Hungary has a strong tradition of pond farming and fish breeding. One of the most important species is common carp (Cyprinus carpio). Another is wels catfish (Silurus glanis).
Some species, such as common carp and wels catfish, have so-called phytophilic eggs with a sticky protein on the surface of the fertilized and swollen egg that facilitates attachment to vegetation in nature.
In nature, the incubation and hatching of eggs takes place while attached, usually to aquatic plants. During artificial breeding this characteristic has a negative effect on the success of hatching because eggs stick together. This means that trapped eggs cannot obtain sufficient oxygen and the developing embryo dies. Different molds can appear on the surface of dead eggs and, when mold starts, it is harmful to adjacent eggs.
The Tannin advantage
Tannin is widely used in common carp hatcheries. Tannin is also used in the breeding of other commercially important fish species, such as catfish and perch.
Tannin, also known as tannic acid or acidum tannicum, is a highly astringent, polyphenolic plant compound. Tannin precipitates proteins and other organic compounds, including amino acids and alkaloids; it also decreases the swelling of collagen. It is not a chemically uniform compound. Pure tannic acid is a white and amorphous powder but exposure to air causes it to become yellowish. It is soluble in water, less soluble in alcohol and glycerin, and insoluble in ether.
During artificial breeding in hatcheries, technicians often add tannin to eggs after fertilization for the termination of stickiness. After treatment with tannin, eggs incubated in Zug jars do not stick together or form lumps, thereby increasing the efficiency of hatching. However, if the concentration of tannin and the duration of use are not appropriate, the quality of eggs is impaired.
In artificial breeding, tannic acid treatment is the last action before fertilized and swollen eggs are placed into Zug jars. As a side effect, tannin also hardens the wall of the egg. Consequently the developing embryo hatches with difficulty. In light of the two effects (removal of stickiness and hardening of eggs), it is important to precisely define the tannin concentration of the solution and the duration of treatment.
Use with caution
Different instructions for use of tannin have spread among practitioners, and many fear the negative effects of the product. The objective of this study was to assess the effect of tannin concentration and treatment duration on the quality and time of hatching of eggs and survival of embryos. These are defined with respect to the characteristics of eggs of particular species. Thus, the effects of tannin were examined on eggs of common carp and wels catfish, at four concentrations and three treatment durations.
Determination of test concentrations and egg treatment durations were based on references from the published literature and personal communication with practical technical experts.
Four tannin concentrations (0.1, 0.5, 1 and 10 g/L) and three treatment durations (5, 20 and 120 sec) were established. The standard practice is treatment with a tannin solution of 0.5 g/L for 20 sec. Each treatment combination was applied to three portions of eggs. Each egg portion was treated by different concentration-time combinations and then rinsed in fresh water.
After application of tannin, 50 eggs from each treatment combination were placed in Petri dishes and incubated under a drip system. Incubation in Zug jars was not practical because the number of eggs was low. Thus, a drip system was constructed that enabled optimization of water flow rate to developing eggs. Water flow rate was adjusted to supply sufficient oxygen to eggs.
A 4-cm high, densely-woven, mosquito net was attached to the sides of Petri dishes to prevent egg washout and escape of newly hatched larvae, thereby allowing accurate counting of larvae. After the first hatching, egg portions were closely monitored and the times of first hatching in each Petri dish were recorded.
Treatment of carp eggs
For carp eggs embryogenesis required three days before hatching began. The concentration of tannin clearly affected hatching rate. Egg hatching rate was significantly decreased at the greatest concentration (10 g/L), but the duration of treatment had no significant effect.
As a basic conclusion for carp eggs, tannin is suitable for preventing stickiness of eggs in the concentration applied in practice (0.5 g/L). Tannin can be used at concentrations greater than 0.5 g/L without any harmful effects and at 0.5 g/L, there is no danger if the duration of treatment extends from a few seconds to one or two minutes. Indeed, concerns about the duration of treatment appear to be unfounded.
Treatment of Wels catfish eggs
Catfish eggs were carefully removed from sides of glass 10-12 hours after spawning. Water for egg incubation was maintained at 21o C and non-fertilized, moldy eggs were removed with a Pasteur pipette. Embryo development required three days and full hatching of each egg portion was completed within five hours of the time of first hatching. Hatched eggs were counted and removed from dishes with a Pasteur pipette.
Tannin concentration and treatment duration did effect the hatching rate of fertilized wels catfish eggs. The 10 g/L tannin solution was lethal, irrespective of treatment duration. However, all other tannin concentrations tested can be applied to prevent egg stickiness without affecting hatching rate. At the lowest concentration tested (0.1 g/L), egg hatching rate increased with treatment duration. At 0.5 and 1 g/L, the effect of treatment duration on hatching rate was negligible, with a slight decline in hatching rate at the greatest treatment duration. The best hatching rates occurred at a tannin concentration of 1 g/L and a treatment duration of 5-20 sec.
The preceding article was prepared by Z. Bokor, I. Ittzés , D. Ács, B. Urbányi and F. Fodor, Szent István University, Faculty of Agricultural and Environmental Sciences, Institute of Environmental and Landscape Management, Department of Aquaculture, Páter K. u.1., 2100 Gödöllő, Hungary. It was reprinted here courtesy of World Aquaculture Magazine