Spatial and seasonal differences in acoustic recordings of the Banda Sea (Indonesia), obtained with a 30 kHz echosounder

Abstract

Within the framework of the Indonesian-Dutch Snellius-II Expedition, acoustic surveys with a 30 kHz echosounder were made in the eastern Banda Sea during the southeast monsoon (August 1984) and northwest monsoon (February 1985). Occurrence, patterns, and migratory behaviour of the sound scattering layers (SSL) and of distinct echopatches (EP) were studied. No significant seasonal differences in number and pattern of SSL's occurred. SSL patterns in the Banda Sea area were typified by the absence of a deep layer at night, which is different from the (eastern) Pacific Ocean but similar to the pattern described for the equatorial Indian Ocean, suggesting a relationship with this area. The thickness of the SSL's was, in accordance with higher biomasses of the micronekton, on average larger in August than in February. However, no clear relation between net catches of mesopelagic fishes and crustaceans and night-time thickness of SSL was found, suggesting that larger organisms than those captured were responsible for the observed backscattering. 90% of the EP's occurred during the day and showed diel vertical migration down to day-time depths of 300 to 400 m. The number of EP's observed was twice as high in August as in February, with a clear concentration in the Aru Basin, the area with the most pronounced upwelling in August 1984. As EP's are generally attributed to schools of pelagic fish, the data suggest a seasonal increase and decrease in the fish stock corresponding with biomass fluctuations of other trophic levels.