Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
The diurnal changes in the acoustic frequency characteristics of Japanese anchovy (Engraulis japonicus) larvae were analyzed based on theoretical scattering models. Larval swim bladders contain gas at night, but not during the day, so the straight cylinder model (no gas in a swim bladder) was used for daytime observations, and the fish bladder resonance model (full gas in a swim bladder) was used for night observations. The sets of acoustic frequencies used were 50kHz and 200kHz, which are those used for shirasu commercial fishing, and 38kHz and 120kHz, which are those commonly used for acoustic surveys using a quantitative echo sounder system. During daytime, differences in the target strength (TS) of the anchovy larvae between the two frequencies were large (especially between 38kHz and 120kHz), but at night, there were few differences between frequencies. These results suggest that acoustic surveys for identifying and estimating the abundance of Japanese anchovy larvae should be performed during the day rather than at night, since this is when differences occur between frequencies and when larvae form dense schools.