By Agricmasters Team
A hatchery can lose a third of a batch between stripping and swim-up, and none of it looks like a disease headline. It happens in the 28 to 32 hours between fertilization and hatch.
Inside that window, two things decide the batch: water and time. Medicine is third.
Know your clock first
At 25 °C, Clarias gariepinus eggs hatch about 28 to 32 hours after fertilization (Delince et al. 1987, after Hogendoorn and Visman 1980). The incubation period shortens as temperature rises, and hatching can be accelerated by pushing water temperature toward about 30 °C. The relationship is close to inverse: warmer water, faster hatch.
That clock is the frame for everything else. When to check the trays, when to count the white eggs, when to expect larvae, when to treat — all of it is set by your water temperature, not by the calendar. A hatchery that does not measure incubation temperature is guessing at all five.
Water is the first tool, not the drug
The FAO/ARAC seed-production manual (Delince et al. 1987) still reads as a sane bench standard:
– Dissolved oxygen 5 to 6 mg/L in the incubator, preferably saturated.
– A flow of 1 to 2 L/min for an 80 to 100 L incubation trough, or an air-lift if you incubate in stagnant water.
– Clean, plankton-free water. Copepods, insects and insect larvae scratch the egg shell and open the door to infection.
– One single layer of eggs per tray. More than one layer appears on the manual’s own list of causes of abnormal egg mortality.
– A load of about 100 to 150 g of eggs per 80 to 100 L trough. Clarias eggs run 600 to 900 per gram, so that is 90,000 to 135,000 eggs per trough, and the manual expects roughly 50 % viable larvae (45,000 to 67,500).
One timing detail that catches people: catfish eggs are sticky, and the stickiness is strongest 30 to 60 minutes after contact with water. If eggs are going onto trays or substrate, do it within the first minute after mixing, not the first ten.
What normal loss looks like
The manual is blunt about it. About 3 to 5 % of the eggs die a few hours after incubation starts, and a slightly higher share, 5 to 10 %, die near the end of development or during hatching. That is normal. Higher losses are not, and the usual causes are listed plainly: a wrong latency period between injection and stripping, stressing the females during that latency, incubating more than one layer of eggs, poor incubation conditions, or a fungicide dose that is too high.
That last one is worth keeping in view. The treatment can cause the problem.
The fungus
Saprolegnia is the water mold that eats a hatch. It does not need a healthy egg to start. It starts on dead eggs, which turn white and opaque within hours, and then spreads onto living ones.
The single most important thing to understand about egg fungus is that the dead egg is the fuel. A tray with poor flow and no dead-egg removal is not unlucky. It is a fungus incubator with a fan.
Which is why the first control is mechanical: look at the trays, take out the white eggs, keep the water moving. It is labor-intensive, and it is still the most reliable thing in the room.
The chemistry, honestly
Malachite green was the standard for decades. The FAO manual recommends 5 ppm for 10 minutes, applied just after incubation and again 10 to 12 hours later, or a 0.1 to 0.2 ppm flow-through prophylaxis. Do not use it.
Malachite green sits on the FDA’s list of compounds with high regulatory priority — drugs that “should never be used in food animals”. It is banned for food fish in the EU and elsewhere. Any manual still listing it as the treatment is describing history, not practice.
What is used now, where the products are available:
– Hydrogen peroxide. FDA-approved as 35 % PEROX-AID for the control of mortality in freshwater-reared finfish eggs due to saprolegniasis. The label dose for warmwater finfish eggs is 750 to 1,000 mg/L for 15 minutes in continuous flow, once a day or on alternate days until hatch.
Cold- and coolwater eggs get 500 to 1,000 mg/L. The label itself warns that species sensitivity differs and recommends a small bioassay before treating the whole batch.
– Formalin. Approved for use in finfish; the historical channel catfish egg protocol is a 15-minute bath at 1,000 to 2,000 µL/L daily.
– Neither is a magic bullet. A 2019 University of Georgia study on walleye eggs found improved hatch at 100 mg/L hydrogen peroxide in one year and no effect the next, and no benefit from treating twice daily (Eroh 2019). The study’s own conclusion is worth repeating in plain language: the most straightforward control remains the removal of dead and infected eggs.
A practical note for West African hatcheries: these are US label indications, written for products that may not be on your shelf. Any off-label use belongs in the hands of a fish health professional, not a pond-side guess. The dose that helps and the dose that kills are close together, and the FAO manual lists too-high fungicide concentration as a cause of mortality in its own right.
Hatching, and separating the larvae
A counterintuitive one: hatching is slightly slower in running water than in stagnant water, because the current washes out the hatching enzymes. If a hatch is dragging, check the temperature and the flow before changing anything else.
Then come the larvae. Healthy ones are about 4 mm and 1 to 1.5 mg, and they head for shadow and shelter. In perforated incubation trays they swim down through the holes, leaving dead eggs and shells behind. In mosquito-net trays the separation is less clean, because deformed larvae fall through too. Once hatching is complete, remove the trays and siphon the crippled larvae on the second day, not the first: on day one the viable larvae cannot swim well yet, and early siphoning wastes them.
The short version
1. Measure water temperature. Know the hatch clock (roughly 28 to 32 h at 25 °C).
2. Oxygen 5 to 6 mg/L, flow 1 to 2 L/min per 80 to 100 L, clean water, one layer of eggs.
3. Move eggs onto trays within a minute of water contact.
4. Expect 3 to 5 % plus 5 to 10 % loss. Investigate anything more.
5. Look at the trays. Remove the white eggs. That is the first fungicide.
6. If you treat, use an approved product at the label dose. Never malachite green.
7. Siphon deformed larvae on day two, not day one.
Fungus in an incubation tray is usually not bad luck. It is a flow problem with a dead egg attached.
Agricmasters is a catfish hatchery extension, working with Rigor Mortis Ventures, Nigeria.
