Management Guide 3: Trout Logic
Troutlodge works hard to ensure you have good results with our eggs. We utilize a combination of advanced machinery and careful personal care to ensure...
Trout Logic Guide: Incubation and Hatching
Troutlodge works hard to ensure you have good results with our eggs. We utilize a combination of advanced machinery and careful personal care to ensure you achieve high hatch rates. Superior genetics provide a good start, but incubation systems and the hatchery process also play a vital role in contributing towards long-term growth and survival of your fish. Understand more about the water quality suggested for incubation and what equipment is required in our third edition of Trout Logic: Incubation and Hatching.
This management guide series covers the following topics:
During the early-rearing stages, fish are more susceptible to disease, so it is imperative to use the best water in your hatchery system.
A good start is critical to the long-term growth and survival of the fish. During the early-rearing stages, fish are more susceptible to disease, so it is imperative to use the best water in your hatchery system. Water should always be “first-use” and should preferably be from an isolated, uncontaminated spring or well (borehole) as stream or river water may often become turbid with fine particles that are not ideal for eggs or alevins. If stream or river water is used, then some form of filtration should be included at the intake point in the hatchery design. Ideally, hatchery systems should be flow-through (single pass) and utilize cold, pathogen-free water. There is increasing use of re-circulated water in hatcheries although it is essential to ensure good filtration and sustained oxygen levels as recommended under water parameters in the next section.
If there is any possibility of incubating water becoming turbid it is recommended to use some form of filtration before the water enters the incubators and particularly the early rearing ponds as suspended solids (even very fine solids) should be avoided as they are harmful to the gills of the alevins and can cause various complications and mortality. Various filters are available including sand and micro-screen drum filters as pictured.
The use of ultra-violet or ozone treatment should be considered in recirculation systems or if there is any possibility of any bacterial contamination in the flow-through system.
Ultra Violet (UV) filters
Water quality parameters
The following are the optimum water quality parameters for incubating eggs, hatching eggs and early stages of the alevin and fry life cycle:
- Temperature. The ideal range of incubating and hatching eggs is 8 – 12⁰ C (46 - 54⁰F) although temperatures of 4 - 19⁰ C (39 - 66⁰F) are tolerable for short periods.
- Dissolved oxygen. The oxygen levels should be at > 95% in the incoming water and > 75% exiting the eggs or alevin ponds.
- pH. pH should be in the range 6.7 – 8.0
- Dissolved gas. (Nitrogen) should be < 105% or problems are likely to occur with gas super-saturation or “gas bubble disease”.
- Alkalinity/Hardness. The alkalinity should be > 75mg/litre.
- Chemicals and minerals. There should be either a complete absence or only a trace amount of elements such as ammonia, cadmium, chlorine, copper, hydrogen sulphide, lead, mercury and zinc.
- Light levels. All incubation and hatching of eggs should be done in low light levels and direct sunlight should always be avoided.
Water Temperature °C
Oxygen Saturation %
Carbon Dioxide mg/ml
Dissolved Gas (Nitrogen) %
Table 1. Water Quality Parameters for Rainbow Trout Eggs and Juveniles
Water flows for hatching eggs
Different hatching systems may demand different water flows but as a general rule, in a flow –through or “single pass” system, a minimum flow of 4-6 litres per minute per 100.000 eggs at temperatures below 15⁰C (59⁰F )is recommended to provide adequate oxygen. The oxygen saturation of water is dependent on temperature so, at temperatures above 15⁰C (59⁰F), the flow should be increased. If possible the oxygen levels should be monitored and should not fall below 6 ppm (parts per million). In some re-circulated systems it may be necessary to introduce oxygen to the water if the levels fall below those recommended.
The amount of water flowing through eyed-eggs should not be excessive, moving the eggs too rapidly or violently, although an agitation or slight tumbling of the eggs in the incubators (especially upwelling incubators as will be described further in this guide) is useful to prevent the growth of fungus such as Saprolegnia sp. and also to remove/wash off egg shells when the alevins start to hatch.
