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

Abstract

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 km2 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.