have a very real impact on the entire agricultural industry.
If you only buy goldfish from the local garden centre, or eat all of your produce, it is probably not strictly necessary. The legislation is designed to protect our fisheries by controlling the live movements of fish. If you would subsequently like to move live fish to or from a fishery you must apply for consent from the Environment Agency. This is a legal requirement with heavy fines for non-compliance.