Main methods for incubating and hatching eyed-eggs
Multiple incubation systems exist, and it is important to find one that is reliable, efficient and best fits your resources and needs.
Main methods for incubating and hatching eyed-eggs
Once you have gone through the steps of receiving and counting the eggs, it is time to introduce them into your incubation units. As with all steps in the hatchery process, this should be done with great care by trained personnel only. Multiple incubation systems exist, and it is important to find one that is reliable, efficient and best fits your resources and needs.
The three most common types of incubation units are:
● Vertical incubators (“Heath trays or stack”).
● Horizontal incubators (“California baskets or trays”)
● Upwelling incubators (“Jars”)
(“Heath trays or stack”)
There are a variety of vertical incubators available. Most modern vertical incubation systems are constructed from GRP (Glass Reinforced Plastic) or reinforced non-toxic plastic, and are durable, easy to clean and disinfect. The most commonly used is the “MariSource” system as illustrated. The principal of a vertical incubation system is that water enters a channel in the top tray, upwells through the egg tray and flows over the front wall into a channel which feeds the next lower tray unit and onwards to the bottom tray.
For hatching, eggs should be placed no more than 2 layers deep in each tray or approximately 12500 – 15000 eggs per tray.
Vertical Incubation System
Advantages of the vertical incubation system include:
- Excellent use of available floor space as most units may be stacked 8 or 16 trays high,
- Efficient use of water supply,
- Ability to isolate individual batches or to remove individual trays for monitoring or management.
- Safety of eggs and newly hatched alevins as trays are covered by a screen mesh to prevent eggs or alevin being washed out.
Disadvantages of the vertical incubation system include:
- Once alevins start to swim up for first feeding they have to be removed to tanks or ponds,
- Air bubbles can be trapped under the mesh causing egg mortality,
- The eggs or alevin are not all easily observable and trays have to be removed for observation.
- Requires cleaning and management of eggs including removal of dead eggs,
- More expensive than other systems.
(“California baskets or trays”)
There are a variety of commercial horizontal incubators available. As with the vertical incubators these are usually made from GRP or reinforced non-toxic plastic. The principal of a horizontal incubator is that baskets (trays) usually about 40cm x 40 cm in size are placed in a trough in series (one after the other). The number of baskets per trough may vary according to hatchery space and water flows but usually 4 – 8 baskets are placed in each trough. The baskets are screened and flat-bottomed and sit off the bottom of the trough. At the end of each basket is a partition that extends to the trough bottom forcing the water upwards through the eggs. It is important that the sides of the baskets fit tightly in the trough to prevent water flowing around the side of the baskets.
Most hatcheries utilise horizontal incubators to hatch their eggs in baskets in which the mesh size at the bottom of the basket is a suitable size to allow newly hatched yolk-sac fry (first hatched alevins with the yolk-sac still unabsorbed) to fall though the mesh into the trough below. Once hatching is complete the baskets are simply lifted out with any remaining dead eggs and egg shells and the alevins are then allowed to swim up and commence feeding in the same trough.
Advantages of the horizontal system include:
- Ease of use,
- Inexpensive and may be custom built on site,
- Ability to see, monitor and work with eggs easily,
- Efficient use of water supply,
- Ability to hatch eggs in the same tanks used for hatched alevin and first feeding,
- Optimum height for hatchery staff to work.
Disadvantages of the horizontal system include:
- Requires more space than other systems
- Requires cleaning and management of eggs including removal of dead eggs.
Figure 1. Horizontal incubator - California System draw
Horizontal incubator - California System photo
Horizontal system with incubation separators
Historically upwelling incubators were used primarily for the incubation of eggs until the eyed-stage. However more and more hatcheries now use upwelling incubators for the hatching of eggs. As the name suggests, upwelling jars are designed so that the water flows in from the bottom and out of the top. It is the upwelling water that delivers oxygen to the eggs, and it is very important that this water flow be equally distributed throughout the jar. This is usually achieved by placing some form of diffuser mechanism beneath the eggs (usually a plate, porous pad, or marbles). The water passes through this diffuser before reaching the eggs.
