Gray whaleEschrichtius robustuspopulation and stock identity

Abstract

1. In response to conservation and management concerns about gray whale Eschrichtius robustus population and stock structure, we provide an overview of the life history and ecology of gray whales as a context for discussion of population and stock structure within the species. Historically eastern and western North Pacific gray whales were managed separately because: (i) their ranges do not overlap; (ii) genetic analyses indicate that the two populations are significantly different; and (iii) eastern gray whales have increased in abundance over the past century while western gray whales have not. 2.Here, we review gray whale migration timing and segregation, feeding and prey species, and reproduction and reproductive behaviour. For the eastern and western gray whale, we review their distribution, history of exploitation, abundance and current status, although most of what is known is founded on the better studied eastern gray whale and only implied for the lesser known western gray whale. Methods to investigate population and stock identity are reviewed including genetics, morphology, chemical signatures, carbon isotopes, parasites, photographic identification and trends in abundance. 3. While the evidence indicates that there is at least some degree of mixing within each of the gray whale populations, no stocks or sub-stocks can be defined. Population structure is not evident in nuclear data, and because selection occurs primarily on the nuclear genome, it is unlikely that there is structuring within each population that could result in evolutionary differences. For western gray whales, there are insufficient data to assess the plausibility of stock structure within the population, owing to its extremely depleted state. Research on eastern gray whales has focused mostly on documenting changes in abundance, feeding biology and behaviour, and suggests separate breeding groups to be unlikely. Both males and females are promiscuous breeders lending little opportunity for the nuclear genome to be anything other than well mixed as is suggested by the high haplotypic diversity of the eastern population. 4. The available data strongly indicate that western gray whales represent a population geographically isolated from eastern gray whales and therefore that the western and eastern populations should be treated as separate management units.

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

“…Very few data are currently available on historical abundance and whaling effort in the western Pacific stock (20), but it is possible that this population was substantially larger than it is now. Future analysis of these two populations using multiple nuclear loci and the IM approach used by Won and Hey (25) to investigate ancestral and effective sizes in chimpanzees might be a profitable way to address these issues.…”

“…Available data show no observable structure within the eastern Pacific gray whale population (reviewed in ref. 20). To further confirm this result, we collected data from six microsatellite loci for the same whales used in this study and estimated the probabilities of one population (K ϭ 1) versus multiple populations (K Ͼ 1) by using STRUCTURE, version 2 (21).…”

Comparing Evolutionary Patterns and Variability in the Mitochondrial Control Region and Cytochrome b in Three Species of Baleen Whales

“…Samples from individual gray whales were taken between 1991 and 2004 from the eastern North Pacific population via biopsy and necropsy sampling (see LeDuc et al 2002, for additional sampling information), which is thought to be panmictic (Swartz et al 2006). DNA extraction of seven additional eastern North Pacific gray whale muscle and blubber tissues was performed using a QIAamp kit (QIAGEN, Valencia, CA).…”

“…Humpback whales show extensive population structure across and sometimes within ocean basins (Baker et al 1993). No strong genetic differentiation has been found within eastern Pacific gray whales (Swartz et al 2006), though the lack of strong structure might reflect whaling history (Alter et al 2008). Population structure within Antarctic minke whales is largely unknown, but recent studies have indicated some differentiation between whales in the Pacific and those in the Indian Ocean sectors of Antarctic waters (Pastene 2006).…”

Predicting seasonal density patterns of California cetaceans based on habitat models

“…Broadscale seasonal migratory patterns of many of the large whale species have been described (e.g. Calambokidis et al 2000, Swartz et al 2006, Barlow et al 2011, and although pronounced seasonal distribution shifts of small cetaceans have been identified (Dohl et al 1986, Green et al 1992, 1993, Forney & Barlow 1998, they are typically not as well understood and may vary considerably from year to year.…”