Evaluation of a Proposed Approach for Offsetting Increases in Underwater Noise from Marine Shipping Using Information on Southern Resident Killer Whales

Abstract

Advice was requested from Department of Fisheries and Ocean Science in the evaluation of a proposed framework for offsetting that could be applied to project-related underwater noise additions. To do this, a theoretical test case using information from southern resident killer whales (Orcinus orca, SRKW) in and around the Salish Sea was reviewed by subject matter experts. Advice was sought on several aspects of the approach, including: the spatial and temporal boundaries of the framework; the measurement of project-related vessel noise additions and the efficacy of management measures relative to a baseline; the use of offset ratios or weighting factors to express aspects of habitat use by the focal species; and how offset credits would be calculated and exchanged. Conceptually, there was reasonable agreement among participants that an offsetting approach could be applied to address residual project-related vessel noise increases on marine mammal species if existing mitigations proved insufficient. However, neither the proposed offsetting framework, nor the demonstration of its use in the test case, were recommended by participants due to concerns regarding the amount of data needed, and the risks and uncertainties in applying it to offsetting. Generally, issues remained unresolved in how the concept may be implemented, particularly in regards to credit calculation and exchange, and in understanding the associated benefits of the offsetting approach to at-risk focal species. High levels of uncertainty and risk were also identified for this, especially in the derivation and use of weighting factors to recognise areas of greater importance or use by the focal species. Guiding principles of credit exchange were discussed, including the need to determine habitat equivalency between regions based on focal species data, to allow for a ‘like-for-like’ exchange. Substantial and essential improvements were suggested to support future considerations of this concept.