Building an Integrated Fish Pond – Integrated Livestock-Fish Farming (Senior 3 Agriculture) | Ganzaa

Integrated Livestock-Fish Farming

Lessons

Senior 3 · Agriculture

Integrated Livestock-Fish Farming

4 lessons

Lesson 2 of 4

Building an Integrated Fish Pond

25 min

Let's begin

A fish pond is a hole that must do two difficult things: hold water for months, and let it all out in a few hours when the fish are harvested. Many ponds dug by hand in Rwanda's valleys fail at the first: they leak through sandy soil, or their banks slump in the rains. MINAGRI counts about 3,000 earthen ponds in the country and plans for them to become more productive (National Aquaculture Strategy, 2023). This lesson shows how to choose a good site and build a pond, with its animal house, that will last.

Definition

Dike (bund, embankment): the earth wall round a pond that holds the water in.

Inlet: the pipe or channel that brings water into the pond; outlet (drain): the pipe at the deepest point that empties it.

Freeboard: the height of the dike above the water surface, which stops waves and heavy rain from overtopping it.

Clay core: a trench under the dike filled with compacted clay, which stops water seeping under the dike.

Spillway (overflow): a protected low point or pipe that lets surplus rainwater out safely, without washing the dike away.

1. Choosing the site

Water: a reliable supply of clean water all year (a spring, stream or irrigation canal) to fill the pond and replace losses, free of pesticides and other pollution.
Soil: clay or clay-loam that holds water. Squeeze test: a ball of moist soil should keep its shape. Hole test: dig a hole about 1 m deep, fill it with water in the evening; if most of the water is still there next morning, the soil holds water.
Slope: gently sloping land, so that the pond can be filled and drained by gravity; avoid the lowest valley floor, which floods.
Safety and access: close enough to the home to be guarded, with a path for feed and harvests; room on the dikes for the animal house.
Permission: marshlands and streams are protected; ask the sector authorities before building in a marshland or taking water from a stream.

2. Parts of an integrated pond

Figure 18. Cross-section of an integrated pond with a pigsty on the dike (not to scale).

Part	Standard (typical)
Size and shape	rectangular, often 200 to 600 m² for a family pond; easy to net from the banks
Depth	about 1 m at the shallow end to about 1.5 m at the deep end
Dikes	built on a clay core, compacted in layers; top at least 1 m wide (wider where the animal house stands); inner slopes gentle, about 2 m across for every 1 m of height; freeboard about 30 cm
Inlet	above the water level, with a screen to keep out wild fish, frogs and rubbish
Outlet	at the deepest point, with a standpipe or a monk to set the water level, and a screen
Spillway	protected with grass or stones, to let flood water out
Animal house	on the dike or on stilts over the water, with a channel or mesh floor so manure enters the pond a little at a time
Dike cover	grass, which holds the soil; vegetables on top; no trees, whose roots open leaks and whose leaves shade the water

3. Tools and materials

Hoes, spades, pickaxes and wheelbarrows; pegs, string and a tape measure; a line level or a clear water-level tube for setting levels; PVC pipes, an elbow and mesh screens for the inlet and outlet; clay for the core; grass splits for the dikes; agricultural lime; and timber or bricks, iron sheets and mesh for the animal house.

4. Building the pond, step by step

Clear trees, bushes and roots from the whole site.
Peg out the pond and the dikes with string.
Strip the topsoil and keep it aside for the dike surfaces and gardens.
Dig a trench along the line of each dike and fill it with clay, compacting it: the clay core.
Dig out the pond and build the dikes with the soil, in layers 20 to 30 cm thick, wetting and ramming each layer firm.
Shape the inner slopes and the bottom, sloping it gently down to the outlet.
Lay the outlet pipe at the deepest point, and the inlet pipe through the dike above the water level; fit screens.
Cover the dikes with the saved topsoil and plant grass.
Build the animal house on the dike, with the channel to the pond.
Spread lime on the bottom, fill the pond slowly, check for leaks, and fertilise; stock fingerlings once the water turns green (about one to two weeks).

Worked example 1: filling a pond

Problem. A pond is 20 m long and 15 m wide, with an average water depth of 1.25 m. The inlet brings in 2 litres per second. How long will it take to fill, ignoring losses?

Solution.

Volume $= 20 \times 15 \times 1.25 = 375\,\mathrm{m^3} = 375\,000\,\mathrm{L}$ (since $1\,\mathrm{m^3} = 1\,000\,\mathrm{L}$ ).
Time $= \frac{375\,000}{2} = 187\,500\,\mathrm{s}$ .
In hours: $\frac{187\,500}{3\,600} \approx 52\,\mathrm{h}$ , a little over 2 days.

