There is considerable interest worldwide in the ‘sustainability’ that aquaponics (the growing of fish and a horticultural crop in the same system), appears to offer in terms of food production, but the actual science behind aquaponics is extremely limited. Much of this interest appears to be emotive, rather than factual, and the purpose of this article is to attempt to clarify where aquaponics can be scientifically proven.
By MIKE NICHOLS
The basic tenet of aquaponics is that the fish are fed with a low value organic product (usually fishmeal, or even plant products), which they convert into animal protein. The waste from the fish is then converted (by bacteria) into soluble nutrients, which are absorbed by the horticultural crop, and this ‘cleaned’ water then returned to the fish tanks.
The grower has very little (no) control over the nutrient content of the solution, apart from a very limited control over pH. In aquaponics, the bulk of the income is derived from the horticultural component, and the fish, although supplying some income, are really present primarily as manure producers. It must be emphasised that that skill set for aquaponics is significantly greater than that required for a conventional hydroponic crop, as both aquaculture and horticultural skills are essential.
Modern hydroponics increasingly demands a precise control of the nutrient content of the solution, but in aquaponics this is not possible. The question therefore arises: does the organic component in aquaponics overcome the increasing need for precision in the nutrient status of the solution?
Comparative horticultural yields from conventional hydroponic systems compared with aquaponic systems tend to favour conventional systems. Thus, it is likely that over time these productivity differences will increase, and therefore the long-term economic prospects for aquaponics would not appear to be good.
It might, however, be argued that aquaponics is an organic production system, as has been accepted in the USA, but not in Europe. Thus a lower yielding crop with organic certification might still be profitable when compared with a conventional hydroponic crop.
Aquaponics in essence is a very simple concept. Fish are provided with ‘waste’ organic material, which they convert into body mass (protein). The waste from the fish is then broken down by bacteria into soluble nutrients, which are then absorbed by plants. The ‘cleaned’ water is then returned to the fish. Bacteria are the key, as they convert the ammonia excreted by the fish into nitrite and then nitrate. Ammonia in high concentrations is toxic to fish, and thus this conversion is critical to the health of the fish, and the potential for recycling the water. Bacteria are also capable of converting fish faeces, and waste fish food into soluble ions for the plants, but this is, in general, a longer and slower but still a very important part of the system.
There can be little doubt that our wild fish stocks are being seriously depleted, by overfishing, and it is only a question of time before fish farming (as opposed to ‘fish hunting’) becomes the norm. One school of thought even believes that the oceans should be left unfished, and be retained as a source of biodiversity. Certainly, it has been a logical step on land to change from being hunter gatherers to farmers, and with diminishing resources it makes sense to consider a similar philosophy for our waters.
One problem with conventional farming is that it uses large quantities of fertiliser, and over the past 100 years the rate at which we have used fertiliser has increased enormously, and (like oil) we are well past peak use (i.e. we are using fertilisers at a greater rate than we are discovering new reserves). Although potassium and phosphorous fertilisers do recycle, the time period is one of many millions of years, as they eventually find their way to the oceans and precipitate. The current source of much of our potassium and phosphorous fertilisers precipitated out many millions of years ago in oceans and lakes… It could be argued that using sea fish waste as a feed stock for aquaculture could assist the re-cycling process, in a small way.
Aquaponics has a large number of problems to overcome if it is ever to become a main stream food production system. First and foremost is the need to evaluate the system from a scientific (and economic) point of view. It is clear from earlier studies that the bulk of the income from aquaponics is normally derived from the plant (horticultural) component, rather than the fish component. Essentially, the fish must be considered as ‘manure’ producers, with a small cash flow, with the bulk of the income being sourced from the plants.
However, the majority of aquaponics operations throughout the world have been initiated by fish farmers. Producing horticultural products requires a distinctly different skill set from aquaculture. In fact, in order to be competitive with conventional hydroponic farmers is likely to require at least two skilled people; a skilled horticulturist and a skilled aquaculturist.
Aquaponic v hydroponics
The absence of much good scientific research is apparent when one examines aquaponics. A simple examination of comparisons between aquaponic and conventional hydroponic yields is a good example. One of the major problems of aquaponics is misinformation. Recently, a large aquaponics operation was established in the United Arab Emirates. One factor for establishing it was the statement falsely attributed to my friend Dr Nick Savidov, that aquaponic crops yield heavier than those grown using conventional hydroponics. In fact, Nick had stated quite clearly that his aquaponic yield were higher than those obtained by local growers using conventional hydroponics. It was NOT a direct comparison between two systems in the same greenhouse. Dr Savidov actually stated that in his study, the yield from aquaponic tomatoes was higher in year 2 than in year 1. He compares this with the commercial yields obtained by local greenhouse tomato producers, but was not a direct comparison between the two systems in a replicated and randomised experiment. The purpose of the figure was to demonstrate how the aquaponic solution needs time to mature before it will perform effectively, due to the build-up of nutrients and bacteria in the system.
