A ROUGH GUIDE ON HOW TO BUILD A GRAVITY FED AQUACULTURE STYSTEM
This is a guide to building a gravity fed aquaculture system the techniques of which can be applied elsewhere. This is such a huge subject that it really requires a book dedicated to it, so here I will tell you only what we did, but I will also point you in the direction of some great resources.
This particular system uses a passive flow of water redirected from a stream into a fish pond, that then over flows down a number of vegetable terraces, supplying them with nutrient rich water. This system was accomplished on the side of extinct super volcano in the western mountains of Guatemala, in a project known as the Yoga Forest.
In addition to the near vertical angle of the project site, we had the monsoon season to battle with, killer bees, my lack of Spanish language skills. But we also had some very skilled indigenous men, our unbending enthusiasm and lots of humor.
Please meet the core team before I begin
From the left Andre, Francisco, me, Whan or El doctor, Miguel and Eddie.
These guys had to have blind faith in me from the beginning and with my limited Spanish it was hard to explain what I meant. To be honest they didn’t have faith in me at all, they believed the plan wouldn’t work but they new they had to do the work, so they tried their best as always, and made me laugh my ass off along the way.
This is an Arial photo of the project site.
The site is located beneath high cliffs, you can see how steep the terrain is. The terraces located beneath the cabins have been constructed out of stone found in the river down below. We built them to grow vegetables on but we also new that they would become desiccated very quickly in the dry season and that the top soil was depleted of nutrients due to the heavy rains, so we would need a method of watering the terraces as well as supplying them with nutrients.
So we invented the the Aquaculture System
This image shows the inflow of water from the stream which flows down the ravine and then comes out a pressurized tap into the pond. The tap helps with oxygenation, this function could have also been achieved by pouring the water from a height into the pond. The pond then has an outflow fed by a simple overflow of the water that goes down a pipe alongside the terraces. This pipe then connects to 4 other pipes which have been perforated and drip feed the terraces with nutrient rich pond water.
The pond built on a slope
This image details the pond, the initial concern was that the pond would not retain water without cement or a liner and that the wall holding in the water would collapse either during the monsoon season or just from the pressure of the water inside. But as you can see it holds.
The vegetable terraces in there first season
The system made a marked difference to the quality and quantity of crops we are able to grow.
Where as before most of leafy greens had gone to seed due to stress, they now provide a harvest for much longer.
The soil on these terraces was poor as it was gathered from the steep slope which had been battered by the rains and sun, and also sour due to the added addition of emergency compost which wasn’t quite decomposed. But despite this most of the crops where thriving within weeks of the system being implemented, and the soil health will only increase with the continuous nutrients that is being added, along with the no dig system which is being applied to maximize soil beneficial soil organism and structure.
How to to design your pond on a slope (contour pond)
When a pond is built on a slope it is called a contour pond as featured in the image below.
2 drainage canal, 3 inlet pipe, 4 diversion drain, 5 overflow pipe (Viveen et al., 1985).
The image shows a pond built on a slope, it is deeper at one end and shallower at the other, this helps with harvesting as when you empty the pond all the fish go to one end, it is also good to provide varying climates in the water to support the needs of the fish, during cold days they will warm themselves in the shallow end and during hot days they will go deeper.
2 The drainage canal which takes the over flow water away, this canal could lead into vegetable your beds. Due to the contours of our land, the water had to be directed to the right of the pond along a hillside and then down the terraces, so we used plastic pipe to do this.
3. The inlet pipe takes water from the diversion drain into the pond in our case we put a pipe into one of the pools in the stream above us and fed it back to the pond through a tap which would can turn on and off to allow for the heavy rains during the monsoon season.
4. Diversion drain; this can be a channel that connects to a water body to act as a source for the inlet pipe. We are located on a very steep slope so there is risk of mud being washed down the slope into the pond, to prevent this we have used the diversion drain to catch run off rain water and divert it back into the main stream.
Selecting a site
You need to consider an number of variables including contours, so how will gravity affect the distribution of nutrient rich water from the pond? (you can use a bunyip to establish this). Also will the pond receive enough sunlight?. Sunlight is a major limiting factor in an aquatic environment and essential for primary producers such as algae which the fish will depend on. Sunlight could be limited by trees, cliffs, buildings, orientation of a slope.
Ponds have multiple functions and support many elements
How many other functions can the pond perform in its given site?. you are looking to support as many elements of your site as you can by allowing the pond to perform many functions
- Water catchment and storage
- Thermal heat store helping to regulate the temperature of the surrounding area.
- Reflecting sunlight onto surrounding crops and trees.
- providing nutrients for surrounding crops and trees from run off water.
- providing organic biomass for mulch compost or animal feed in the form of water weeds.
- providing a haven for wildlife including beneficial pest predators such as dragon flies
- providing a safe zone for water foul. In turn their droppings fertilize the water and help algal growth which is food for fish. (be careful the water does not become over fertilized!)
- Aquatic vegetables and crustaceans can be grown as well as fish.
