Target Strength of Southern Resident Killer Whales (Orcinus orca): Measurement and Modeling

Abstract

A major criterion for permitting the deployment of tidal turbines in Washington State’s Puget Sound is management of risk of injury to killer whales from collision with moving turbine blades. An active monitoring system is being proposed to detect and track killer whales within proximity of turbines and alert turbine operators of their presence and location to permit temporary turbine shutdown when the risk of collision is high. Knowledge of the target strength (TS) of killer whales is critical to the design and application of active acoustic monitoring systems. In 1996, a study of the TS directivity of a 2.2-m-long bottlenose dolphin at an insonifying frequency of 67 kHz was performed. Noting that killer whales, which are dolphins, are morphologically similar to bottlenose dolphins and then assuming allometry, we estimated the relative broadside and tail aspect TS of a 7.5-m-long adult killer whale at an insonifying frequency of 67 kHz to be −8 and −28 dB, respectively. We used a three-layer model for plane wave reflection of sound at 200 kHz from the lung of killer whales to estimate their TS. We assessed the accuracy of our killer whale TS estimates by comparing them with TS estimates of free swimming killer whales obtained using a split-beam active acoustic system operating at 200 kHz. The killer whale TS estimates based on the preliminary model were in good agreement with those obtained for free swimming killer whales.