Overview: The IMOS Bio-Acoustic Ship Of Opportunity (BASOOP) sub-facility is part of a major international effort that aims to develop a global ocean Mid-trophic Automatic Acoustic Sampler (MAAS) being proposed as part of the CLimate Impacts on Oceanic TOp Predators (CLIOTOP) program. This SOOP dataset covers the Indian Ocean and waters south and east of Australia and across to New Zealand. BASOOP commenced on the 1st of July 2010 to collect underway acoustic data while vessels are transiting ocean basins. At present, nine vessels are participating in the BASOOP program. Six are commercial fishing vessels that have agreed to record data during transits to and from fishing grounds. The remaining three are scientific research vessels collecting underway acoustic data during transits and science operations. Bio acoustic signals allow understanding how mid-water prey species (known collectively as micronekton) such as small fish, squid, krill and jellyfish are distributed. Micronekton form the core of the ocean food web, transferring energy from primary producers at the ocean surface to top predators such as tunas, billfish, sharks, seals and seabirds. The mass and distribution of micronekton reflects broad-scale patterns in the structure and function of the ocean, as well as the dynamics of marine ecosystems. The mapping complements established observing systems such as physical sampling of ocean currents, surveys of ocean chemistry and biology (plankton and zooplankton), and electronic tagging and tracking of large marine fish and mammals. The combined information greatly enhances the capacity of marine scientists to monitor shifts in food availability over time, assisting in the monitoring and modelling of oceanography, ecosystems, fisheries and climate change, and in understanding the behaviour of top predators. Data Collection: All BASOOP vessels collect 38 kHz acoustic data from either Simrad EK60, ES60 (split beam) or ES70 echosounders. In all cases the 38 kHz echosounders are connected to Simrad ES38B transducers. This is a narrow-beam (7 o) ceramic transducer with good long term stability and manufacturer supplied calibration parameters. Research vessel Southern Surveyor also collects concurrent acoustic data at 12 and 120 kHz. The research vessel Aurora Australis collects concurrent acoustic data at 12, 120 and 200 kHz. The primary data-type recorded from the vessel-mounted echosounder systems is georeferenced calibrated water column volume backscatter, Sv [dB re 1 m-1], (Maclennan et al. 2002) . The raw acoustic data is post processed to (i) identify on-transit data and prioritise processing, (ii) apply calibration offsets, (iii) apply semi-automated filters to identify and reject bad data and (iv) create output stored in netCDF format, mean echointegrated Sv for cells of 1000 m distance and 10 m height. A full metadata record is also stored in each netCDF file. Reference: *Maclennan, D.N., Fernandes, P.G., and Dalen, J. 2002. A consistent approach to definitions and symbols in fisheries acoustics. ICES Journal of Marine Science: Journal du Conseil 59(2): 365.
