Early Rearing

In the rare case that you experience premature hatching either in the box or immediately after transfer into your hatchery, it is important to note that in most cases these fish are still viable and 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 opened and the stock inspected at least once daily. Any eggshells and dead eggs should removed. It is important to ensure that the fry are at the appropriate development stage before placing in the rearing ponds. This is usually when 90% of the population are ready to swim up and take feed, which corresponds with 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. This will allow remaining fry to fall through before removing tray together with any eggshells. If the mesh size at the bottom of 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 placed in the rearing ponds at the optimum strength and size.

With upwelling incubators, the eggs shells and dead eggs should have flowed out of the incubator during the hatching process as they are lighter than living eggs or yolk sac fry. These shells and dead eggs should be removed from the rearing troughs into which they flowed. Ensure that the flows are not sufficiently strong or the depth of the eggs not so great that healthy fish flow out of the incubators together with the eggshells. Once the yolk sac fry have completed absorption, become more active and start swimming they will be carried with the flow of water upwards and out the exit of the incubator 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 minimum mortality and maximum growth rates. If ponded too early, as in Pictures 1, 2 and 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.

1-day post-hatch

4 days post-hatch

14 days post-hatch

21 days post-hatch
Swim-Up Fry ready for first feeding

It is very important to ensure that once the alevins have been placed in the early rearing ponds that the water flows are correct. The flow should be adequate to supply all the alevins with sufficient oxygen but not too much that will cause the alevins to become exhausted from swimming.

Ideally the alevins should be well distributed throughout the early rearing pond. If the alevins are all observed crowded at the inlet it is likely that the flow is inadequate to supply sufficient oxygen. If the alevins are all observed crowded at the outlet or pushed against the outlet screens it is likely that the flow is too strong. In both cases high mortality is likely to occur.

It may be useful to place a “substrate mat” in the bottom of the rearing tank at this point of development. The principal is that the alevins can find respite from the flow for some periods of time and are able to rest.

Substrate mat in bottom of early rearing pond.

The first feeding of trout is critical to ensure the best production from each batch of fish. The type of feed, frequency of feeding and the way the feed is administered are all important. It is highly recommended that the fry are fed a diet specifically designed in content and appearance. Using a reputable feed producer is highly recommended. Often these diets may seem expensive but little feed is required and low-cost diets are rarely the best ones for your fish. See Troutlodge study report “The Effects of Feed Rate and Feed Type on the Early Growth of Troutlodge Fry” located on the resources section of our website.

Feed suppliers provide feed rate charts for each of their diets. Diets for fry and fingerling trout require a higher protein and energy content than diets for larger fish. Fry and fingerling feed should contain approximately 50 % protein and 15 % fat. Avoid the common mistake of grinding down (cheaper) feed for larger fish as this often does not contain the correct amounts or ratios of important ingredients needed by first feeding fish.

Once a quality feed has been selected and the amount of feed determined, the next consideration is how to feed the fish. First feeding fry should be fed a small amount by hand at least ten times per day until all the fish are actively feeding. The principal is less feed more frequently as the fry need to be challenged by the feed to start feeding.

After this period, an automatic feeder is most practical, with two or three hand feedings daily to observe the fish. As the fry grow, frequency of feeding can be gradually decreased to about five times per day. Trout can hold roughly 1% of their body weight in dry feed at each feeding, so frequency should be adjusted accordingly.

Fry gain weight rapidly so they should be sample counted weekly for the first 4 to 6 weeks on feed and the daily feed ration adjusted according to their weight.

Feed should be distributed over at least 2/3 of the water surface when fry are less than 5cm (2 inches). If feed is only introduced at the inlet it is likely that only the strongest and largest fish will eat and there will not be uniform growth. Do not introduce the feed too close to the outlet screens or it may be washed out before the fish have a chance to eat it.

Though the use of a published feeding chart is strongly recommended, charts are only guides and individual judgment should be exercised based on observations. Do not overfeed. Feed settling at the bottom of the tank will remain uneaten. Excess feed leads to deterioration of water quality and promotes disease. Remove excess feed from the pond promptly.

Once all the fish are actively feeding an automatic feeder can be used. Automatic feeders come in a variety of makes from several equipment producers. The principal in all automatic feeders for fish is that small amounts of feed are distributed into the rearing tank constantly during the day.

A possible drawback of any automatic feeder is that it is advisable to have more than one in the early rearing ponds as a single feeder will distribute the feed in a single place and often the larger and stronger fish dominate the area precluding smaller and weaker fish from obtaining adequate feed. It is also important to check frequently that the feeder is working and that the fish are receiving a regular supply of feed.

Clockwork belt feeder