ACOUSTIC SURVEYS OF PELAGIC FISH RESOURCES IN THE BANDA SEA DURING AUGUST 1984 AND FEBRUARY-MARCH 1985

Acoustic surveys with a 38 kHz echosounder were carried out during the Snellius-II Expedition by R.V. 'Tenggiri' in the eastern Banda Sea and northwestern Arafura Sea. The area of approx. 360 000 km2 was surveyed both during the southeast and the northwest monsoon season. The objective was to make a first rough assessment of the distribution and abundance of the pelagic fish stock in the area, using a digital echointegration system. Significant differences in depth and volume of backscattering were observed between night and day, possibly related to diel vertical migration and avoidance of the vessel. Using the target strength of Decapterus of 14 cm length, the mean densities of the pelagic fish stock in the upper 100 m varied between 5.38 (day) and 8.82 (night) tonnes per square nautical mile in August and between 1.41 and 2.46 in February. The seasonal difference corresponded to biomass differences detected at lower trophic levels. Highest concentrations of backscattering were found in the eastern part of the area in both seasons, near the Outer Banda Arc and the Arafura Shelf. Total fish biomass estimates, based on the recording by daylight in the upper 100 m for the area surveyed, were 570 000 tonnes for August and 150 000 for February. No identification of fish species was accomplished. If daylight recordings mainly involve small pelagic fish, the total stock in the eastern Banda Sea is very large compared with local annual landings of non-tuna fish. © 1990.