A comparative acoustic-abundance estimation of pelagic redfish (Sebastes mentella) from hull-mounted and deep-towed acoustic systems

During an international acoustic survey on pelagic redfish (Sebastes mentella) in the Irminger Sea in June–July 2001, a deep-towed vehicle carrying a dual-frequency SIMRAD EK60 Mk1 echosounder was used experimentally to observe and estimate redfish abundance at depths greater than 500 m. Comparison was made with the hull-mounted SIMRAD EK500 echosounder. The deep-towed echosounder improved the spatial resolution of fish in scattering layers as well as the ability to scrutinize and interpret fish echoes. The signal-to-noise ratio was better than that of the hull-mounted system. The fish abundance measured by the deep-towed sounder was consistently high compared to the hull-mounted sounder, particularly, at greater depths. For redfish within the deep scattering layer (DSL), the abundance was, on average, 5% higher, increasing to 55% for redfish below the DSL. From these findings it is concluded that a deep-towed echosounder is superior to hull-mounted ones for estimating the abundance of fish swimming in deep water.