In attempts to enthance fish stocks and increase the harvest of grown fish after release in Japanese coastal waters, classical conditioning of fish to sound before the release has been tried with red sea bream and some other species, The common sound used in the conditioning is a 300Hz, 1s tone burst, but this does not seem to be effective in attracting conditioned fish after release. This paper describes a key component of the sound to which red sea bream were conditioned. A group of one-year old and three groups of younger juvenile red sea bream were conditioned to the 300Hz sound during feeding in separate floating cages. Upon completion of the conditioning, the fish were exposed to 1s tone bursts of frequencies 100 to 5000Hz and of various sound pressures. The juveniles showed positive responses at 100 to 3000Hz and best responses at 300Hz and at 1500Hz, they were attracted to the sound origin upon projection but left the sound origin soon after. A similar response pattern was observed at 200 to 3000Hz in the one-year old sea bream, with best responses at 300Hz. The one-year old fish showed typical avoidance response to 100Hz, 1s tone bursts, which showed that the lateral line system mediated the response but that the lateral line is still poorly developed in the juveniles. When 300Hz, 1s tone bursts were applied at various pulse intervals, the juveniles responded positively to bursts at 0.04 to 5s intervals. These results imply that the key component of the sound is the rising pressure and thus the conditioned fish would be attracted to any intermittent sound after release.
