Multibeam Acoustic Detection of Fish and Water Column Targets at High-Flow Sites

Abstract

The potential rapid development of tidal energy and accompanying structures in the coastal waters in several regions of Canada, and the world, will require subsurface sampling tools to monitor and evaluate the biological and environmental effects of this new industry. Proposed development sites are characterized by large tidal ranges, strong tidal flows, extreme turbulence, and in some cases sediment-laden water restricting visibility, imposing monitoring challenges. Turbulence-induced acoustic backscatter typically occurs in the top 10–20 m, but can extend from the surface to bottom during some phases of the tide. In Minas Passage, turbulence accounted for 78 % of the total water column backscatter making the separation of fish difficult if not impossible at times. In this paper, we examine operational capabilities of two high-frequency multibeam sonars to meet these monitoring challenges in the nearshore high-flow shallow coastal waters (1–100 m). Observations from proposed sites are used to illustrate the challenges as well as the advantages, and limitations, of multibeam sonars compared to conventional split beam technology in strong tidal currents and turbulent waters. Acoustic multibeam hardware necessary to meet the monitoring demands is currently available, but the processing and analytical software required to advance their application is insufficient to meet the requirements for effects and impact monitoring. It is anticipated that the required advancements will be available within the next 5 years. Challenges associated with equipment deployment, power supply, and communications are also discussed.