Mass mortality of commercially cultured marine species in summer is the most serious problem for the longline culture. Reducing crowding stress induced by high stocking density on the basis of high biomass is proposed to be an important approach to solving this issue. A laboratory experiment with multi-layer culture (18 sea urchins total, 6 urchins in each layer) was conducted to investigate gonad yield, gonadal development, thermal tolerance (CTmax) and disease transmission for the juvenile sea urchins Strongylocentrotus intermedius for -7 weeks at elevated temperatures. The present study found that precocious puberty occurred both in those sea urchins cultured in multi-layer and those not (control group), indicating that gonadal development is accelerated by crowding stress (even in a low density of 6 sea urchins/1834.56 cm3). The CTmax showed a significantly negative correlation with gonadal development, suggesting precocious puberty probably is responsible for the decrease of resistance ability. We further assessed whether segregation in multi-layer culture improves these important traits of sea urchins. This approach significantly reduced gonadal development, improved CTmax and lower mortality and morbidity after disease challenge assays in sea urchins than those not separated in multi-layer culture, which efficiently reduces the gametogenesis, improves resistance ability and decreases disease transmission. The present study establishes a new approach to improving the survival of cultured organisms in the longline culture at high temperatures.