Answer: about 52 hours. The same sum tells a farmer whether a spring can keep up with evaporation and seepage in the dry season.

Worked example 2: the slope of the pond bottom

Problem. A pond bottom is 20 m long. The water is 1.0 m deep at the shallow end and 1.5 m deep at the outlet. What is the slope of the bottom, as a percentage?

Solution.

Fall $= 1.5 - 1.0 = 0.5\,\mathrm{m}$ .
Slope $= \frac{0.5}{20} \times 100 = 2.5\%$ .

Answer: 2.5%: enough for all the water to run to the outlet when the pond is drained, so no fish are stranded in puddles.

Warning

Safety. Pond digging is heavy work: lift with bent knees, work in teams and take water breaks. Deep, steep-sided ponds can drown young children: fence ponds near homes and never let children play or swim in them. Never dig near power lines or under unstable banks.

Tip

Memory hook: "Water in high, water out low, clay below the wall, grass on top, no trees at all."

Farm Africa: "Earthen fishpond construction". Watch how the pond is marked out and the dikes are built and compacted, and compare it with the steps above.

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Practice

Give five factors to consider when choosing a site for a fish pond.
Describe two simple tests that show whether a soil will hold water.
State the function of: (a) the inlet screen, (b) the outlet at the deepest point, (c) the freeboard, (d) the clay core.
Why are trees not planted on pond dikes?
List, in order, eight steps in building an earthen pond.
A pond is 25 m by 16 m with an average depth of 1.2 m. How many litres of water does it hold, and how many hours will an inflow of 3 L per second take to fill it?
A pond bottom 30 m long is 0.9 m deep at one end and 1.5 m at the outlet. Calculate the slope of the bottom.
A farmer builds a pond on sandy soil at the lowest point of a valley, with no outlet pipe, and plants banana trees on the dikes. Evaluate his choices.

Show answersHide answers

Any five: reliable clean water all year; clay soil that holds water; gentle slope for gravity filling and draining; no flooding; security and access near home; room for the animal house; no pollution; permission from the authorities.
Squeeze test: a ball of moist soil keeps its shape if it has enough clay. Hole test: a hole about 1 m deep, filled in the evening, still holds most of its water next morning.
(a) keeps out wild fish, frogs and rubbish; (b) lets the whole pond be drained for harvesting and cleaning; (c) stops waves and heavy rain overtopping and breaking the dike; (d) stops water seeping under the dike.
Roots open channels that cause leaks and weaken the dike, and leaves shade the water and fall in to rot.
Clear the site; peg out; strip topsoil; dig and fill the clay core; dig the pond and build compacted dikes in layers; shape slopes and bottom; fit outlet and inlet pipes with screens; cover dikes with topsoil and plant grass; build the animal house; lime, fill, check for leaks and fertilise (any eight, in order).
Volume $= 25 \times 16 \times 1.2 = 480\,\mathrm{m^3} = 480\,000\,\mathrm{L}$ ; time $= \frac{480\,000}{3} = 160\,000\,\mathrm{s} \approx 44\,\mathrm{h}$ .
Fall $= 0.6\,\mathrm{m}$ ; slope $= \frac{0.6}{30} \times 100 = 2\%$ .
Poor choices: sandy soil leaks, so the pond will not hold water; the valley floor floods, so fish escape and wild fish enter; with no outlet the pond cannot be drained for harvest or cleaning; tree roots cause leaks and leaves shade and foul the water. He should choose clay soil on the valley side above the flood line, fit an outlet at the deepest point, and plant grass on the dikes.

Key points

Site: reliable clean water, clay soil (squeeze and hole tests), a gentle slope above the flood line, security, access and permission.
Parts: compacted dikes on a clay core with about 30 cm freeboard; screened inlet above the water; outlet at the deepest point; spillway; grass on dikes; animal house on the dike or over the water.
Build in order: clear, peg, strip topsoil, clay core, dig and compact, pipes, grass, animal house, lime and fill.

Sources and credits

National Aquaculture Strategy for Rwanda 2023–2035, Ministry of Agriculture and Animal Resources (MINAGRI), May 2023: number of earthen ponds. Facts cited.
Photo Fish Ponds 01, by Geossegawa, via Wikimedia Commons, licensed under CC BY 4.0.
Video: Earthen fishpond construction, Farm Africa, embedded from YouTube.

Written by Ganzaa: the text, the table, Figure 18 and the worked examples. Dimensions are typical values; follow RAB or your fisheries officer's advice for your site.

11. Choosing the site

22. Parts of an integrated pond

33. Tools and materials

44. Building the pond, step by step

1. Choosing the site

2. Parts of an integrated pond

3. Tools and materials

4. Building the pond, step by step