Difference of resistance to bacterial coldwater disease in natural environment between two hatchery populations of ayu Plecoglossus altivelis with different number of generations subjected to selection based on character of growth

We compared the resistance of two hatchery populations of ayu Plecoglossus altivelis having a different number of successive hatchery generations (F2 and F18 in 2007, F3 and F19 in 2008) to bacterial coldwater disease in a river. Both populations originated from the Yoshino River in Tokushima Prefecture, Japan, and they were selected by character of growth. Hatchery-reared juveniles of two populations were released in April and May 2007 and in April 2008 in a branch of the Kaifu River, Tokushima Prefecture, in the stream above a dam, which prevented access by wild ayu. The percentages of diseased ayu in the total numbers recovered were 14.9% and 45.9% for F2 and F18, respectively, and 22.1% and 47.2% for F3 and F19, respectively. After the disease outbreak, the percentage of F2 and F3 in recoveries exceeded recoveries of F17 and F18. These results indicate that F2 and F3 ayu have superior resistance to bacterial coldwater disease compared to F18 and F19.