Tilapia Aquaculture nursery Phytoplankton Secchi Disk

Urban Tilapia Nursery producing fingerlings and the importance of Phytoplankton concentration in Aquaculture fish ponds

Came across this thriving small Aquaculture fingerling grow out operation on the outskirts of Maputo, Mozambique, during my travels in 2012.

10 m x 10 m PVC lined ponds – 6 Hapas per Pond

The whole setup is very well laid out and neatly constructed. Ponds measure around 10m x 10m with 6 hapas per pond, each measuring around 3m x 1.8m. Pond depth is 80 cm.

Ponds were lined with PVC sheets to mitigate soil acidity being transfreerd to the water and thus eliminate the the need to lime it. All ponds are then fertilized to produce phytoplankton for the fry/fingerlings to feed on.

Sex Reversal of fry for Monosex

All fry are first reared from egg stage in a separate facility before being transferred here. In that nursery they are fed with testosterone laced feed for sex reversal purposes to ensure an all male batch. An all-male batch of tilapia fingerlings is essential to achieve uniform growth throughout the grow out phase. Failing to sex reverse the fry would result in erratic growth and non uniformity in size, as they would start breeding while still juveniles, resulting in young consuming feed that would otherwise be used for adult fish growth. The sex reversal phase last 21 days. When the fry reach 0.22 grams they are transferred to another system until they reach 0.44 grams and are then transferred to this grow out facility.

Stocking Density & Growth

Fry are transferred into this facility based on a stocking density of 100 – 1000 per m3 until they reach 5 grams. There after they are moved to much larger adjacent green (fertilized) pond without hapas at a stocking density of about 3.5/m3.

How to measure Phytoplankton Levels – Secchi Disk

After ponds are fertilised, its important to measure the concentration (or production) of phytoplankton. Phytoplankton gives the water a green tinge. Its a micro algae plant-like cell that serves as feed for the fish. During the day, phytoplankton through photosynthesis produces Oxygen. Phytoplankton accounts for approximately 50% of all photosynthesis on Earth and thus, up to 50% of all the oxygen on earth.

The levels of concentration of phytoplankton in fish ponds is therefore a good indicator of whether the ponds have enough nutrients or not, and by association whether the Ph levels need adjusting.

So how does one measure the concentration of phytoplankton in the pond? The cheapest and best way is by using a Secchi Disk. A Secchi Disk is a flat white disc measuring 30 cm in diameter, attached to a tape-measure, or piece of string, weighted from below by a 200g weight. One can also attach a handle (wooden dowel) to the disk, with distinct measurement marks, which then does not require it to have a weight below.

Measurements take place between 10:00 and 14:00 for best results. The operator ensures that the disk is bright white and clean before taking the measurements. The measurement is a visual one. By dipping the Secchi Disk into the water you will be able to see the transparency change at different depths

Dissolved Oxygen – Aeration

During day time, phytoplankton will through photosynthesis produce oxygen. At the night, the reverse occurs, so D.O levels in the water tend to drop, as does pH/Alkalinity.

To ensure adequate levels of D.O. above 6mg/l this Aquaculture operation uses a simple petrol powered water pump that re-circulates and splashes water back into the pond, breaking the surface tension of the water and infusing it with Dissolved Oxygen.


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