Insights into recruitment in the Pacific Coast Feeding Group of gray whales based on resightings of mothers and their calves
Perez, AB, B Gisborne, D Goley, J Jacobsen, W Szaniszlo, A Lang, J Calambokidis. 2015. Insights into recruitment in the Pacific Coast Feeding Group of gray whales based on resightings of mothers and their calves. Abstract (Proceedings) 21st Biennial Conference on the Biology of Marine Mammals, San Francisco, California, December 14-18, 2015.
The majority of eastern North Pacific gray whales feed in the Bering, Chukchi, and Beaufort Seas. Photo-ID and genetics have shown that a subset of this population, the Pacific Coast Feeding Group (PCFG), is a somewhat distinct group of about 200 individuals known to feed through the summer and fall from N California to SE Alaska. Recruitment to this group is an important consideration in understanding its status and management. A large scale collaborative effort was initiated in 1998 to monitor PCFG whales. We describe the occurrence of mothers and calves observed in the PCFG range, their patterns of use, and degree to which calves born in this region served as internal recruitment to this group. A working definition of whales seen after 1 June has been used to separate PCFG whales from those migrating beyond the PCFG though the accuracy of this cut-off point for mothers with calves that migrate later has not been evaluated. Estimates of calf numbers were low in early years of this study, potentially in part due to not all calves being recorded. Identifying mothers and calves was emphasized during 2012 and 2013, and 41% and 50% of known mothers were documented with a calf suggesting a higher and more typical birth rate, though this will need to be confirmed with additional years data. A high proportion of PCFG whales were seen annually so we examined patterns of occurrence for 52 photo-identified mothers seen multiple years through 2013 with one to four calves (total 72 calves). PCFG mothers were seen with a calf as early as 19 April and as late as 26 September. We calculated an average weaning date of 31 July based on the midpoint between when mothers and calves were last seen together and apart. Early weaning dates were associated with calves that were not seen in subsequent years. For 56 calves of known PCFG moms seen prior to 2013, 36 were documented returning to the PCFG, close to the 39 that would be expected with all calves being internally recruited to the PCFG and a 70% 1st year survival. These data provide the first indications of a more typical birth rate for PCFG mothers and survival rate for their calves.