Gray whale Eschrichtius robustus population and stock identity



    Corresponding author
    1. NOAA Fisheries Service, Office of Science and Technology, 1315 Eastwest Highway, SSMC 3, Rm 12552, Silver Spring, Maryland 20910, USA,
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    1. Southwest Fisheries Science Center, NOAA Fisheries Service, PO Box 271, La Jolla, California 92038, USA,
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    1. National Marine Mammal Laboratory, Alaska Fisheries Science Center, NOAA Fisheries Service, 7600 Sand Point Way, NE, Seattle, Washington 98115, USA
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  • Editor: PC

S. L. Swartz. E-mail:


  • 1In 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.
  • 2Here, 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.
  • 3While 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.
  • 4The 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.