Analysis of systematic source level measurements of small vessels

Abstract

Improving our knowledge of how sound impacts marine mammals is particularly important where the spatial distributions of vessels and marine mammals overlap, as exemplified by the critical habitat for the endangered Southern Resident Killer Whale (SRKW). In this study, two acoustic recorders were deployed in transboundary Haro Strait (British Columbia, Canada and Washington State, USA) from July to October 2017 to measure sound levels produced by whale-watching vessels and other small boats. During this period, 20 different volunteer vessels were assessed operating at a range of speeds—nominally 5 knots, 9 knots, and cruising speed. The measurement protocol was designed based on ANSI S12.64-2009. For all vessels, we observed positive correlations between source levels and speed; however, the speed trends (slope of curves) were not as strong as those of large commercial vessels. Mean source levels were computed for each vessel type in the broadband frequency range (0.05–64 kHz), the SRKW communication band (0.5–15 kHz), and the SRKW echolocation band (15–64 kHz) at each speed. Here we discuss how source levels were affected by vessel speed, hull shape and propeller type, as well as the positive and negative aspects of the protocol design.