The spillway that sets the height of the water and allows for passive
discharge of surplus water during large rainfall events
We can build a dam to serve specifically as a fish pond and which can be designed to be more productive for aquaculture systems generally, compared with stocking an existing farm dam with fish. As most of the production occurs in the upper levels of water, a depth of under 2 metres allows you to feed and harvest the fish easily and bring them to a desirable size as quickly as possible. Using an example of the chicken tractor, infrastructure design can also be applied to fish to create a more intensive system where resources such as the animals’ manure are cycled and productivity is increased whilst benefiting the surrounding systems. The ideal style of dam for the purpose of fish production is the contour dam, which is dug into the side of a shallow sloping hill (on a reasonable flat landscape) with a dam wall of a semi-circular curve or a semi-square shape. The profile of the dam floor can be easily constructed so that it is flat, and the inner walls and back-cut of the dam can be reasonably steep, maximising the volume and minimising the challenges of harvest, whilst maintaining a consistent temperature.
Large rocks positioned at waterline in the corners for
rod and line fishing for meal size harvests
Water quality: Water quality is an important factor in aquaculture, therefore it is not ideal to have the dam positioned on the water catchment itself (due to sediment runoff), but rather to gravity syphon or pump (if you have to) the water from another catchment. One of the inputs that raises the production is increasing the oxygen content in the water and one of the most passive ways to do this is to drop falling water from a height from one of the syphons from an overcharged uphill dam and/or swale system, which are often overcharged, so you can allow the surplus water to splash into the fishpond, oxygenating it in the process. An additional benefit is that as the water passes through the fishpond dam it collects additional nutrient from the fish and carries it further down the landscape. As the fish mature there is an increased amount of nitrite in the water from their droppings, which is converted into nitrate via the beneficial soil organisms.
Fish food: Another system input is food for the fish. Trees such as mulberries can be established around the pond to attract insects which fall into the pond, an ancient Chinese method. The mulberry leaves can also be inoculated with silk worm eggs which, once hatched, can be knocked into the pond. Crotileria grahamiana, a legume bush that grows well by the water’s edge where the soil is slightly anaerobic, attract the Monarch butterfly which lays its eggs on them and the caterpillars that cover these bushes can be flicked into the pond as fish food and the pruning used as mulch. There are many other types of tree that are suited to growing by the water’s edge that are also susceptible to becoming inundated by caterpillars, and they can be an asset in these systems. Solar lights can be used to attract moths and if the lights are low to the water the moths get trapped underneath and stuck in the water and it becomes a night time fish feeding system. Compost worm farm and vegetable waste systems can be set up to float on the pond to trap black soldier fly larvae as a direct harvest for the fish. Meat maggots can also be produced easily from dead animals, road kills, or meat wastes but the this can be quite odorous.
Water plants: Water plants are beneficial to have around a fishpond, as it improves water quality and gives additional habitat for fish. Water lilies, emergent plants and edge-side plants all have a an advantage in keeping the water clean and adding to the aquaculture ecology. Beneficial algae will naturally occur with sunlight and organic matter. The dam can be seeded with zooplankton by adding a bucket of mud from below the waterline of a good healthy dam, which only needs to be done once, and crustacea such as crayfish and particularly freshwater shrimp can be added which will feed on the zooplankton before being eaten by the fish. A way to tell if the pond is oversupplied with fish is if the shrimp population is greatly diminished.
Attachments and accessories: If you intend to keep fish that potentially run down stream in breeding times (like Australian Bass for example), you need to put a fence across the spill-way that is rounded out into the dam so that it doesn’t get clogged with vegetation etc during rain events. If you intend to stock eels (which would have to be trapped live as baby eels, or "elvers"), or to keep eels out, you need an eel fence (one that they can’t get through or over by propping themselves up). A platform or jetty where you can feed and/or catch fish from is a very handy attachment to a fish pond dam.
Stripping off the top soil from the site to be saved
and used to dress the site at the end
Construction: The construction process is much the same as any other dam, the first thing you need to do is strip the top soil off the area where the dam will be, including the dam wall, and put that to one side while you begin the construction process, this is used later to top dress the dam wall (and the back of the dam if there is a cut above the water line) so that it can be easily revegetated and thus restabilised afterwards. You then create the keyway trench by digging down in a slot shape below and along where the dam wall will stand until you get down to good solid clay (100% if possible).
Digging the hole and building the wall
Cutting the keyway below where the dam wall will stand,
to lock the dam water tight
Compacting the keyway trench by track rolling
As you start to dig into the dam, the clay from the main hole is packed into the keyway and rammed down very hard with track roll compaction or vibrating sheep’s-foot roller compaction (if you have marginal material that is a good process to use).
Track rolling the dam wall, to compact it until water tight
Dry material needs to be wet down. Once the keyway is dug and well compacted you have cut off any potential water flow going under the dam wall. The next process then is to dig out the dam main hole, using the material to build and compact the wall, making sure the inside and outside walls are not too steep (a 3-1 or 2-1 slope maximum) and are nicely compacted.
Flat bottom with a depth of 2.2 meters, ideal for a productive fishpond
Interns learn to survey the future water line accurately
The bottom of the dam needs to remain flat and perfectly level with a little over 2 metres as an ideal depth, allowing for a little bit of anaerobic soil creation on the bottom of the dam and an average of 1.5-2 metres of water depth for your fish.
Stocking the dam: Once the dam is built and filled with water from an uphill syphon, the next thing is to stabilise the dam and ensure a healthy dam ecology with algae (which will happen naturally from sunlight & organic matter), zooplankton (from a bucket of mud from a healthy dam), some crustaceans and water plants. You know when your aquaculture ecology is working well when the water becomes clear with 1-1.5 metres of visibility. It will always have a bit of a tannin look at first, but the clarity will increase. When this takes place you are now ready to stock your fishpond dam with fish. Your choice of fish depends a lot on where you are and what requirements you have. Here at the PRI on Zaytuna Farm we use Australian Bass, some eels, and local catfish, and they work pretty well all together.
Harvest: We often obtain a meal sized harvest simply using a rod and line and general fishing techniques as the fish are very easy to catch. We can also use a net, or lower the level of the water to harvest all the fish at once.
Fishpond dam wall with cover crop emerging
The back of the dam wall, with cover crop seed sprouting as the first
vegetative cover for stability and erosion control
Maintenance: Similar to most other dams, the edges need to be kept clean, it is good to get a perennial ground cover growing around the edges. Our choice here in the subtropics is pinto peanut, a perennial prostrate ground cover legume, but any other perennial ground cover gives you a carpet style cover for a clean edge to the dam, making it easier to maintain, feed and harvest your fish. This goes for spill ways too, they need to kept clean and open so they don’t clog and cause problems in large rain events. One thing that is a little bit different with a fishpond, compared to a regular dam, is the amount of nitrite in the water and thus the abundance in life encouraged and enhanced, creating a faster build up of anaerobic material converted to soil on the bottom of the dam, which needs to be maintained every few years. Perhaps every 5-10 years you will need to drain the pond and remove the nutrient rich soil, laid out to dry and used as a top-dress fertiliser on gardens and fruit trees. It will naturally convert from anaerobic to aerobic soil when it is dried out, or it could be used in compost as an enhancing agent as food for aerobic organisms. In a highly a productive aquaculture system, you can manage to get 1 kg of fish produced in 1 cubic meter of water.
The fishpond in the photo is roughly 800 cubic metres of water which is approximately 800 kgs of fish we can harvest within 18 months.