Keeping fish in earthen ponds

Fish farming is one of the key agribusiness activities in the country, with the government sometime back realising its potential and pumping millions of shillings in the venture.

However, despite many people embracing the business, farmers still get it wrong. Here is how to get it right.

SPECIES SELECTION

This depends on climatic conditions of an area, water quality and supply and most importantly, market needs. Fish species suitable for culturing in Kenya can be categorised into two; the cold and warm water species.

Cold water species require low water temperatures, relatively low dissolved oxygen and continuous supply of high quality water. An example of cold water species is trout.
Warm water species, on the other hand, require less intensive management.

Common species include tilapia, cat fish and common carps. These species are hardy and can be easily cultured in earthen ponds with average level of management thus suitable for smallholder producers.

Each species has its own advantages and challenges on management and marketing. The challenge with tilapia is over-reproduction that results in small and emaciated fish stock. Keeping monosex fish or introducing a predator species like catfish or common carp helps to check tilapia population.

On the other hand, catfish is difficult to breed in pond environment and, therefore, farmers must rely on hatcheries for fingerings. A technology to induce reproduction artificially at the farm level can be adopted after nursery management training to ensure optimal fingerings survival.

THINGS TO CONSIDER

Presence of pin bones in common carp meat discourages most consumers despite its massive body. However, when choosing a suitable species to culture, consider the following;
The ability to reproduce in captivity.

The growth rate.

Whether it feeds on both commercial or supplemental feeds.
Ability to tolerate high stocking density.

Average management.

Highly desired in the market.

Site identification and pond construction
The following factors should be considered;

Terrain: Fish ponds should be constructed in areas with a slope of not more than 1 per cent and near a steady water source. This helps to use gravity in inflow and draining of the ponds. Higher inclinations increases the cost of construction and predisposes the ponds to collapse of dykes.

Soil type: Ponds should be constructed on soils that are 20 to 35 per cent clay. Sandy soils are porous while clay soils are too compact and absorptive, thus, may deprive water some essential nutrients for fish to flourish.

Water source: Water used in fish ponds should be from natural sources like rivers, lakes, boreholes or rain. The water should have high level of dissolved oxygen and free of silt, poison, predators and unwanted fish. Ponds should not be sited in areas prone to flooding as this may cause heavy losses from washed or escaped fish.

Size of the pond: The size of the pond depends on purpose, production system and the intended scale of the venture. Ponds intended for breeding purposes are generally smaller compared to the ones used for raising fish. The size of the pond also depends on the target population one intends to produce.

Once you identify a proper site, the next step is to construct the pond. The shape of the pond doesn’t matter much as this is intended to fit the topography. However, rectangle ponds are easy to construct and manage. The depth of the pond depends on the fish species to keep, the size and production system.

As a rule of thumb, deep end should not exceed a depth of 1.2 metres at water level and the shallow end should not be less than 0.4 metres. Shallow ponds predispose the fish to predation, weed encroachment and reduces stocking capacity thereby lowering productivity.

On the other hand, very deep ponds limit light and heat penetration to the floor thereby interfering with food production. Further, the deep areas have low dissolved oxygen and cooler waters and are avoided by fish making them a wasted space and resource.

The floor slope should be in such a way that it allows easy draining of the pond. A slope of about 0.02 per cent (2cm for every 10 metres along the pond) is recommended.

Around the pond, strong dykes (walls or embankments) should be constructed to avoid walls from collapsing and water overflowing when the pond is filled.

Draining the pond is a routine management practice intended to help in preventing overflowing, harvesting, eradicating predators, de-silting, and improving the bottom conditions of the pond. The drainage should be from where the pond is deepest.

ROUTINE POND MANAGEMENT

After constructing a fish pond, what follows is a series of routine practices to maximise production. Proper and accurate records should be kept for every activity taking place in the pond.

Routine management practices include stocking, feeding, fertilisation, controlling water levels, quality, pH, weeds, predators, diseases, harvesting and marketing.

