Assessing Target Strength, Abundance, and Biomass for Three Commercial Pelagic Fish Species along the East Coast of Peninsular Malaysia Using a Split-Beam Echo Sounder

Decapterus maruadsi (round scad), Megalaspis cordyla (torpedo scad), and Selar boops (oxeye scad) are commercially important small pelagic fish species of Malaysia. Despite their commercial and ecological value, little is known about their target strength (TS), spatial and vertical distributions, and biomass estimates. A split-beam acoustic survey was performed in the east coast of Peninsular Malaysia (ECPM) in March-June 2009. The calibrated echo sounder allowed determination of TS, estimation of fish density (spatial distribution) and biomass, and produced density maps along depth gradients (vertical distribution). The TS-length relationships indicated that the logarithmic TS was proportional to the squared length. Additional water parameters that included salinity, temperature, and dissolved oxygen (DO) were collected to examine fish density changes in relation to water quality and depth. Results revealed that the small pelagic fish formed small schools that were scattered over a wide area of the ECMP; mean density varied between 1.1 and 1.9 t/km2. The maximum fish density was in the spatial coverage between 102.875° E, 6.000° N and 103.125° E, 6.500° N. The mean total biomass was about 18,610 t. Optimum fish density was estimated around water depths between 40 and 60 m. Fish density mainly decreased with decreasing DO and with increasing depth. The demonstrated acoustic method can be used as a monitoring tool for national fish stock surveys