Abstract: With Simrad EY60 split-beam echosounder, the spawning stocks of Megalobrama hoffmanni in Qingpitang spawning grounds were observed from April to May, 2006. The echoes from M. hoffmanni were identified by its distinctive characteristics, and the ancillary information from gill nets sampling and tethered fish measurements of target strength were also important. The results showed that the aggregation and lingering behaviors of M. hoffmanni were obvious. During the period of aggregation, the average swimming speed of M. hoffmanni was 0.24 m·s^-1, being lower than the corresponding value of ther fishes (0.49 m·s^-1). The target strength of M. hoffmanni in situ ranged from -33.8 to -52.3 dB, with an average of -42.2 dB. From April 27 to May 1, M. hoffmanni migrated to Qingpitang spawning grounds gradually, and the peak density appeared. In May 1, the number of spawning fish was about 98 000, and the total mass was 48 tones. Our results indicated that the stationary acoustic observation method for M. hoffmanni identification and discrimination was effective.

Key words: Biological phosphorus removal, Polyphosphate, Microorganism