Evaluation of a Floating, Two-Vessel Towed Transducer System for Detection of near-Surface Fishes

Mobile hydroacoustic surveys typically use a downward-looking transducer, either mounted on the hull of a boat or on a vehicle towed alongside or behind a boat.  Such deployments are ineffective for surveys of near-surface fishes, both due to boat avoidance and the conical nature of the acoustic beam that provides limited sampling capability near the transducer.  Trawl surveys have experienced similar problems from boat avoidance.  A solution for trawl surveys has been a two-boat operation where the net is towed between two boats.  We explored the use of a similar approach for hydroacoustic surveys.  In order to stabilize the tow and maximize near surface detection, we mounted the transducer just below a floating towed device.  A small zodiac was used as the second vessel to reduce charter costs.  With this approach we were able to detect fish immediately below the surface with no indications of avoidance.  We were also able to use this approach to evaluate the extent of avoidance with our standard, single-vessel deployment.