Apply lime and organic manure in the pond before refilling new stock. Agricultural lime is most preferred and should be evenly applied on the floor. Rates of application depend on the alkalinity of the water or soil pH. Where the soil pH or alkalinity is not known, the minimum recommended amount of lime (1kg per 10m2 of pond surface) should be applied.

Where organic manure is not applied before filling the pond, continuous application at the rate of about 5kg per 100m2 per week should be maintained. Animal manure (cow, pig, chicken, goat or sheep) is the most preferred, but, compost manure and plant matter (hay or humus) can also be used.

To further increase productivity, inorganic (chemical) fertilisers should be applied in the water taking precaution not to deposit the mineral directly at the bottom. Commonly used inorganic fertiliser include DAP and urea. Application rates for each are two and 30 teaspoons full of DAP and urea respectively per 100m2 per week.

STOCKING RATES

Stocking density of a fish pond depends on fish species and pond productivity. However, in extensively managed earthen ponds, a stocking rate of about one to two fingerings per m2 is recommended for monocultures of tilapia.

This rate can be increased with improved management to optimise pond productivity. Where catfish is introduced in tilapia ponds to control population, a ratio of 1:10 (a catfish per every 10 tilapia) should be considered.

Catfish should be introduced when tilapia is at least three times the size of former. The stocking rate for catfish monocultures is about two to three fingerings per m2 but can be increased to about five fingering per m2 if there is good management. With proper management, the fish should attain market weight at about six to nine months depending on water temperatures.

FEEDING YOUR FISH

Tilapia fingerings are omnivorous, feeding mainly on tiny water organisms as well as dead organic matter (detritus) and as they increase in size, they become entirely herbivorous and feed on small water plants. Similarly, at young stages the catfish and common carp fingerings are omnivorous and as they increase in size, they become predatory.

Tilapia are surface water feeders and prefer feeding during the day, while catfish and common carp are bottom water feeders and prefer darkness when feeding.

Fish can be provided with supplemental or commercial feeds especially where the stocking rate is high to avoid starving. Kitchen and slaughter house waste, oil seed cakes and grains are among supplemental feeds.

When feeding the fish, ensure time, place, quantity and quality are maintained as disruption may induce stress to the fish.

WATER QUALITY AND QUANTITY

Water quality is determined by turbidity, temperatures, pH, dissolved oxygen and minerals. Highly turbid water impairs vision especially for surface water feeders like tilapia interfering with their feeding.

Catfish and common carps, on the other hand, depend on their smell and touch in search of food and can tolerate reasonable turbidity. Regardless of the fish species, low dissolved oxygen lowers their activity particularly feeding.

Low dissolved oxygen can arise when the water temperature decreases or there is invasion by algal bloom. Algal blooms can be distinguished by discolouration of pond water and usually happens due to increased application of fertiliser in the pond.

Overstocking and excess pond vegetation can lower the pH of the pond significantly and should be avoided. Rise in pond water pH is usually caused by acidic soils and can be controlled using agricultural lime.

WEEDS, PREDATORS AND DISEASE CONTROL

Weeds and predation are the most serious challenges in earthen ponds. Diseases are not very common partly due to the low rates of fish farming and because cultures used are native to the tropics and, therefore, tolerant to common diseases.

Weeds can be controlled by continuously trimming unnecessary vegetation along the banks of the pond and deepening shallow ponds to recommended depths. Common predators include birds (pelicans, kingfishers and herons), frogs, tadpoles, predator fish, dogs and wild carnivores and human beings.

To control predation from land, a perimetre fence should be installed and a net placed over the pond. Drying the ponds prior to restocking is a good way of killing frog eggs. Common indicators of stressed fish include gulping air, abrupt change in feeding behaviour, whirling or swimming erratically away from others, distended stomach, discolouration, peeling skin and ulcerations, among others.

Harvesting and marketing fish

After six to nine months, fish is ready to be harvested and sold or consumed at home. At this age, fish from properly managed ponds should be about 250 to 500g.

Harvesting should be done early in the morning or late in the evening when fish are not very active. Always harvest fish after identifying a ready market to avoid losses.

The most common method of harvesting involves draining the ponds. In cases where draining of the pond is not possible, seine, cast, lift, gill or hoop nets can be used.