Farming fish in earth ponds

Earth ponds have long been used as culture tanks for fish farming due to the numerous advantages they offer over alternative options.  They are relatively inexpensive to construct and offer a low-risk option for farming fish.
 
Under the right conditions, earth ponds can be a cost-effective and lucrative solution; these conditions start with ensuring that the climate in which the ponds are erected suits the temperature requirement of the fish being farmed for most of the year.  Fish grow optimally within a narrow temperature range outside of which their growth rate quickly reduces.  For example, the optimal temperature range for tilapia is around 28-30°C and at a water temperature of 24°C the rate of growth halves.  Under ideal conditions tilapia will take around 7 months to grow to market size whereas at 4°C cooler the fish take 14 months to attain market size.  This clearly has massive financial implications on the viability of the farm in terms of return on capital generally ruling out earth ponds for farming tropical fish in temperate climates.
 
Another important consideration is that harvesting an earth pond is a slow process.  As such ponds are not well suited to farming catfish as they need to be size sorted frequently to limit cannibalism.  If catfish are to be reared in ponds they are only stocked at a size exceeding 100g, as growth is then more even and cannibalism reduced, and smaller ponds are used to maximise the density at which catfish should be farmed.  On the other hand tilapia can be stocked from fry all the way through to harvest size, with only 2 harvests being essential during this entire period.
 
Ponds are usually stocked at lower densities than tanks or cages, requiring a large area for a commercial operation.  The construction of earth ponds requires a flat site with a high clay content in the soil to ensure that the ponds hold water.  In order to keep construction costs down the area for the ponds should be flat with a gradient of less than 1:10.  Finally, ponds require a large amount of water to be filled and maintained.  As such, the number of sites that are well suited to constructing earth ponds is limited.
 
The downsides of earth ponds include the lack of temperature control, the ambient temperature determines the water temperature, which in turn determines how quickly the fish can grow.  Harvesting is slow, incorrect pond management can result in the fish having an off-flavour and predators need to be kept out of the ponds.
 
On the positive side though, where the climate is suitable the growth of the fish in ponds can be quick, microorganisms growing in the pond providing natural feed for the fish and the farming risk is very low, making ponds a first choice where the climate, soil conditions and water availability allow for their use.