Feeding home ranges of Pacific Coast Feeding Group gray whales

Abstract

ABSTRACTThere is a lack of detailed information about the range and habitat use of gray whales (Eschrichtius robustus) during their seasonal occupation off the Pacific Northwest (PNW) coast from northern California to southeast Alaska, USA. These data are important for management because of anthropogenic pressures (e.g., indigenous harvesting, fishing gear entanglements, ship strikes, naval exercises, siting of marine renewable energy facilities). We applied satellite tags to 35 gray whales in the eastern north Pacific (ENP) off the coasts of Oregon and northern California from September to December 2009, 2012, and 2013. These whales are members of the Pacific Coast Feeding Group (PCFG), a subset of gray whales in the ENP that feed off the PNW, during summer and fall. Tracking periods for the satellite‐tagged whales in this study ranged from 3 days to 383 days. We applied a Bayesian switching state‐space model (SSSM) to locations for each whale track to provide a regularized track with 2 estimated locations per day and associated movement behavior (either transiting or area‐restricted searching [ARS]). We isolated the portion of the SSSM track in the feeding area for each whale by removing all southward and northward migration locations. We calculated home ranges (90% isopleths) and core areas (50% isopleths) for these non‐migrating, feeding‐area tracks with >50 SSSM locations using local convex hull utilization distributions. Feeding‐area home ranges for the resulting 23 whales covered most of the near‐shore waters from northern California to Icy Bay, Alaska, and ranged in size from 81 km2to 13,634 km2. Core areas varied widely in size (11–3,976 km2) and location between individuals, with the highest‐use areas off Point St. George in northern California, the central coast of Oregon, and the southern coast of Washington, USA. Nearshore waters off Point St. George were a hot spot for whales in the PCFG in late fall, close to where most of the whales were tagged; 19 whales had overlapping home ranges and 15 whales had overlapping core areas there. One whale, a male tracked for 383 days, did not migrate, spending the entire winter off Point St. George and the California‐Oregon border. Residence times (portions of the track with a minimum of 3 successive locations in ARS behavioral mode) ranged from 1 day to 142.5 days; 19 whales had residencies >30 days in some areas. Because most of the whales in this study were tagged in the fall in the southern portion of the feeding area, off northern California, results are weighted toward fall and winter movements. Although some whales were tracked into the spring and summer, additional tagging earlier in the year and in more northerly locations would provide an even clearer picture of gray whale use of feeding areas in the PNW. Nevertheless, these results constitute valuable information about high‐use areas for gray whales in this region, providing baseline home range data for future comparisons with regard to year‐to‐year variability, potential responses to climate change, and exposure to anthropogenic activities in the marine environment. © 2019 The Authors.Journal of Wildlife ManagementPublished by Wiley Periodicals, Inc. on behalf of The Wildlife Society.