Twenty-Two Per Cent Hatch: Diagnosing a Rohu Spawning Failure
A private hatchery in Bankura runs its first rohu spawning of the season in the last week of April. A 9 kg female and two 4 kg males are selected from a stock tank where they have been held for weeks together with silver carp broodstock. Water for the breeding pool comes from a tube well in which iron floc is visible, mixed 1:1 with pond water; the pool temperature is 24 °C. The female receives two intramuscular doses of pituitary extract six hours apart, at 2 mg/kg and then 5 mg/kg. She is stripped six hours after the second dose. Stripping yields only about 250 g of eggs, which are hard and greyish. The eggs are incubated in a Chinese circular hatchery at a flow of 4 L/min. By 30 hours about 22% have hatched, and the unhatched eggs are covered with white cottony tufts that spread to the neighbouring eggs. The owner blames the hormone dose and wants to triple it for the next spawn.
1. Is the hormone dose the likely cause? Assess the dose used and identify the more probable reasons for the low egg yield.
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The dose is not the problem. A two-dose pituitary extract regime of about 2–3 mg/kg followed by 5–8 mg/kg for a female, with 2–3 mg/kg as a single dose for males at the second female dose, is a normal protocol for Indian major carps. Tripling it would risk over-dosing, which produces egg hardening, poor ovulation and sometimes broodstock mortality; it must not be done.
The yield is the real signal. A rohu female produces roughly 1.0–1.2 lakh eggs per kg, so a 9 kg female should give on the order of 9–11 lakh eggs, or about 2–2.5 kg of eggs. Only 250 g were obtained - perhaps 5% of expectation. Two features explain it. First, the eggs were hard and greyish, which points to incomplete ovulation, most often from stripping too early. At 24 °C the latency after the second dose should be longer than the standard 5–6 hours at 27–28 °C; at 24 °C waiting 8–10 hours is more appropriate. Second, broodstock condition is suspect: the female was held in a tank shared with silver carp, so competition for feed and space, and possibly poor ovarian development, would depress fecundity.
Other contributing faults: tube-well water with visible iron floc was used without settlement or aeration; and 4 L/min in a circular hatchery is too low to keep eggs rolling, so they settle in clumps and die.
2. Explain the 22% hatch and the spreading cottony tufts, linking them to specific failures in the incubation system.
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Three failures compound each other.
1. Water and temperature. Iron in the tube-well water precipitates as ferric hydroxide when it meets oxygen, coating the egg membrane and blocking the pores through which the embryo exchanges gases. It also consumes oxygen as it oxidises. At 24 °C development is slow, so the eggs sit in the incubator far longer than they should - days instead of the roughly 18–24 hours expected at 26–28 °C - and the exposure window for pathogens lengthens.
2. Flow and egg density. At 4 L/min the eggs are not kept in gentle suspension. Settled eggs suffocate, die, and become substrate for Saprolegnia, the water mould producing the white cottony tufts. Once established on dead eggs, hyphae spread to adjacent live eggs; that is why hatch fell to 22% rather than simply matching the fertilisation rate. Low flow also concentrates metabolic ammonia around the eggs.
3. Dead-egg management. The tufts spreading indicates that dead eggs were not being removed. In a working hatchery, dead white eggs are siphoned out or the tank is treated with a salt dip, and the flow is increased so no egg sits still.
The 22% figure is therefore not a single-cause number: poor egg quality from premature stripping, iron-coated water, sub-optimal temperature and inadequate flow all push in the same direction.
3. Rewrite the spawning and incubation protocol for the next attempt, with the numbers you would use.
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Broodstock
- Keep male and female rohu in separate broodstock ponds, not mixed with silver carp, and feed a protein-rich brood diet for at least 4–6 weeks before spawning.
- Select females with a soft, distended abdomen and a reddened vent; check that eggs sampled with a catheter are round and translucent.
Water
- Store tube-well water in a settlement tank for at least 24 hours with aeration so iron oxidises and settles, then filter before use; do not mix freshly pumped well water straight into the breeding pool.
- Hold the breeding pool at 26–28 °C, ideally by using a greenhouse cover or by spawning later in the season when ambient temperature is higher.
Hormone
- Female: pituitary extract 2–3 mg/kg first dose, 5–8 mg/kg second dose 6 hours later; males 2–3 mg/kg at the time of the second female dose. A commercial GnRH preparation at its label rate (of the order of 0.3–0.5 mL/kg for females) is an acceptable alternative.
- Strip 6–8 hours after the second dose at 27–28 °C, and 8–10 hours at 24 °C; strip into a dry bowl, fertilise immediately with milt from two males, and add water only after mixing.
Incubation
- Stock eggs at a moderate density and raise the flow to roughly 8–10 L/min per tank so eggs roll gently and never settle.
- Siphon dead white eggs twice daily; if fungus appears, give the eggs a short salt dip and increase flow.
- Expect hatch in 18–24 hours at 26–28 °C; record fertilisation at the 8–10 hour stage so problems are identified early.