Subsections

• • Hatching trays
  • Roe barrels
  • Water requirement and light
  • Eye stage
  • Bathing against fungi
  • Shock treatment
  • Removal of dead roe
  • Roe care and timing
  • Transportation of roe

Temperature, roe maturation and degree-day

The temperature on the roe affects the maturation greatly but also mortality, for it is not advantageous to use too warm or too cold water. Experience has shown that 4-8°C suits arctic charr roe best. Much higher or lower temperature greatly increases mortality losses. Probably the greatest losses occur at too high temperatures, especially during the first few weeks. It might be better to increase the temperature first after the eyes have developed, if judged beneficial. Too high temperature may lead to smaller yolk sac fry at hatching, and the alimentary tract may be immature when the fry shall receive artificial feed, leading to great losses. Too much of the energy supply in the yolk sac may not be used as needed for tissue differentiation and maturation.

If the temperture is 4-6°C the maturation period from fertilization to hatching is commonly 410-450 degree-day. degree-day are the number of days multiplied with the temperarture, which means that the period is 80-90 days at 5°C, it is 100 days at 4°C, but only 55-60 days at 8°C.

Hatching trays

There are many types of equipment available for roe hatching. Hatching trays which are kept in hatching trough/channel/runway are most common though. In each tray approximately 1.5-2.0 L of roe can be accommodated.The roe are spread out so not much more than two layers of roe cover the tray. The tray has a grid on the bottom and on one side and a partitioning weir, so that the water flow is forced up through the bottom grid, out through the side and then it flows on to (beneath) the bottom of the next tray.The trays are thus arranged in a row in the hatching channel, where the water level is controlled by overflow. Sometimes air pockets are formed beneath the trays, hindering normal flow. They have to be lifted and shaken now and then to get rid of this air.

Copper or zinc (galvanized) tubing, fittings or taps should not be used in fish farming. These metals are poisonous at very low concentrations and the roe and small fry are especially vulnerable.

Figure: Hatching tray with four hatching boxes. In the upper tray there is a sieve on an angle in front of the drainage which is convenient for first feeding alvins.

Figure: Hatching box

Figure: Hatching rug. The yolk sack fries fall through the rips on the upper layer and stay between the layers. There it is dark and quiet and the fries move less.

Roe barrels

Specific roe barrels are much used, especially by large producers. Many liters of roe may be put in a barrel and they do not require much space. The water flow is through an inclined louver or grid at the bottom. At the middle there is a pipe/tube that collects and lets out air bubbles formed in the water. At the top there is an overflow channel. The main drawback is that the roe have to be moved into another container before hatching takes place. These barrels are therefore best suited for roe producers.

Figure: Egg barrowl. It takes 40 liters, is 850 mm in height and 500 mm Ø.

Water requirement and light

In a hatching channel with 4 hatching trays, the water flow should be at least 5-7 L/min. Often 1 L/min per each liter of roe is used as reference. The water requirement of roe is not great, so very little is gained by cutting down on the water supply. Nor should there be so much water used that there is movement of the roe in the trays. It is prudent to check the oxygen content of the outflow now and then, especially if and when changes in temperature occur. The oxygen saturation should be between 95-100% in the outflow water.

The trays are covered during maturation, so the roe are mostly kept in total darkness. It is not beneficial to keep them in too much light. High energy light (e.g. blue and ultra violet light) may damage the genetic material of the embryos, especially in the earlier phases of maturation, increasing the likelihood of malformations. So commonly the roe in the trays are covered until hatching. When working with the roe in the trays, strong light should be avoided, but soft working lights are probably harmless.

Eye stage

When enough pigment has collected in the eye of the developing roe, that it can be discerned through the egg membrane, the eye stage is reached.This happens at midway of the maturation period prior to hatching, at 200 degree-day, and the most sensitive stage of the development is behind. The roe suffer treatment and transport better than before (before the eye stage they suffer handling badly).

Bathing against fungi

Fungi of the genus Saprolegnia are very common in freshwater. The spores therefore are transmitted into the farms with the water, whether it is surface or spring water usually makes no difference, although naturally the amount of infective spores may be variable depending on the source. This type of fungi thrives in circumstances where content of organic matter is high, such as on dead roe or residue in a roe tray. It forms a cotton-like growth covering the dead roe, and if not kept at bay/deterred, it will grow on the live ones and smother them. Its growth must be prevented and most commonly a bathing treatment is used as a remedy, adding the active chemical agent into the hatching channel, allowing it then to slowly wash out with the water (cfr. chapter on diseases). The inflowing water may be turned off for a short while to increase the effect of the bathing treatment.Malachite-green has been much used for this purpose (1:300.000-1:600.000), but in many countries its use is restricted, due to environmental reasons. A dip for a short while (15-30 seconds) in a stronger solution (1:15.000) may be used. It should be mentioned that malachite is a dangerous substance and a protective mask and gloves should be worn during its handling.

