Managing egg death

The factors that can cause dead eggs (eggs that have stopped developing) include excessive handling, overcrowding, high temperature, water hardness, and transport delays. Important hatchery water quality parameters will be reviewed in my other post, please subscribe to this blog to get new information about your farm.

Excessive handling
Embryos in the early improvement stages are sensitive to handling and should be handled as little as possible to prevent mechanical injury.

Overcrowding
Many factors affect the maximum loading rate a hatchery can sustain. Generally, 1 to 2 pounds (0.45 to 0.9 kg) of egg mass can be incubated in a single hatching basket 8 inches wide x 16 inches long x 4 inches deep (20 cm x 41 cm x 10 cm). Egg masses should not overlap substantially. Over-crowding causes poor water circulation and makes it easier for diseases to transfer between egg masses.

Temperature
Temperature is a significant environmental factor that affects egg improvement, hatch rates and disease susceptibility. Newly spawned eggs are more sensitive to temperature changes than eggs more than 24 hours old, in which the embryonic outline (the beginnings of a catfish fry) can be observed (Fig. 4b). If eggs less than 24 hours old are moved from a pond to hatchery water of a substantially different temperature, they must be acclimated to prevent a high mortality rate.

If the water temperature of the pond, transport container, and hatchery differ more than 5 to 7 °F (2 to 3 °C), eggs should be water-tempered for 15 to 20 minutes for each 5 °F (2 °C) of difference. Eggs can be tempered by using a hose to slowly run hatchery water into the transport container until the water temperature in the container matches that in the hatchery. The optimal temperature range for incubating catfish eggs is 78 to 82 °F (26 to 28 °C). At temperatures above and below this range, hatch rates will be reduced by egg death and disease (Fig. 5).

Transport delays
On larger farms, difficult logistics may cause eggs to sit in spawning cans at the pond side or in transport containers on vehicles for prolonged periods of time. Eggs should not be left on the pond bank for more than 15 to 30 minutes because long transport times and poor water conditions during transport (e.g., temperature and dissolved oxygen) result in egg death. In fact, it has been reported that egg masses left unprotected in cans on the pond bank for 30 minutes have up to 25 percent lower hatch rates than eggs transported quickly.

If transport time may exceed 30 minutes, eggs should be put in insulated containers with well-oxygenated water (> 5 ppm dissolved oxygen). Fill transport containers with pond water to help prevent shock caused by differences in water quality and temperature. When the quality of pond and hatchery water is significantly different, be sure to temper the eggs by slowly exchanging the transport container water with hatchery water.

Water stability
The calcium component of water hardness plays an important role in catfish fry development. Hatch rates from eggs incubated in water with less than 10 ppm calcium hardness during the first 24 hours after spawning are reduced by as much as 70 percent. Low calcium stability during later stages of development can cause up to a 25 percent reduction in hatch rates. For this reason, it is important to maintain adequate calcium hardness in the hatchery water—a minimum of 20 ppm, especially during the first 24 hours after spawning. During periods of low calcium concentration, as when a metering pump fails, newly spawned eggs (less than 24 hours old) can be left in the pond an extra day or a calcium chloride solution can be added to hatching troughs designated for new spawns.