Fisheries and Aquaculture: Pond Management, Polyculture and Seed Production
Fisheries questions test pond preparation, species combination, water quality and seed production. A calculation of stocking number or expected production is a common item, so keep survival percentage in mind.
Pond preparation
Drain and dry the pond, remove weeds and predatory fish, then apply lime (commonly 200-500 kg/ha depending on soil acidity) and only afterwards manure or fertilizer. Fill with water and check pH (about 6.5-8.5) and dissolved oxygen (above 5 mg/L before stocking). Liming before drying or liming together with heavy manuring wastes the treatment.
Composite fish culture
Indian major carps occupy different niches: catla feeds near the surface, rohu in the column, mrigal and common carp near the bottom. Stocking them together uses the whole water body. A typical ratio is catla : rohu : mrigal = 3 : 4 : 3 at about 10,000 fingerlings per hectare. Supplementary feed is given at roughly 3-5% of body weight daily and growth is checked monthly by sampling.
Seed and breeding
Spawn goes to a nursery, then a rearing pond, then the stocking pond. Induced breeding by hypophysation uses pituitary gland extract; eggs are incubated in a hatchery and spawn reared in hapas.
Worked example
A 1 ha pond is stocked at 10,000 fingerlings with the ratio 3 : 4 : 3. Total ratio parts = 10. Catla = 3/10 x 10,000 = 3,000; rohu = 4/10 x 10,000 = 4,000; mrigal = 3,000. If survival to harvest is 70%, about 7,000 fish are harvested; at an average 800 g each, expected production is about 5,600 kg.
Traps that cost marks
- Manuring before liming, or liming a pond that is still full.
- Overstocking without checking dissolved oxygen.
- Measuring oxygen at midday and calling it the daily minimum.
- Forgetting to acclimatize seed to pond temperature before release.