Estimation and validation of optimal density of juvenile sea cucumber Apostichopus japonicus in a rearing facility with air-pocket fences

Abstract

Apostichopus japonicus has been an important export of Japan since the seventeenth century. However, with increase in demand, overfishing of this sea cucumber has become a concern. Consequently, we designed a nursery facility for the juvenile sea cucumber A. japonicus. However, growth stagnation occurred depending on population density. Therefore, we investigated the optimal density of juveniles based on the food supply. Sediment traps were installed and sediments were collected monthly from two facilities in a fishing port, Hokkaido, Japan, for a year. Settling particle flux and sediment mass were measured. It was found that the food supply was the lowest in June, and calculations were based on these values. The optimal density for juveniles with a length of 20 mm was calculated to be 141-151 individuals in a facility with a lower surface area of 5.8 m². In addition, 150 juveniles with a mean length of 23.3 mm were housed in one facility and 1,500 in the other facility in October 2019. After 1 year, the length and survival rate were 46.3 mm and 87.3% of the former and 34.5 mm and 33.2% of the latter, demonstrating the validity of the optimal density calculated.