Choosing the size of your pond
The pond needs to be at least 30cm deep at the shallow end and 1 meter at the deep end. A pond of 300m2 will provide enough fish for a average family, and a pond this size can be dug without machines. If you do not have a large enough area for this, multiple ponds can be built. Our pond is only 4m2 but it will still provide us with occasional fish, plant matter for mulch and compost, nutrients for the vegetable beds, water storage, reflected sunlight, increased humidity for the surrounding fruit trees and a habitat for wildlife.
How suitable is your soil ?
Ensure the soil has a high clay content so that it retains water or that there is high ground water. If the soil can be molded into a sausage then it contains a good amount of clay. Test the ground water by digging a hole a meter deep, cover it, then check on it the next day. If it has filled with at least a foot of water then you know you have a high enough level of ground water to make a pond
Mark out your site
You can use wooden stakes or sticks hammered into the ground, then rap string around them to provide a guide line. In permaculture we often build ponds that are convoluted in shape. This is because by maximizing the surface area and varying the shape we are creating micro-climates which allow a great diversity of species. However when building a pond on a steep contour this is quite difficult, as curves to the supporting wall can jeopardize the strength of the wall, however this is fine on the inner wall. Our pond was built rectangular in shape as it was the easiest shape to make within our limitations.
Remove top soil.
Remove all the top soil and put to one side. This top soil is nutrient rich and also highly water permeable. It will be used later, to plant into after it has been spread over the surface of the dyke.
Remove the sub soil and put to one side this will be used for building up the wall.
Also separate any clay into a pile this will be used in the core trench which i will explain about shortly.
Our site marked out with the topsoil removed.
Put in a drainage pipe
This is a pipe that will allow you to completely drain the pond, some aquaculture systems are drained to harvest the fish. It is also useful to have this pipe in place encase you need to do repairs or alter the inside of the pond in some way. For example a flash flood fills it full of mud and you need to scoop it out. Or your pond isn’t water tight and you need to either line it with clay or banana leaves to create a bio-film.
(Pictured above, pipe installation) Install the pipe once you have reached the desired depth. Be sure that the outflow leads to a suitable place, for us this is the river. And locate the pipe about a foot above the bottom so that it does not clog with debris.
Dig the clay core trench
This trench will prevent water seepage where the new dyke meets the original ground level. The trench needs to come half way up each side, be around a foot and a half thick and a foot deep. It needs to be about a foot or two away from the actual hole you have dug for the pond.
The Diagram shows a dyke without and with a clay core and the effects on water retention.
The image shows the trench for the clay core and separated clay and top soil.
Filling the clay core
Once the trench has been dug out completely it then needs to be filled with clay, do not include any sticks or organic debris and try to use clay with as little stones in as possible to ensure it acts as an effective barrier against water seepage. With every 30cm of clay added to the trench puddle it in so it compacts. Use water to help it form a homogeneous, easily manipulated mixture, it needs to be moldable in your hands, not to sloppy, not to dry.
This photo shows the team filling up the trench with clay and Whan with his wellington boots puddling in the clay.
The clay core with more height and the team still compacting, they were very thorough in this process.
The clay core complete, its doesn’t need to be quite as smooth as this but the team seemed to really enjoy smoothing the clay. I told them to ” acer lo como un grande lombrese! (make it like a giant worm!) and that is precisely what they did.
We then mounded up the sub soil on top and compacted it with shovels, stamping our feet and a lump of wood attached to a pole which made a very effective battering device.
Andres pictured above compacting the soil with the home made compacting device.
We then added top soil to the outside edge to be planted in later.
The strongest dyke wall slopes 2 meters for every 1 meter in height, a slope like this ensures strength of the dyke minimizing leakage, and erosion. You can easily test this by assembling a triangle from sticks as pictured above.
Please note we did not allow our pond to slope this much as we where limited for space, it works fine but during the rainy season there has been some erosion which we have had to repair.
Put in an overflow pipe
An overflow pipe will prevent the pond from overflowing and will also be used as the outflow for the vegetable terraces.
Noe used a piece of taught string and a spirit level to determine the maximum water level in the pond. We then bored a hole through the wall, inserted the over flow pipe which was the diameter of a hose pipe then sealed around it with clay. It is important that both the over flow and drainage pipe have mesh covers over them to stop them from being blocked with debris.
In this picture you can see Noe and Gaspar putting in the overflow pipe but also the drainage pipe on the left hand side, note how we have adjoined an additional vertical piece of pipe to it that comes just below the surface level of the water. This is so we can easily reach the pipe if we need to pull the plug so to speak.
Inflow and out flow
The picture above details water inflow, over flow and the outlet pipe (drainage pipe) as well as a silt catchment basin, this is to be used if your inlet pipe feeds from a water channel fed by a stream or river. If your pond is at the bottom of a hill it is very important to prevent rain water run off carrying debris and sediment from entering the pond, as this could quickly clog up your system. The best way to do this is by digging a drainage canal at the bottom of the hill to divert the water away.
