Subsections

• • Stripping time
  • Stripping
  • Fertilization
  • Water hardening
  • Roe disinfection
  • Resumé: Stripping and fertilization
  • Number and quality of roe
  • Weight tests

Spawning and care of roe

Stripping time

Spawning period of charr in its natural environment is very variable and depends on the breed. Most commonly spawning takes place in autumn (Sept.-Dec.) but there are breeds that spawn at another time of the year.The spawning period seems to be genetically controlled. One could plan to culture different breeds, to be able to produce roe the whole year. The drawback is that uniformity of the product would be lost.It seems more advantageous to use one breed and modify the timing of spawning by control of the environmental factors that trigger it, such as light and heat. This has become quite common in the farming of trout and salmon. With two or three spawning periods, fluctuations in the production can be evened out, and the use of the production facilities of fry are much improved, lowering the production costs. The control of spawning time is extremely useful/of great interest for larger roe and fry producers.

Stripping

The receptacles/vessels/containers for roe/sperm should be clean and dry.The roe and sperm should not come into contact with water until after the fertilization. The broodfish may be anaesthetized for the treatment to be less taxing. Benzocain (0,7mL/L, 20%) og chlorbutanol (30%), phenoxyethanol (0.25 mL/L) and MS-222 are common drugs for this purpose.

When the drugs are in effect, the female is taken and dried in a towel.The tail is grabbed by one hand and the fish stripped with the other from the pelvic fins backwards.A woolen mitten or glove on the tail hand may prove useful for a good grip. Usually one does not have to use much force when stripping. If the roe seem stuck inside the fish and the cloaca retracts between strokes, the fish should be allowed a period of 2-3 days more before another stripping attempt is made.

Roe that are fully mature or more, may become fragile/sensitive with a tendency to burst. If many such roe are mixed with good quality roe, it has a negative impact on the fertilization process. So each female should be stripped into a separate container. Roe of inferior quality may then be partially removed or thrown away totally. Afterwards the usable roe batches may be mixed together, at will.

Figure: Female being stripped

 

The males are usable and fertile for a few months in the spawning period and the same male can be used many times. They should not however be used many times per week as this leads to inferior sperm. Also the quality of the sperm produced reduces as spawning progresses.

Fertilization

The males are stripped directly over a roe batch, or into a separate container and the sperm then collected into a vial or syringe. The fish should be dried in a towel to prevent water from mixing with the sperm. Approximately 2 mL of sperm is sufficient for fertilization of 1 L of roe.Sperm from two or more males are preferably used for each batch, in the case of low quality sperm from individual fish.

The sperm should be mixed with caution/care into the roe batch. The greatest part is probably fertilized within the first 15 seconds, but one should wait for 1-2 minutes before adding water into the mix. Then one should wait for 2 more minutes before excess sperm and roe husk (? peel?) is washed off the batch.

Water hardening

After washing, the eggs must be allowed to water harden in peace. During this process, water is drawn in between the outer and inner membrane of the egg/roe and the egg increases in size up to 40%.So sufficient water must be added in the first place.The temperature should be similar (the same as of the broodfish itself) throughout the whole process of stripping and water hardening, so the roe should be kept in a cooling bath or a cooler/refridgerator. Strong sunlight and/or light affect the roe negatively, so they should be covered. The water hardening takes 2-3 hours.

 

Figure: Water hardening

Roe disinfection

When roe are stripped from the female, a thick layer of bacteria may surround the roes. These bacteria are not necessarily infectious, but they may have a negative effect on the chemical equilibrium and exchange between the roe and the surroundings/environment. To remove this bacterial coating the roe are bathed in iodine solution (100 mL Buffiodine in 10 L of water) for 10 minutes. The Buffiodine solutions should be replaced often as it becomes inactive after a few batches. After disinfection the roe are put into the hatching trays, which have been prepared by washing and disinfection. If the roe have been allowed to water harden in the hatching tray it is convenient to have the iodine bath of a size that fits/contains/holds the tray, and renew the solution after every three trays.

There are many farmers that cut the roe disinfection process and hope for the best. As the effort involved is slight, this is not advisable. Many infective agents may be carried in roe or sperm liquid (kidney disease is carried within the roe, so bathing does not prevent the infection).One can expect fungal infections to be more common and striking earlier in batches that have not been thoroughly bathed and washed.

Resumé: Stripping and fertilization

1. The fish is anaesthetized to ease handling.
2. The fish is dried in a towel.
3. The female is squeezed in a dry and clean container. If the roe come into contact with water before fertilization, the egg mouth/opening will close up preventing fertilization.
4. The male is squeezed directly over the roe batch. Use two males for each batch for safety's sake.
5. The sperm is gently mixed with the roe, waiting 2 minutes.
6. Water is added to the container (same temperature as broodfish) and allowed to stand for 2 minutes.
7. Excess sperm and slime is washed off with cold water.
8. The roe are place in a cool dark storage for 32-33 hrs during water hardening.
9. The roe are then disinfected in a Buffiodine solution (100 mL/10 L water) for 10 minutes (everything at same temperature).
10. Roe are placed in a hatching tray and covered.
11. 3-5 days from fertilization the treatment against fungi may be begun. It is ceased at the stage of eye development.
12. At eye stage, dead roe may be removed from the roe batch.

Number and quality of roe

The size of charr roe varies between breeds and depends also on the age and size of the brooding female. Feeding and environment may also affect the size. The roe are usually larger and in greater number from large females. Most commonly the diameter of a single roe is 4-5 mm.From females that are being squeezed for the first time, roe size is usually smaller and more variable, than from fish that has been squeezed before. The weight ratio of roe to body weight is 10-25%. In each liter of roe there may be 6000-15000 roe or even more, as the number is linked to the size/diameter of the roe. Larger fry are produced from larger roe, with greater chances of survival, as they are better able to assimilate artificial feed at primary feeding.

The number of roe can be estimated in various ways. A score or two of roe may be counted into a measuring cylinder containing water, measuring the displacement. Another method uses the number that can be put in a row of 25 cm as a reference to tabulated values (see appendix no. ??). The roe may also be counted on a rack/grating frame that counts 500 roe at a time.

Number of roe may be found by ratio measurement, either of volume or mass. Then samples are taken from the batch, a ratio factor found and multiplied for the whole batch. First, the number of roe in a mL is measured. Use a graduated cylinder, fill to a mark (below the maximum) with water and then count 50 roe into it. The volume increase is recorded and calculated per roe, or the roe number per mL can be calculated. This can be repeated a few times for better precision.

Example: A roe batch from one female displace 600 mL of water and 50 roe displace 11 mL of water.How many roe are in the batch?

Number of roe/mL =$${\frac{{\mbox{Number of roe}}}{{\mbox{volume increase}}}}$$ = $${\frac{{50 roe}}{{11 mL}}}$$ = 4.5roe/mL

Total no. of roe = Roe volume x No. roe/mL = 600mL x 4.5roe/mL = 2700 roe in batch

If a counting rack is at hand, the number in a volume of 100 mL can be quickly counted and used for estimation of the roe number per mL. The total number can then be calculated in the same way as above.

Weight tests

The whole batch is weighed and then grab samples of roe are taken, 100 roe counted out from each grab and weighed to an accuracy of 0.1 g.From this the average weight of 100 roe is found. The accuracy depends on the thoroughness/exactitude of removal of water. It is good practice to dry them on paper.After the overall average weight is found the number can be calculated thus:

Number of roe = $${\frac{{\mbox{Total weight of batch} \times 100}}{{\mbox{Overall average wt. of 100 roe}}}}$$