While the fish may be hatching early, in most cases they are still viable fish that with proper care will behave and grow like a fish that hatched on a normal schedule.
Once hatching begins, normally 6-7 days after arrival, it will normally be completed within 2 to 4 days after starting. The exact amount of time it takes for all of the eggs to hatch depends on water temperatures and hatchery conditions, as well as the individual lot of eggs. It is important to quickly remove the shells left behind by the hatched eggs.
In the rare case that you experience premature hatching either in the box or immediately after transfer into your hatchery, it is important to try to save these fish. While the fish may be hatching early, in most cases they are still viable fish that with proper care will behave and grow like a fish that hatched on a normal schedule.
With vertical incubators the trays need to be carefully opened at least once daily and any egg shells either allowed to wash/flow-out, taking care not to let any fry escape, or the trays need to be manually cleaned to remove any egg shells, dead eggs or saprolegnia fungus. It is important to ensure that the fry are strong enough before placing in the rearing ponds which is usually done between 14 and 20 days post-hatching depending on hatchery water temperatures.
With horizontal incubators it is optimum to have a mesh size at the bottom of the tray that allows the yolk-sac fry to fall through into the rearing trough below. Once all hatching has taken place the trays can be gently shaken to allow remaining fry to fall through before removing the trays together with any egg shells. If the mesh size at the bottom of the tray is too small to allow the fry to fall through then the egg shells and any dead eggs should be manually removed and, as with vertical incubators, the fry should be placed in the rearing ponds at the optimum strength and size.
With upwelling incubators the eggs shells and dead eggs will flow out of the incubator during the hatching process as they are lighter (specific gravity) than living eggs or yolk-sac fry. These shells and any dead eggs should be removed from the rearing troughs. Ensure that the water flows are not too strong or the depth of the eggs not so great that healthy fish flow out of the incubators together with the egg shells. Once the yolk-sac fry have absorbed the yolk and start to become more active and start swimming they will also swim or be carried with the flow upwards and out the incubator exit into the rearing trough. Any remaining fry in the incubator will need to be carefully poured out into the rearing trough.
Alevins that are placed into the proper rearing environment (ponded) at the optimum time experience little mortality and good growth rates. If ponded too early, as in Pictures 1, 2 and 3 on page 3, the yolk sac of the alevin is highly susceptible to abrasion and physical damage. In extreme cases, this can cause the yolk membrane to rupture, resulting in coagulation of the yolk material (evidenced by turning white) and subsequent fry death at later stages. Alevins ponded at this age still have not attained neutral buoyancy and often crowd at the bottom of the tanks, increasing the risk of suffocation. The situation becomes even more critical if the alevins are now introduced to starter feeds, primarily due to environmental fouling. If fry are ponded after the yolk reserves are completely exhausted the effects are just as problematic. Energy reserves are insufficient to survive the learning phase of feed initiation, and starvation results. Research has frequently demonstrated that fry ponded and introduced to feeds at time of “swim-up” and just prior to MAWW (Maximum Alevin Wet Weight) achieve maximum growth rates and maintain optimum health through this difficult transition period. It is important for every hatchery facility to determine site-specific values for the length of time needed to reach this optimum stage.
The development of these yolk sack fry fish was carried out at 10C. Take this in consideration since the speed of development depends on the temperature.
Picture 1. Day 1 post Hatch
Picture 2. Day 4 post hatch
Picture 3. Day 14 post hatch
Picture 4. Day 20 post hatch
Picture 5. Day 21 post hatch
Picture 6. Day 22 post hatch
