Practical ultraviolet irradiation equipment for the reduction of bacterial loads in rotifers [Brachionus rotundiformis]

The effect of ultraviolet irradiation on the reduction of bacterial loads in rotifers (Brachionus rotundiformis) was investigated. In a small-scaled laboratory experiment, UV-irradiation at 112 mW/cm2 for 20 min reduced bacterial loads in rotifers, which were measured by TCBS agar (Thiosulfate Citrate Bile Salts Sucrose Agar) medium, from l0(7) to l0(4) CFU/g without apparent damage to the mobility of the rotifers. In a hatchery-scaled experiment using newly devised equipment and 30l of stirred seawater (10(5) individuals/ml) , UV-irradiation at about 120 mW/cm2 for 30 min reduced the bacterial loads in rotifers by 1 or 2 orders of magnitude (from l0(7) to 10(5) or l0(6) CFU/g) . These results demonstrate that the UV-treatment with the present equipment brings about the same effect as nifurstyrenate bath treatments, which has long been used for the reduction of bacterial loads in rotifers in Japanese hatcheries.