Zion, B., Institute of Agricultural Engineering, Agricultural Research Organization, The Volcani Center, P. O. Box 6, Bet Dagan 50250, Israel
Barki, A., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, P. O. Box 6, Bet Dagan 50250, Israel
Grinshpon, J., Institute of Agricultural Engineering, Agricultural Research Organization, The Volcani Center, P. O. Box 6, Bet Dagan 50250, Israel
Rosenfeld, L., Institute of Agricultural Engineering, Agricultural Research Organization, The Volcani Center, P. O. Box 6, Bet Dagan 50250, Israel
Karplus, I., Institute of Animal Science, Agricultural Research Organization, The Volcani Center, P. O. Box 6, Bet Dagan 50250, Israel

Groups of St Peter's fish Sarotherodon galilaeus were trained during 28 days of classical conditioning to respond to an acoustic signal by approaching a feeder. Their ability to retain the acoustically conditioned behaviour was tested after eight different intervals ranging from 12 to 188 days from termination of training. Retention tests comprised 7 consecutive days of retraining and the fish response was compared with that of naive fish. Fish response drastically diminished with time. After 55 days, there was no apparent expression of memory on the first test day but fish response was restored on subsequent retraining days, indicating the existence of latent memory. Six months from the end of training, there was no indication of behavioural retention and the response completely vanished. The study was conducted in the context of sea ranching, with classical conditioning used to gain control over fish location. In light of the present results, it is suggested that periodic reinforcement of the acoustically conditioned behaviour should be used. © 2011 The Authors. Journal of Fish Biology © 2011 The Fisheries Society of the British Isles.