Large schools of Japanese sardines,Sardinops melanostictus, mate in single pair units at night

Synopsis We observed the behavior of the spawning group of Japanese sardine,Sardinops melanostictus, in the field at night, and also confirmed the spawning behavior in the darkness in captivity. The chasing and mating were done in a pair of 1 male and 1 female. The 1-to-1 mating was a definite finding irrespective of the group density in the field, the density in the tank in captivity, or the sex ratio. This maybe because there is little visual information during spawning at night.