The only comparisons of conventional hydroponics and aquaponics that I am aware of are:
• by Wilson Lennard in New Zealand (see Nichols and Lennard 2010), and
• by Panatella in Italy (Panatella et al, 2012).
In the New Zealand study, the yield of some herbs and lettuce were higher using aquaponics in the summer, and lower in the winter when compared with conventional hydroponics. This was attributed to the lower winter temperatures reducing the feeding rate of the fish, and therefore, reducing the nutrients available for plant growth in the solution.
In the Italian study, Panatella found that it took time for the nutrient content of the aquaponic solution to build up, and that the lower the fish stocking rate, the poorer the growth of the lettuce. He found a numerical (but not a significant) difference in favour of conventional hydroponics in one study, and a numerical and a significant difference in favour of conventional hydroponics in another study.
However, in both cases the fish stocking rates, and the ‘maturity’ of the nutrient solution may have played a major role. Certainly, Savidov has stated that yield increases as the aquaponic solution matures, and this may be due to an increase in the nutrient content over time, or due to the increasing activity of the bacteria. These studies were all undertaken with leafy vegetables (lettuce and herbs), which must be regarded as of minor importance when compared with the big three greenhouse crops—tomatoes, cucumbers and capsicums. The only study with one of these crops was by Vergote & Vermeulen (2012), who demonstrated that fruiting vegetables (such as tomatoes) require larger quantities of nutrients than an aquaponic system was capable of supplying.
Perhaps the answer is a more active bacterial system or else some form of reverse osmosis, in which the fresh water is returned to the fish, and the concentrated nutrients supplied to the crop? In the final analysis, it may well be that much of the nutrients in the solution are still in an organic form, and not available to be absorbed by the plant roots. Improved aeration of the nutrient solution might enhance the organic breakdown by means of bacteria? However, as the majority of the fruit vegetables are grown in solid media and irrigated via drippers, then the ability not to clog the drippers would be an essential pre-requisite.
Clearly, there is an urgent need to determine the yield potential of aquaponics compared to conventional hydroponics before too many people invest (and possibly lose) their hard earned savings.
Organic certification may well be the key to whether aquaponics has any potential or not. The world body for organic certification is IFOAM (International Foundation of Organic Agricultural Movements), but this organisation has no overall authority to decide on what shall (or shall not) be organically certified, although, de facto it has taken this role. It appears to be the acceptable standard in Europe, but not in North America where the USDA has a different set of standards. From an aquaponic view point, IFOAM refuses to consider the horticultural component, because it is not grown in the soil—a key plank to the IFOAM philosophy. According to IFOAM, hydroponics is not an acceptable organic system, even though aquaponics (being a recirculating system) is much more sustainable than any soil-based system, and the nutrients are all derived “from the back end of the fish”, and no pesticides can be used in aquaponics because of the danger to the fish.
Where to from here?
Clearly, if, as appears likely, the yield from the horticultural component of aquaponics is no better (and probably) lower than from conventional hydroponics, then there must be some advantage (financially) from aquaponics, or the system will not go any further.
There are (in my view) a number of possibilities:
• If aquaponic produce becomes acceptable for organically certification, then there may well be a premium paid for this product, and the economics will stand up, or
• There is little doubt that the sea and oceans are being overfished, and that aquaculture will play a major role in the future in providing mankind with fish protein. Whether this will be a land-based aquaculture or seaborne aquaculture will depend on circumstances, but it is certain that the efficient disposal of fish waste products (ammonia and fish faeces) will be an important component of any aquaculture production, either on land or in the sea. I am reminded of visiting a large land-based barramundi operation near to Melbourne, Australia, in which the waste product from the fish is disposed into the local waste water system, at some considerable expense. Using the waste water in an aquaponic situation would save this expense.
Dr Nick Savidov has shown that by using an oxygen generator to supply the fish tanks with additional oxygen, that the fish stocking rate can be increased, and the additional oxygen in the system means that the performance of the bacteria (to convert solid waste into soluble nutrients (and ammonia to nitrate) is greatly enhanced. The growth of the plants may also be better, because the anaerobic conditions in the nutrient solution are reduced, as most (if not all) of the organic particular matter has been oxidised.
