Each winter, the entire Eastern Pacific (or California) population of gray whales migrates from its primary feeding grounds in the Arctic to the lagoons and near-shore areas of Baja California, Mexico to breed, bear and nurse their young, and cavort before returning to the Arctic. The northbound migration is composed of two distinct pulses of whales. The first is composed of adults and juveniles, and the second is made up almost entirely of cows with their three month old calves. 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. See estimates of gray whale calf abundance from 1994-2006. 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.