Monitoring and impact mitigation during a 4D seismic survey near a population of gray whales off Sakhalin Island, Russia

Abstract

A 4D seismic survey was conducted in 2010 near the feeding grounds of gray whales off Sakhalin Island, Russia. To minimize disruptions to the whales' feeding activity and enhance understanding of the potential impacts of seismic surveys on gray whales Eschrichtius robustus, an extensive monitoring and mitigation plan (MMP) was developed. Typically, mitigation plans involve observers on seismic vessels to monitor for the presence of marine mammals in an exclusion zone so as to prevent physical injury to the animals. Due to the protected status of western gray whales, an additional protection zone based on a behavioural disturbance threshold of exposure of 156 dB re μPa 2 -s per pulse was applied for whales within their feeding habitat defined by the estimated 95% abundance contour. Real-time radio-transmitting acoustic recorders were deployed along this contour to verify modelled acoustic footprints within the feeding grounds. Shore-and vessel-based observation teams monitored for the presence and activity of whales. A real-time GIS workflow tracking procedure was developed that integrated acoustic and whale positioning data to determine if sound levels at a whales' position within the feeding area exceeded the behavioural threshold, in which case a shut-down of the seismic source was implemented. Additionally, behaviour and distribution surveys were conducted before, during and after the seismic survey to evaluate the effectiveness of the MMP. No large changes in whale movement, respiration, or distribution patterns were observed during the seismic survey. This could be interpreted to mean that the MMP was effective in reducing the sound exposure and behavioural responses of gray whales to seismic sounds.

Delineation of a coastal gray whale feeding area using opportunistic and systematic survey effort

“…Conversely, using only opportunistic surveys with most effort in deeper waters would place the feeding boundary too far offshore, possibly resulting in delays of the seismic survey to avoid disturbing gray whales in deeper waters that may have been transiting instead of feeding. Such delays could have conflicted with the primary mitigation of completing the survey as quickly as possible before many gray whales arrived on their feeding ground (Bröker et al 2015). Critical habitat demarcation can be difficult, particularly for mobile and cryptic species such as marine mammals that can also have large geographic ranges (Wheeler et al 2012).…”

“…Consequently, a monitoring and mitigation plan (MMP) was developed that used an equivalent per-pulse sound exposure value of 156 dB re µPa 2 -s (156 dB SEL) as a threshold level for disturbance of gray whale behaviour (Bröker et al 2015). The MMP required estimation of the Piltun feeding area boundary to delineate areas ('A-zones') within the feeding ground where sound levels exceeded 156 dB SEL.…”

East or west: the energetic cost of being a gray whale and the consequence of losing energy to disturbance

“…For example, disturbances could be limited to specific times of day or at the beginning, middle, or end of the foraging season. The current mitigation strategy is to try and avoid or reduce any interactions by conducting activities as early in the foraging season as possible before the gray whales arrive (Nowacek et al 2013, Bröker et al 2015. In addition, disturbance earlier in the season may allow exposed animals to compensate for lost foraging by increasing their foraging effort during non-disturbance periods later in the season.…”

Monitoring the gray whale sound exposure mitigation zone and estimating acoustic transmission during a 4-D seismic survey, Sakhalin Island, Russia

“…The acoustic monitoring team communicated with teams of behavioural observers posted onshore at the northern and southern boundaries of the PML, as well as on a vessel within the survey grid. The observation teams employed a GIS utility that combined modelled sound exposure contours with visually localized animal positions to enable sound exposure threshold assessments (Bröker et al 2015). The ob servers loaded the appropriate (as determined by the acoustic team) sound exposure contour into the GIS utility and, based on that boundary, determined whether any observed whales might be exposed to sound above a threshold level.…”

“…It was recognized that airgun sounds from the 2010 survey would have the potential to cause disturbance to the gray whale feeding activities. As such, a detailed acoustic monitoring study formed part of the monitoring and mitigation plan designed by SEIC and the IUCN 's Western Gray Whale Advisory Panel (WGWAP) through its Seismic Survey Task Force to minimize survey impacts (IUCN 2008b, 2009, Bröker et al 2015. The acoustic monitoring infrastructure included a line of telemetric real-time bottom-anchored buoys transmitting full waveform data to shore, an evolution of a conceptually similar design used to monitor a seismic survey in an adjacent region in 2001 (Rutenko et al 2007).…”