Elimination of adhesiveness in the eggs of shishamo smelt Spirinchus lanceolatus using kaolin treatment to achieve high hatching rate in an environment with a high iron concentration

Using tannic acid treatment to eliminate egg adhesiveness results in a low hatching rate because of iron attachment to the egg surface in an iron-enriched environment in shishamo smelt, Spirinchus lanceolatus. This is an issue affecting hatcheries in northern Japan. Therefore, we tested kaolin as an alternative to tannic acid for the elimination of egg adhesiveness. In a small-scale experiment, we found that treatment with kaolin at 5.0 g/L for 5 to 30 min and at 10.0 g/L for 10 s to 30 min is more effective in eliminating adhesiveness than treatment with tannic acid. In a large-scale experiment in an environment with high iron concentration, the total iron amount on the egg surface increased in the nontreated, tannic acid-treated and the 5.0 g/L kaolin-treated groups, where treatment was conducted for 5 min, although the amount of iron in the tannic acid-treated group was significantly higher than in the kaolin-treated or nontreated groups just before hatching (P<0.05). The hatching rate of the tannic acid-treated group was significantly lower than in the kaolin-treated and the nontreated group just before hatching (P<0.05). Furthermore, the tannic acid-treated group showed a significantly higher mortality during hatching compared to the kaolin-treated and the nontreated groups (P<0.05). There were no differences in starvation and seawater tolerance of the larvae among all groups. These results suggest that a 5-min treatment with kaolin 5.0 g/L is a more suitable method of eliminating egg adhesiveness than treatment with tannic acid in the case of egg rearing in an iron-enriched environment.