One of the problems with aquaponics is that the major greenhouse crops are fruit/vegetables (tomatoes/capsicums /cucumbers) and that these appear to grow best using hydroponics based on a solid media, such as rockwool, coir or pumice. These systems all use drippers to irrigate the plants, and drippers tend to block when using solutions containing particulate matter. No doubt, particulate matter could be eliminated from the system, but this would involve considerable amounts of filtration.
The most successful aquaponics systems have been those that produce leafy vegetables, such as lettuce and herbs using a floating raft system. Even then it is common to have airstones to provide additional aeration, as there is still unoxidised particulate matter in the system, and this can result in anaerobic conditions, and therefore poor plant growth.
Probably a key factor in aquaponics is that the majority of the income from the enterprise is derived from the plant (horticultural) component, and fish are primarily there simply to provide the fertiliser.
Omega 3 Fatty Acids
Apart from producing protein, fish are also an important source of the essential long chain fatty acids (omega 3 and omega 6). However, these are not produced by fish, but are dependent of the fish diet. The statement that we are what we eat is equally true for fish as for humans. For example, in New Zealand recently it was found that the omega 3 content of farmed salmon was considerably lower than that of wild salmon, and this was attributed to the different diets, with the wild salmon eating only fish, while farmed salmon consume a wide range of different proteins, including considerable quantities of abattoir waste, rather than the much more expensive fish meal.
A recent statement from Australia has suggested that the ratio of omega 3 to omega 6 in Tilapia is extremely low (as much as 1:24), and this again may well be a function of what the Tilapia are being fed.
The original source of omega 3 is phytoplankton and algae, which are consumed by the smaller vegetarian fish, which are then eaten by the larger carnivorous fish such as wild salmon.
It is possible also that the ratio of omega 3 to omega 6 is important in human health; not just the actual level of omega 3. This is being influenced by the increased use of omega 6 fatty acids derived from vegetable oils such as corn, safflower and soya beans, but the jury is still out on this…
There are, in fact a number of different omega 3s including the major ones, namely: ALA (a-linoleic acid), which converts very inefficiently into the two key w 3s, namely:
• EPA (eicosapentaenoic acid), which is implicated in good heart health
• DHA (docohexaenoic acid), which is implicated in good brain health.
We are what we eat, and we are only able to convert ALA to the useful fatty acids EPA or DHA at a very inefficient rate of about 5%, so that essentially we are dependent on the levels of EPA and/or DHA that we absorb in our food.
The same is also true of fish, their ability to produce omega 3 fatty acids is very limited, and they too are dependent on their food sources.
In developing countries, the concept of fish and fresh vegetables (Nichols and Savidov, 2012) from the same system is targeted at fish protein and not omega 3, but in developed countries the real value of the fish may well be directed towards the omega 3 content, rather than protein.
Towards a hybrid system
There is little doubt that high quality fruit vegetables are not very easy to produce using aquaponic systems. Modern greenhouse tomato growing (for example) requires control of the solution conductivity to ensure quality produce, and this is difficult with aquaponics. One possible solution may be to use the fish to provide the basic plant nutrients, but to supplement these nutrient levels where necessary. One difficulty may well be actually measuring the nutrient levels (via a conductivity meter) when much of the nutrients may well be in non-ionised form as organic particulate matter.
The fish and tomato project undertaken at Bleiswijk, Netherland, some years ago demonstrated that it was possible to produce highly acceptable tomato crops along with Tilapia fish in the same greenhouse system, but it should be noted that the water from the fish was sterilised by UV light, and supplemented by additional fertiliser before being used on the tomatoes. In addition, not all the solution was recirculated, as some was dumped. Note also that Tilapia is a very tough fish, and capable of living in far lower pH levels than many other fish.
Finally, it was stated at an aquaponics discussion group held during the Australian Protected Cropping Conference in Melbourne this year, that the only people currently making any money out of aquaponics are the people selling equipment and/or running training courses!
Nichols M.A & Lennard W. (2010)—Aquaponics in New Zealand. Practical Hydroponics & Greenhouses, Issue 115, 46-51.
Nichols M.A. & Savidov N.A. (2012) —Aquaponics: Protein and vegetables for developing countries Acta Hort, 958, 189-193.
Pantanella, E., Cardarelli, M., Colla, G., Rea, E. & Marcucci, A. (2012)—Aquaponics vs hydroponics: Production and quality of lettuce crop. Acta Hort, 927, 887-893.
Vergote, N. & Vermeulen J. (2012)—Recirculation Aquaculture System (RAS) with Tilapia in a Hydroponic System with Tomatoes. Acta Hort, 927, 67-74.
About the author
Dr Nichols is a retired University teacher from Massey University, New Zealand,and a regular contributor to Practical Hydroponics & Greenhouses. Email: email@example.com Ω
November 2013 / Issue 137