Formalin and hydrogen peroxide (500-1000 ppm) have also been used with relative success as well as salt which has also proved of some use.

Bathing against fungi is usually only done in the interval between the water hardening stage up to the stage of eye development. Some farmers bathe two or three times a week as a preventive measure, but others manage with less. Possibly the roe must be bathed two to three times a week during aggressive spells of fungal infections. Usually the bathing is started a week after fertilization and stopped at the eye stage. At that stage the roe tolerate handling better and dead roe may be removed, using suction in a narrow soft tube for picking or syphoning the dead roe away.

Shock treatment

Dead roe whiten in colour when water gains access through its protective membranes, coagulating its protein supply.In every batch of roe there are always some roe that are sterile or not fertilized, and they may be recognized from their lightyellow colour, being slightly paler than the fertile ones. To make them easier to spot and remove, their protective membrane is disrupted using a so-called shock treatment, whereby they turn white, distinguishing them clearly from the healthy roe. The protective membrane of non-fertilized roe is more sensitive to harsh treatment. The whole roe batch is allowed to drop in running water between two containers. The batch may be poured into another container from a height of 50 cm, or it may be convenient to use a syphon between the two containers, allowing the drop to be ca. 50 cm from the lower end of the syphon tube. The day after, the white roe are easily spotted and may be removed. Shock treatment takes place after eye stage is reached.

Removal of dead roe

To minimize the growth of fungi in the hatching tray, it is important to remove the dead roe.The roe should however not be subjected to much handling in their most vulnerable stage. They seem to tolerate handling for a few hours after water hardening, but from then on should not be handled until after the stage of eye development is reached.

Automatic equipment is available for removing the dead roe, rather expensive though, which makes handpicking them a common choice. Then, applying suction to a suitably wide/narrow tube, the dead roe are syphoned into another container. The flow is controlled manually by squeezing the tube. It is a very time consuming and tiresome work/process.

If a lot of dead roe are found in the batch, it may be a question if it is worth the effort to save it. It is also a possibility to put them in a solution of sugar (ca. 2:8) or salt (1:9).The dead roe have less density than the live ones and float up, and the opportunity to remove them by floating may be used. The roe should be kept in the solution for a short while only.

Roe care and timing

As mentioned above the roe tolerability of handling is different depending on maturation stage. The first 1-3 days after fertilization the roe can take some handling and the dead ones may be removed. They may be recognized by their whitish colour as described above. Non-fertilized roe are also paler in colour than the fertilized ones, but it is a very exacting and tiring work to remove them at this stage. Three days from fertilization up to the eye stage it is best to leave the roe in peace.

Bathing against fungi is usually only done in the interval between the water hardening stage up to the stage of eye development. Commonly the bathing is started a week after fertilization.

Once the eye stage is reached it is safe to subject the roe to shock treatment so the dead ones will whiten up. One can then use the pouring or syphoning process described above for the shock treatment.

Transportation of roe

Usually roe are not moved or subjected to transport until at the eye stage, when they have developed some handling tolerability. At this stage the dead roe have usually been sorted out and removed and losses may be expected to be minor to the hatching stage.This means that only a small excess/surplus is needed when selling and transporting a batch of roe to a buyer to ensure delivery of the amount asked for.

During transport it is most important that the temperature is constant and that the roe are kept moist. It is not necessary to keep them in a bath of water though. Specially made boxes or trays of styrofoam/polystyrene for roe transport are available. Compartments in the tray keep different batches of roe separate and the trays may be stacked together into convenient units for transport. In a package unit there are usually 5 boxes or styrofoam trays, and each styrofoam tray has 4-8 compartments. The topmost foam box is filled with ice, but in each compartment of the lower trays there is place ca. half a litre of roe.At the bottom of the trays there are small holes or perforations. When the ice in the topmost tray melts, the water drips onto the roe in the trays below, making its way towards the bottom, keeping the roe cool and moist in the process during the transport.The isolation may be increased to extend the possible duration of transport without adding ice, if needed.

Transport of newly fertilized eggs may also be done, if less than 50-100 time degrees have elapsed from the fertilization. (Time degree = time (hrs) x temperature (°C)).Losses increase with longer transport and when 100-120 time degs. from fertilization are exceeded. At that stage the first cell divisions of the zygote/developing embryo take place and the roe is sensitive to any kind of impact. Newly fertilized roe are preferably moved in water filled containers, keeping acceleration and sudden movement at minimum.

Sometimes roe and sperm are transported before fertilization. The sperm is cooled to 0-4°C to increase its capacity for storage.The sperm has to have access to air/oxygen during transport, so the surface area has to be large and the sperm layer thin (ca. 6-8 mm).If the container is filled with oxygen a longer storage time is possible, even some days.

The unfertilized roe are packed in a similar way, having a thickness of only four layers of roe in the compartment. In this way roe may be kept for a few days before being fertilized. The ratio of fertile roe decreases with increasing storage time.