A seismic survey was conducted adjacent to the nearshore feeding ground of gray whales Eschrichtius robustus off northeastern Sakhalin Island, Russia. Scan surveys were conducted at 7 shore stations before, during and after the seismic survey. We investigated whether gray whales shifted their distribution with respect to distance from the shoreline in response to acoustic pulses from the seismic source. To do this, we used linear mixed effects modelling that included effects of detection, space and time. Data were tested for effects of magnitude and presence/absence of sound from seismic activity on whale distance from shore. Sound covariates were estimated over 3 temporal scales (8 h, 3 d and since the start of seismic activity) at locations 500 and 5000 m offshore each observation station. Sighting distance from shore was less in poor visibility and at earlier times of day. No significant effects of sound were identified, although data suggest that at most stations, sighting distance from shore increased slightly over the 2 wk of the seismic survey. The analysis was limited, however, by several factors that included low numbers of sightings throughout most of the study, non-availability of data on biomass of gray whale prey and sources of error that could not be accounted for in the model. Sensitivity to potential errors in sighting distance estimation was assessed using a correction factor based on known locations of vessels and gray whales when sighted. The model was refitted using distance-corrected sightings. Results were consistent with the original model.
A seismic survey took place during June and July 2010 adjacent to the gray whale (Eschrichtius robustus) coastal feeding area on the northeast Sakhalin Shelf, Russia. Seismic surveys produce underwater sound that can cause hearing injury and behavioural disturbance in marine mammals. In addition to common mitigation measures to prevent acoustic injury, mitigation measures to avoid behavioural disturbance to gray whales within the feeding area were applied. This behavioural mitigation required delineation of the feeding area; however, no clear boundary was obvious because gray whale distribution within the feeding ground varies within and across years. We estimated the feeding area's offshore boundary using a 1.0 km 2 gray whale relative density surface derived from systematic and opportunistic survey data collected during June and July 2005 to 2007. We calculated a separate surface for each of the systematic and opportunistic data sets, then calibrated and merged the 2 surfaces. We evaluated 3 geostatistical kriging methods (ordinary, simple, and co-kriging) that were applied to the merged surface to estimate a smoothed surface across areas with and without survey effort. Simple kriging was most suitable due to its ability to transition over sharp gradients in whale abundance and provide reasonable predictions in data-void areas. A 95% abundance contour of the kriged surface was used as an estimate of the feeding area boundary. Our approach provided an objective and quantitative basis to delineate the feeding area boundary to support measures taken to mitigate the potential impacts of the seismic survey on the whales.
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