Low discovery rate of sea cucumber Apostichopus japonicus juveniles after seed release in the field

Low discovery rates are common at initial stages of sea cucumber Apostichopus japonicus juveniles release, and possible causes, such as drifting away by waves, predation, burying by mud, and emigration have been suggested. In order to clarify causes of the low discovery rate, the body length and juvenile occurrence were examined by releasing juveniles into artificial reefs. We compared discovery rates among four releasing treatments; using oyster shells as attachment devices, covering whole artificial reefs with different net mesh size as 2 mm and 4 mm, and control groups which were directly released into artificial reefs. The discovery rate decreased quickly during the first 7 to 14 days after release, after that it decreased slowly. Comparison of juvenile body length between just released and 28 days after releasing groups, indicated no size-dependent disappearance pattern. Discovery rates did not differ significantly among the four releasing treatments. We discuss the other possible causes contributing to the observed low discovery rate.