GRAY WHALES (ESCHRICHTIUS ROBUSTUS) OFF SAKHALIN ISLAND, RUSSIA: SEASONAL AND ANNUAL PATTERNS OF OCCURRENCE¹

Abstract

The annual return, seasonal occurrence, and site fidelity of Korean‐Okhotsk or western gray whales on their feeding grounds off northeastern Sakhalin Island, Russia, were assessed by boat‐based photo‐identification studies in 1994‐1998. A total of 262 pods were observed, ranging in size from 1 to 9 whales with an overall mean of 2.0. Sixty‐nine whales were individually identified, and a majority of all whales (71.0%) were observed in multiple years. Annual sighting frequencies ranged from 1 to 18 d, with a mean of 5. 4 d. The percentage of whales reidentified from previous years showed a continuous annual increase, reaching 87.0% by the end of the study. Time between first and last sighting of identified individuals within a given year was 1‐85 d, with an overall mean of 40.6 d. Annual calf proportions ranged from 4.3% (1997) to 13.2% (1998), and mother‐calf separations generally occurred between July and September. The seasonal site fidelity and annual return of whales to this part of the Okhotsk Sea emphasize its importance as a primary feeding ground for this endangered population.

DNA evidence for historic population size and past ecosystem impacts of gray whales

“…In the absence of a direct estimate of this parameter from lifetime measures of reproductive success in males and females, we assumed the most conservative ratio of 2. Juvenile abundance can be more confidently estimated from census data (13,16).…”

“…However, two other populations may have contributed genetic diversity to the eastern Pacific population in the past: the western Pacific population (16) and an extinct Atlantic population (22). A recent study comparing mitochondrial control region data between the eastern and western Pacific gray whale populations indicates an F ST value of 0.087 between these two populations (23), potentially inflating our estimate of population size in the eastern population by about a factor of Ϸ9.5% [1/(1 Ϫ 0.087) ϭ 1.095].…”

“…However, census size of animal populations is typically higher than effective size because not all adults successfully breed. We converted effective size N e into total census estimates (N) by multiplying by a conservative 2:1 ratio of total adults to breeding adults (15) and the ratio of total population size to total adults, estimated between 1.58 and 1.78 (13,16) based on census and fisheries data.…”

Survival estimates of western gray whales Eschrichtius robustus incorporating individual heterogeneity and temporary emigration

“…Western gray whales were proposed to be extinct as recently as the early 1970s (Bowen 1974), but are presently known to survive as a remnant population (Brownell & Chun 1977, Blokhin et al 1985, Weller et al 1999. Recent mark-recapture abundance estimates indicate that the population may currently consist of approximately 100 individuals (Wade et al 2003).…”

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

“…The migration timing, routes, and breed-ing grounds of western gray whales (WGW), however, are poorly understood. 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.…”