Species imbalance and overfeeding before a festival market
Mr Patil has a 1.2 ha pond, average depth 1.8 m, in Kolhapur. In March he stocked 6,000 catla (40 g), 4,000 rohu (30 g), 3,000 mrigal (25 g) and 2,000 common carp (20 g), a total of 15,000 fish. He fertilises with cow dung weekly and feeds a 28% protein pellet at 3% of body weight. After five months, average weights are catla 350 g, rohu 420 g, mrigal 300 g and common carp 600 g. The farmer is pleased with common carp but complains that catla are not growing and that the pond surface is crowded at feeding time. Secchi depth is 35 cm, early-morning dissolved oxygen is 4.2 mg/L, pH is 7.8, and there is no aeration. He wants to add another 3,000 catla fingerlings and raise feeding to 5% of body weight to 'push growth' before the festival market in two months.
1. Assess the current stocking and feeding plan. Which species is most likely to be limited, and why?
Read the worked answer
The pond is heavily stocked: 15,000 fish in 1.2 ha is 12,500 fish/ha, and the standing biomass is already substantial. Using the given average weights, catla biomass is 6,000 × 0.350 = 2,100 kg; rohu is 4,000 × 0.420 = 1,680 kg; mrigal is 3,000 × 0.300 = 900 kg; common carp is 2,000 × 0.600 = 1,200 kg. Total biomass is about 5,880 kg, or roughly 4,900 kg/ha.
Catla is the species most likely to be limited. It is a surface and mid-water plankton feeder, but the pond is also carrying common carp, which is a bold, omnivorous feeder that competes aggressively at feeding time. The crowding at the surface during feeding suggests that catla are not getting their share. Catla at 350 g after five months are growing more slowly than rohu at 420 g, which is the opposite of what one would expect if plankton and feed were adequate. The early-morning oxygen of 4.2 mg/L is not yet lethal, but it is not a comfortable margin for a dense pond with no aeration.
Adding 3,000 more catla would intensify surface competition and raise total biomass. Raising feed to 5% would add waste and oxygen demand in a pond that already has a low dawn oxygen margin.
2. What practical changes would you recommend for the next two months, and what would you measure to judge success?
Read the worked answer
First, do not add the extra 3,000 catla. The pond is already crowded and the limiting species is the one he proposes to increase. Second, partially harvest common carp now. At 600 g they are marketable, and removing 1,200 kg of biomass would reduce competition, lower oxygen demand and generate cash before the festival. Some rohu at 420 g may also be partially harvested if the market accepts that size, but catla at 350 g are not yet the best festival fish.
Third, reduce feeding from 3% to about 2.5% of standing biomass and use floating pellets so that feed intake can be observed. Stop or sharply reduce cow-dung application; the pond already has a 35 cm Secchi depth and the dung adds oxygen demand. Fourth, install night aeration or at least run a paddle wheel from late evening to early morning, because the 4.2 mg/L dawn reading shows the pond is close to stress.
To judge success, measure dawn dissolved oxygen at least twice a week, ammonia, Secchi depth, average weight of catla and rohu, and survival at harvest. A rising catla growth rate, stable dawn oxygen above about 4 mg/L and no surface gasping would show that the changes are working. For the next cycle, reduce total stocking density and adjust the species ratio so that catla are not crowded by common carp.
3. Calculate the approximate daily feed requirement at 3% and at 5% of standing biomass using the given average weights, and explain why the 5% figure is risky.
Read the worked answer
Standing biomass is calculated as follows:
- Catla: 6,000 × 350 g = 2,100 kg
- Rohu: 4,000 × 420 g = 1,680 kg
- Mrigal: 3,000 × 300 g = 900 kg
- Common carp: 2,000 × 600 g = 1,200 kg
- Total = 5,880 kg
At 3% of biomass, daily feed = 0.03 × 5,880 = 176.4 kg/day. At 5%, daily feed = 0.05 × 5,880 = 294 kg/day. The increase is about 118 kg of feed per day.
That extra 118 kg is not free. It adds faecal and uneaten feed waste, which consumes oxygen and releases ammonia. The pond already has a dawn oxygen of only 4.2 mg/L and no aeration. A 5% feeding rate would also be expensive and may not be eaten efficiently if catla are crowded and stressed. The feed conversion ratio could worsen, so the farmer would pay more for less growth. A better approach is to reduce biomass by harvesting common carp, feed at about 2.5-3% with careful observation, and improve aeration before considering any increase.