Effectiveness of low rearing density was examined against red sea bream iridoviral disease (RSIVD) in juvenile red sea bream Pagrus major. In an experimental challenge test, in which the number of fish and the volume of rearing space were set in three levels, low-density groups (2.1 or 2.2 kg/m3) showed significantly lower mortalities than medium- (4.2 kg/m3) and high-density groups (8.4 kg/m3). In a field trial, fish were reared in neighboring net pens (6.4 m3) at three densities (0.8, 2.8, 9.6 kg/m3) for 73 days. The mortality due to natural infection with RSIV showed a clearly inverse correlation with the rearing density. Additionally, hemoglobin content and plasma component levels were measured in the before-mentioned field trial and an indoor experiment, in which the number of fish and the aquarium size were set in three levels (2.8, 5.3 or 5.8, 11.3 or 11.6 kg/m3). The levels of plasma total cholesterol and phospholipid in high-density groups were significantly lowered than those of low-density groups. Low rearing density was effective against RSIVD in red sea bream, and plasma lipid components may play a role in the disease resistance.
