Seismic surveys near gray whale feeding areas off Sakhalin Island, Russia: assessing impact and mitigation effectiveness

Abstract

In 2015, two oil and gas companies conducted seismic surveys along the northeast coast of Sakhalin Island, Russia, near western gray whale (Eschrichtius robustus) feeding areas. This population of whales was listed as Critically Endangered at the time of the operations described here but has been reclassified as Endangered since 2018. The number and duration of the 2015 seismic surveys surpassed the level of previous seismic survey activity in this area, elevating concerns regarding disturbance of feeding gray whales and the potential for auditory injury. Exxon Neftegas Limited (ENL) developed a mitigation approach to address these concerns and, more importantly, implemented a comprehensive data collection strategy to assess the effectiveness of this approach. The mitigation approach prioritized completion of the seismic surveys closest to the nearshore feeding area as early in the season as possible, when fewer gray whales would be present. This was accomplished by increasing operational efficiency through the use of multiple seismic vessels and by establishing zones with specific seasonal criteria determining when air gun shutdowns would be implemented. These zones and seasonal criteria were based on pre-season modeled acoustic footprints of the air gun array and on gray whale distribution data collected over the previous 10 years. Real-time acoustic and whale sighting data were instrumental in the implementation of air gun shutdowns. The mitigation effectiveness of these shutdowns was assessed through analyzing short-term behavioral responses and shifts in gray whale distribution due to sound exposure. The overall mitigation strategy of an early survey completion was assessed through bioenergetics models that predict how reduced foraging activity might affect gray whale reproduction and maternal survival. This assessment relied on a total of 17 shore-based and 5 vessel-based teams collecting behavior, distribution, photo-identification, prey, and acoustic data. This paper describes the mitigation approach, the implementation of mitigation measures using real-time acoustic and gray whale location data, and the strategy to assess impacts and mitigation effectiveness.

Keywords: Bioenergetics; Eschrichtius robustus; Mitigation effectiveness; Russia; Sakhalin Island; Seismic survey.

Fig. 1

Overview of the 2015 seismic survey areas and their position relative to the 95% kernel contours of the nearshore and offshore feeding areas. The Chayvo and Arkutun Dagi seismic surveys were adjacent to each other and are therefore shown as one area. The insets on the upper right show the mitigation zones for the seismic survey areas. Shore-based acoustic stations received data from the Autonomous Underwater Acoustic Recorders with VHF and Iridium data transmission capability (IR-AUAR) (Rutenko et al., 2022a)

Fig. 2

Seasonal gray whale abundances within the nearshore feeding area based on scan data from aerial (2001–2005), vessel-based (2002–2013), and shore-based (2004–2013) surveys were used to establish seasonal criteria for behavioral shutdowns: 1 July for Zone A1 (solid line) and 15 July for Zone A2 (dotted line). Whale count per survey is the average number of whales counted per survey per day

Fig. 3

Integrated real-time processed acoustic and gray whale location data and vessel positions as received by the Central Post. The 95% kernel contour (blue line) represents the nearshore feeding area boundary. The pre-season modeled and field adjusted 163 dB re 1 µPa² SPL footprints were displayed for both the entire survey line (orange) and for the active air gun array progressing with the moving seismic vessel (red). Gray whale location data from shore-based teams are shown as green dots