The young alevins (yolk-sac fry) can be left in these incubators almost until the time of swim-up. The upwelling incubators are placed in the rearing tanks and when the fish become more active the majority will swim out on their own whilst the remainder will need to be poured out. The pouring out of the remaining eggs needs to be done gently and with extreme care to ensure that the young alevins are not harmed. The water height of the trough into which the alevins will be reared should be ½ the height of the upwelling incubator so that alevins swimming or washed out are not harmed by the fall. Some commercially available incubators also provide a mesh screen that may be placed on the top of the incubator in the event you do not want the fish to swim out into a rearing tank and may want to move them after hatching into an alternate tank.
As you will see in the photos that follow there are additional pipes, one external and one internal, which are used to allow for trapped air bubbles to escape. If these are not installed and air becomes trapped in the bottom of the incubator this air will usually escape as a large bubble which can result in a significant amount of eggs being blown out of the incubator. Upwelling hatching jars maintain adequate circulation by using the water flow to partially suspend the eggs. Many upwelling incubator manufacturers will advertise a total amount of litres that can be incubated but these numbers are often exaggerated and trials should be conducted in individual hatcheries. When hatching eggs upwelling incubators should contain no more than two -thirds of the total incubator volume in eggs. The flow rate in upwelling units should be adjusted so that eggs are suspended approximately 50% of their static depth (i.e. if eggs are 10 cm deep with water off, they should be approximately 15 cm deep with water flowing). A correctly adjusted water flow will create a gentle rolling effect, ensuring that dead eggs are moved about and washed out of the incubator to prevent the development of fungus such as Saprolegnia sp.
Figure 2. Basic diagram of upwelling incubator
Advantages of the upwelling incubation system include:
- Ease of use
- Efficient use of water supply
- Self-cleaning and removal of dead eggs and egg shells with outflow
- Labour saving
- Reduction in fungal infections
- Ability to hatch eggs in rearing tanks with minimal transfer needs
- Efficient use of space
Disadvantages of the upwelling incubation system include:
- Constant monitoring of flow rates is required to ensure eggs are suspended at correct level
- Possible loss of alevins due to air bubbles or uncontrolled increase in water flow.
Upwelling incubators in rearing trough
Care of eggs during the hatching process
Whatever type of incubation system is used it is important to care for the eggs during the final stages of their incubation and hatching process. This includes
- Monitoring the water flows to ensure that the eggs are being provided with adequate oxygen. If possible the oxygen levels should be regularly monitored and maintained at the levels recommended on Chapter 2 - Water Sources.
- Keeping the eggs clean of dead eggs and any fungus – especially in vertical or horizontal incubators. There are a variety of ways to do this but it requires manual removal of all dead eggs and any fungus as frequently as possible and not less than once a day. Specially designed egg “tweezers” or a suction bulb and pipette are the best ways of removing dead eggs or fungus as these can be used with minimal disturbance to the live and healthy eggs or newly hatched yolk-sac fry.
Saprolegnia is a freshwater mould or fungus (sometimes called “cotton mould” due to its likeness to cotton wool) that infects dead fish eggs and alevins in the hatchery. However the fungus grows very fast and if not removed from the incubation units will quickly cover healthy live eggs and suffocate or infect them.
The best method of keeping a hatchery free of saprolegnia is to keep the incubators clean of all dead matter by constantly picking out all dead matter manually. Treatments of formalin and even salt are reported to be effective against saprolegnia infections.
Extreme care should be taken when using such treatments, especially formalin, since the wrong dose or exposure can be toxic to eggs and alevins, and the use of formalin raises concerns for both users and the environment. A commercially available fungicide, Pyceze (Bronopol), for use with trout eggs is available, but reports on its efficacy are inconclusive.
