WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 65 Tannin concentration and treatment duration affected the hatching rate of fertilized wels catfish eggs (Fig. 3). The 10 g/L tannin solution was lethal, irrespective of treatment duration. 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. Notes 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. B. Csorbai, Jászkiséri Halas Ltd., 5137 Jászkisér, Vágóhíd út 15., Hungary Z. Bokor, Corresponding author: Tel.:+36 28 522000/1912 Fax.: +36 28 522927 E-mail: Bokor.Zoltan@mkk.szie.hu References Bíró P. 1993. Halak biológiája. Egyetemi jegyzet. Pages 46, 48 In: MTA Balatoni Limnológiai Kutatóintézet. Tihany, Hungary. Horváth L. and G. Tamás. 1981. Ivadéknevelés. Mezőgazdasági Kiadó. Budapest. Page 52 Straub, F.B. 1975. Biológiai Lexikon. Akadémiai Kiadó. Budapest, Hungary. Tamás, G.H. and L. Horváth. 1982. A ponty, a növényevő halak és a compó szaporítása, valamint ivadék-előnevelése. Page 43 In: G. H. Tamás, L. Horváth and I. Tölg, editors. Tógazdasági tenyészanyag-termelés. Mezőgazdasági Kiadó. Budapest, Hungary. number of eggs was low. Thus, a drip system was constructed that enabled optimization of water flow rate to developing eggs (Fig. 1). 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 observed hatching, egg portions were closely monitored and the times of first hatching in each Petri dish were recorded. Tannin Treatment of Common Carp Eggs Embryogenesis required 3 days before hatching began (H. Tamás and Horváth 1982). The concentration of tannin clearly affected hatching rate (Fig. 2). Egg hatching rate was significantly decreased at the greatest concentration (10 g/L). Although tannin concentration affected hatching rate, the duration of treatment had no significant effect. As a basic conclusion, tannin is suitable for preventing stickiness of eggs in the concentration applied in practice (0.5 g/L). In the case of common carp, tannin can be used at concentrations greater than 0.5 g/L without any harmful effects. At 0.5 g/L, there is no danger if the duration of treatment extends from a few seconds to one or two minutes. Concerns about the duration of treatment appear to be unfounded. If tannic acid treatment requires 10-20 sec longer than the standard treatment duration (20 sec), egg hatching rate would not be affected. Tannin 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 21 C. Non-fertilized, moldy eggs were removed with a Pasteur pipette. Development of embryos was observed with a microscope. Embryo development required 3 days. Full hatching of each egg portion was completed within 5 hours of the time of first hatching. Hatched eggs were counted and removed from dishes with a Pasteur pipette. FIGURE 2. Hatching rate of common carp eggs at different tannin concentrations and treatment durations. FIGURE 3. Hatching rate of wels catfish eggs at different tannin concentrations and treatment durations.
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