Chapter 5: Incubation and Post Hatching Care of Fry
Better hatching success can be expected when it is possible to incubate the eggs in a hatchery settling, that is, if the eggs can be collected safely. Some fish build nests that are not practical to transfer to the hatchery or attach their eggs so firmly to a substrate that it is not practical to remove the eggs. In such cases, it may be best to leave the eggs in their natural settling, allow the parents to guard the eggs, then collect the fry after hatching. Another alternative would be to provide a substrate for egg deposition, then transfer the substrate to a better environment for incubation (See Chapter 3 on egg/fry transfer). Eggs and the associated substrate can be transferred to a prepared nursery pond where the eggs will hatch and the fry can develop. This approach is disadvantageous in that the magnitude of disease problems during hatching are hard to quantify and even harder to control. This results in an unknown quantity of fry in the nursery pond. Past production records from ponds where similar production practices were used can be used as a guide as to the quantity of fish that can be expected.
An alternative is to transfer the substrate holding eggs to the hatchery where the substrate can be rinsed to remove any sediment or debris and the eggs treated with formalin to control fungus. The substrate with eggs attached can be held in aerated aquaria or tanks that receive a 2 times/h water exchange. A given aquaria or tank should be stocked with egg-bearing substrates holding eggs of similar age. This will facilitate the harvesting of similar age fry. If the eggs take ≥48 h to hatch they should be treated twice a day with formalin at 100–250 mg/L for an hour until 12 h before the anticipated hatching. In some cases where the eggs can be removed from the substrate, i.e. Channel Catfish Ictalurus punctatus, the eggs can be collected from the spawning pond and transferred to the hatchery for incubation.
A cleaner way for a natural spawn is to bring the brooders into the hatchery, provide the appropriate substrate and water quality, and then allow the fish to spawn naturally, or stimulate the fish to spawn naturally following the administration of an exogenous hormone. In either case, the substrate and its eggs will be cleaner than natural spawns collected from a pond and the eggs should have less disease problems and a higher hatch rate. Once the egg-bearing substrate is collected, it can be incubated as described in the previous paragraph.
Many species of fish with adhesive eggs have fry that initially will remain in the nest or substrate until more of the yolk sac is absorbed, then the fry will start swimming up into the water column. Where the substrate cannot be easily transferred to the hatchery for incubation it is often easier to collect the fry after they have begun to swim-up versus trying to collect recently hatched fry often hidden in the nest or substrate where the eggs were attached. Where the substrate can be transferred to the hatchery and suspended vertically in a clean aquarium, the sac fry will drop to the bottom of the aquarium and can be siphoned out daily and stocked into new tanks along with other sac fry of a common age.