Movements of gray whales between the western and eastern North Pacific

Abstract

The western North Pacific (WNP) population of gray whales Eschrichtius robustus is redlisted by the IUCN as Critically Endangered. As part of a long-term study on whales off Sakhalin Island, Russia, photo-catalog comparisons of gray whales in the western and eastern North Pacific (ENP) were undertaken to assess population mixing. These comparisons involved 2 ap proaches: (1) a systematic comparison of the WNP 'Sakhalin Catalog' to an ENP 'Pacific Northwest Catalog' that consisted of images from the northwest coast of North America and (2) a non-systematic comparison of the WNP 'Sakhalin Catalog' to an ENP 'Laguna San Ignacio Catalog' that consisted of images from central Baja California, Mexico. The Sakhalin to Pacific Northwest comparison consisted of 181 and 1064 whales, respectively, and resulted in 6 matches (3 males, 2 females, and 1 whale of unknown sex). All sightings of 'Sakhalin whales' in the Pacific Northwest occurred off southern Vancouver Island, British Colum bia, Canada. The Sakhalin to Laguna San Ignacio comparison consisted of 181 and 2514 whales, respectively, and re sulted in 4 matches (2 males and 2 females). As the Pacific Northwest and Laguna San Ignacio catalogs represent only a small fraction of the total estimated number of individuals in the ENP population (~19 000), it is likely that more WNP/ENP ex change has occurred than was detected by these photo-catalog comparisons. Although these matches provide new records of movements between the WNP and ENP, recent observations of gray whales off Japan and China suggest that not all gray whales identified in the WNP share a common wintering ground.

Threshold foraging by gray whales in response to fine scale variations in mysid density

“…The capacity of gray whales to respond to scarce or depleted resources is of particular interest as their populations have rebounded to historical levels in the case of the eastern Pacific stock, while western stocks remain critically endangered (Weller et al . ). Whether these populations employ different foraging strategies or thresholds that could limit their ability to access marginal prey resources may be another area of interest for further study.…”

Gray whale densities during a seismic survey off Sakhalin Island, Russia

“…However, other populations, such as all North Atlantic and North Pacific right whale populations, and gray whales in the western North Pacific, remain depleted and continue to be of high conservation concern (e.g. Kraus et al 2005, Weller et al 2012). These small populations are particularly susceptible to anthropogenic threats such as entanglement in fishing gear, exposure to contaminants, habitat degradation, ship strikes, disturbance from vessel traffic, and hearing injury and acoustic disturbance from underwater sound (Clap ham et al 1999, Reeves et al 2003.…”

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

“…If a substantial number of the females that forage off Sakhalin (WGW) do indeed migrate to BajaC to breed, the western population that breeds in BajaC may be more sensitive to disturbance compared to EGW if they cannot compensate for the additional energy needed for a longer migration. Although some data suggest a fully western migratory route for some animals, this seems plausible, as we have yet to identify a contemporary western breeding ground (Weller et al 2002(Weller et al , 2012. Both eastern-and western-foraging females that breed in BajaC appear to be spending similar amounts of energy with regards to reproduction (pregnancy and lactation) given that lactation costs were only 1.5% greater for the longer-migrating western whales.…”

“…Their primary feeding habitat is located off the northeastern coast of Sakhalin Island, Russia (Weller et al 1999, 2002, Ty urneva et al 2010. Some animals travel ~10 000 km from their summer feeding grounds to BajaC (photo identification and tracking data; Mate et al 2011, Weller et al 2012; Fig. 1).…”