Potential for broadband acoustics to improve stock assessment surveys of midwater fishes

Acoustic-trawl surveys rely on a combination of backscatter measured with echosounders and species composition data from trawls to apportion the backscatter to different species and size classes. Narrowband echosounders have been widely used in this context for decades. Multi-frequency analysis of narrowband echosounder data has been shown to be effective for discriminating between diverse taxa (e.g., euphausiids vs. swimbladdered fishes) but distinguishing morphologically similar species (e.g., swimbladdered fishes) remains a major challenge. Previous work indicates that broadband backscatter techniques have the potential to improve such acoustic target characterizations by exploiting nearly continuous frequency spectra, but these methods have not been widely applied in fisheries surveys. The recent commercial availability of broadband transceivers is accelerating the evaluation of this technology. We present operational data from two broadband acoustic scattering systems: (1) 14-160 kHz and (2) 3-10 kHz used during surveys of walleye pollock (Gadus chalcogrammus) in the Gulf of Alaska and Eastern Bering Sea. The presentation focuses on the potential for: 1) discrimination among common species in the area and 2) utilization of swimbladder resonance to estimate the dominant size class in single-species fish aggregations. The implications for fish stock assessment surveys are considered.