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-eggsOnce 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”)Vertical Incubators(“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.

Horizontal Incubators (“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

Upwelling Incubators("Jars")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 processWhatever 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.

Suction bulb and pipette

Egg Tweezers

Saprolegnia sp.

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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