Gray Whales: A Celebration of Conservation
Survey of Northward Migrating Gray Whales
The eastern North Pacific gray whale population summers and feeds mainly in the Chukchi, Beaufort and the northwestern Bering Seas. The population migrates south along the coast in the autumn to wintering grounds on the west coast of Baja California, Mexico, and the southeastern Gulf of California to breed, bear and nurse their young, and cavort before returning to the Arctic. SWFSC scientists monitor this migration each year, conducting shore-based surveys from two sites on the California coast.
The northward migration begins about mid-February and is also segregated according to age, sex and reproductive condition. The first phase of this northern migration includes: (a) newly pregnant females followed later by (b) adult males and anestrous females and then (c) immature whales of both sexes. The second phase consists mostly of mothers with calves. These pairs are observed on the migration route between March and May and generally arrive to the summer feeding grounds between May and June.
Since the spring of 1994, scientists from the Southwest Fisheries Science Center have been monitoring this northbound migration of cows and calves from Piedras Blancas, a point of land just north of San Simeon, California and just south of the Big Sur coast. This site is ideal for counting these whales because these animals generally pass within 200 m of the point and often stop to nurse their young in the lee of the rocky point.
Counts of northbound cows and calves have revealed surprising variability in calf production for this population. It appears that the number of calves born each year is related to environmental conditions in the Arctic that limit prey populations and/or the availability of prey to reproductive females. We have hypothesized that the timing of the melt of sea ice in the Arctic may control access to primary feeding grounds for newly pregnant females and thus impact the probability that existing pregnancies will be carried to term.
This project, along with the SWFSC survey of southbound gray whales and analyses of ice distribution information from the Arctic, enables us to study the link between reproduction in this population and inter-annual climate variability in the Arctic where these whales feed in the summer months. This work has the potential to shed light on important questions about the impacts of climate change on Arctic ecosystems, and on gray whales and other species that depend on these ecosystems for their survival.
Pregnant female gray whale (left) compared to a non-pregnant female (right)
Top: Pregnant female gray whale (cow)
Middle: Cow with newborn calf
Bottom: Cow with older calf
Gray whale breeding and calving are seasonal and closely synchronized with migratory timing. Sexual maturity is attained between 6 and 12 years of age. Breeding is highly synchronous, with females coming into estrous during a three-week period from late November to early December, which coincides with the onset of the southward migration. Conception is restricted to a short period between late November and early January. If there is no conception, a second estrous may occur 40 days later when the whales are on the wintering grounds. The gestation period is estimated to be 12 to 13 months, with a mean calving date in mid-January. Some calves are born during the southward migration while others are born near or on the wintering grounds. Females normally reproduce at intervals of two years, producing a single calf every other year. At birth, calves are about 15 to 16 feet (4.6 to 4.9 m) long and weigh about 1500 to 2000 lbs (680 to 920 kg). Calves are weaned and become independent by seven to nine months old while on the summer feeding ground.
Figure 1. Estimated number of gray whale calves present during northbound migrations from 1992 to 2013
Perryman, W.L., Reilly, S.B. and Rowlett, R.A. 2011. Results of surveys of northbound gray whale calves 2001-2010 and examination of the full sixteen-year series of estimates from the Piedras Blancas Light Station. Paper SC/62/BRG1 presented to the IWC Scientific Committee.
Perryman, W., Donahue, M., Perkins, P., and S. Reilly. 2002. Gray whale calf production 1994-2000: Are observed fluctuations related to changes in seasonal ice cover? Marine Mammal Science, 18(1):121-144.