Stabilizing the soil and preventing erosion
Andre finished of the outer walls of the dyke by planting a perennial ground cover of false peanut. This plant will form a dense Matt which will reduce the damage on the dyke by heavy rains.
How to make nature come and feed your fish
If you want nature to come to your pond and feed your fish, the first thing to do is maximize the surface area within the pond. Create as many different habitats as possible to provide ecological niches for a diverse range of organisms. This primarily will involve placing a variety of rocks, sticks and logs withing the pond; creating habitats for for invertebrates and algae to grow, which will be an important food source for the fish.
Volunteer Ben pictured here, wedging branches under rocks. We did this until the whole of the pond was covered.
Periphyphyton based feeding
Periphyphyton based feeding is a means of feeding fish with algae that naturally grow on sticks. This is called a bio-film and it naturally accumulates on sticks and rocks; so long as there is adequate sunlight and nutrients available. Periphyphyton is highly nutritious and also improves the immunity of the fish.
To kick start this process add nitrogen, this can be in the form of manure from rabbits, chickens, hooved animals or urine. Another good way to get life kick-started is to take some sludge from another pond and add it to yours.
Selecting the right plants for your pond
Plants provide additional food for fish and also a habitat for wildlife, when you come to select the plants, be sure to match their growing needs to the services you wish them to provide. For example plants in the reed and rush family remove toxins from the water but also have deep tap roots so be sure not to plant them anywhere where they may break up the dyke. Plants in the Millifolium family are submergents with no subterranean root system, they float beneath the surface and will provide a ideal habitat for young fish and insects to hide, they also aerate the water. Hyacinth also aerates the water, but float on the surface and provide areas of shade.
If plants become over grown, they are not a problem but a solution, use them to make compost or mulch your beds with. The fine green plant you can see in this picture is duck weed, it is highly nutritious for chickens and makes their eggs very golden.
Filling your pond
Once you have built the structure, increased your surface area with sticks and rocks, and planted your plants you can fill your pond.
The pond filling up with the view of the volcano behind. Our pond has a continuous feed of water entering into it at a slow rate to help with oxygenation, the overflow then trickles down the vegetable terraces, then into the forest garden.
STOCKING THE POND
Which plant and animals?
The best way to stock your pond is with a variety of different species to fill all ecological niches, ideally you would have mollusks, crustaceans and different species of fish feeding in the various trophic levels and ecological niches as well as edible plants such as lotus, arrow root, water cress and water chestnut.
With a poly culture like this you can have a continuous yield, which is reliable, because if one crop fails you have another to fall back on. It also ensures you are utilizing all the available resources. However for us, many of these are hard to come by in Guatemala, so it will be time before our pond is as productive as it can be. But that also gives us time to observe it, accept and value its feedback. For now we have several tilapia, these fish are highly adaptable and can modify their body shape to suit various environment, however cat fish would be better as they are even more resilient and are tasty too.
The technique we used for stocking this pond was to put a bunch of fish in and let them figure out for them selves how many this pond could support, none have died. Their is argument that they will not grow big if it is over stocked as they will be competing for food, their is also argument that if they reproduce; none of the fish will grow as they will all be competing for food, however there seems to be plenty of food in our pond and some of the best agriculturists i have seen rely on the off spring of their fish to repopulate stocks.
INSTALLING THE DRIP SYSTEM
Determine the level of slope.
If there is enough pressure in your system, i.e enough gravity or you have pressurized it using a tap, then a mild slope should not make to much difference to the drip irrigation. Ideally your terraces or area will be sloping slightly downwards from your pond and the direction from which the water is coming. To determine this use a bunyip, a bunyip is a length of clear plastic attached to two poles with measuring increments. It allows you to measure contours. please find details on how to make and use a bunyip here.
Jeremy the intern coordinator and forest manager blowing out the bubbles from the bunyip tube. Jeremy is the bunyip master.
Install the pipe work
Once you are sure that the slope will allow the water to be equally distributed you can attach your piping. Attach one end of the pipe to the outflow of the pond and zig zag the rest of it over your vegetable beds, Jeremy made small holes along the pipe to allow the water to drip through. be sure to seal the other end of the pipe to create back pressure. If you dont have enough pressure then you will need to put in a tap.
Before taking on this grand task i strongly advise you to read the document i have suggested below. it will give you more details on all the aspects i have covered and more, please also check out he work of Sep Holzer who was also a great inspiration for me during this project.
Good luck to you all, i hope your aquaculture systems no matter how big are small are as diverse as they are abundant.
I would like to thank the core team Francisco, Whan, Eddie, Aundres, Miguel and the volunteers for all your hard work and humor, Jeremy for all your useful criticism and amazing pipe skills, Noe who helped with everything, Charlie for helping me to deal with rebel rouses and Hayley for making it possible
Small scale fresh water fish farming, Agrodoc 15, 2008 Carballo. E, Eer. T. , Schie and T., Hilbrands. A. , Agromisa foundation and CTA, Wageningen.
Raising fish in ponds, a farmers guide to tilapia culture, 1990 Murnyak. D. and Murnyak. M, Evangelical Lutheran church of Tanzania.