It should be noted that the use of malachite green, which was widely and effectively used for the treatment of saprolegnia, is now banned and illegal due to its carcinogenic properties.
Therefore, the most effective way to keep a hatchery free of saprolegnia is to constantly and manually remove dead eggs and alevins.
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Post hatching care and ponding
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.
Post hatching care and ponding
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.
Different incubation systems will require different handling:
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.
Time of ponding alevins
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.
Pictures showing alevin development
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
First feeding of alevins
As the alevins consume (metabolise) the yolk to meet their energy needs their wet weight increases. This occurs because the tissue (muscles, organs, etc.) have a higher moisture content than the yolk does.
First feeding of alevins
At the time of hatching, trout alevins have a large reserve of yolk remaining from the eggs which is why they are referred to as yolk-sac fry (Picture 1 & 2 in the previous chapter). In a yolk-sac fry the alevin wet weight is made up of approximately 70% yolk and 30% embryo. This yolk is denser than water causing the yolk-sac fry to live at the bottom of the incubators or early rearing pond. The membrane that surrounds the yolk is very sensitive to damage and external abrasion. Consequently yolk sac fry should be handled sensitively or not at all at this stage. As the alevins consume (metabolise) the yolk to meet their energy needs their wet weight increases. This occurs because the tissue (muscles, organs, etc.) have a higher moisture content than the yolk does. 1 gram of yolk is converted to 2 – 3 grams of tissue. The alevin weight continues to increase until just before the completion of the yolk absorption. This stage is called the Maximum Alevin Wet Weight (MAWW) and occurs at the optimal time of “ponding” the fish into the early rearing tanks and the start of feeding.
Figure 1. Maximum Alevin Wet Weight (MAWW)
Water flows in early rearing ponds
It is very important to ensure that once the alevins have been placed in the early rearing ponds and at the time before and after “swim-up” 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 against too strong a current. 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 of inadequate or excessive flows high mortality is likely to occur. A useful tool is to use a substrate mat in the bottom of early rearing tanks. A picture on the following page illustrates a typical substrate mat. These are commercially available in a variety of styles and can also easily be made on site. The principal is that the alevin can find respite from the flow for some periods and are able to rest.The first feeding of trout is critical to ensure they get the best possible start in life. The type of feed, frequency of feeding and manner in which the feed is presented are all important. It is recommended that the alevins are fed the best diet available. Often these specialty diets may seem expensive but little feed is required and low cost diets are rarely the best ones for your fish.
It is recommended that a recognised feed supplier gives advice on the best feed suited to the local conditions of the site. Feeding charts are also available from most feed suppliers. 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.
Substrate mat in bottom or early rearing pond
Substrate mat in bottom or early rearing pond
Once a quality feed has been selected and the amount of feed determined, the next consideration is how to feed the fish. First-feeding alevins 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 alevins need to be challenged by the feed to start feeding. After this period, an automatic feeder is most practical, with two or three additional 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 consume 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).
Good feed distribution assures easy access to the feed and will help to achieve size uniformity within the population. 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 of the fish during feeding. Do not overfeed. Once feed settles to the bottom of the tank, small trout will ignore it. Excess feed leads to deterioration of water quality and promotes disease. Remove excess feed from the pond promptly.
Once all the alevins are actively feeding an automatic feeder can be used. Automatic feeders come in a variety of makes from a number of equipment producers. The principle in all automatic feeders for alevins is that small amounts of feed are distributed into the rearing tank constantly during the day. The most commonly used automatic feeder for alevins is the clockwork belt feeder. These are usually produced in a 12 and 24 hour option and work on the simple principle that a belt attached to a clockwork mechanism is evenly loaded with the day’s feed allocation. During the feed period the belt moves forward distributing small amounts of feed throughout the desired period. The only drawbacks 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.
Automatic clockwork belt feeder
Clockwork belt feeder