The number of gray whale calves migrating with their mothers along the California Coast this year was one of the lowest on record. The population is still regaining ground after an Unusual Mortality Event that resulted in a sharp decline in overall population numbers.
While the eastern North Pacific gray whale population that migrates annually along the West Coast has rebounded in overall abundance, the production of calves has remained low. A team from NOAA Fisheries’ Southwest Fisheries Science Center found that this year’s count was close to the record low reported in 2022 with an estimated number of 221 mother-calf pairs.
Most of the gray whale population feeds in the Bering and Chukchi seas off Alaska during the summer. Gray whale calf production has been tied to ice cover and food availability in the region. Females must find enough food there to successfully produce and sustain calves.
“While we are no longer seeing the high number of gray whale strandings that the population experienced during the Unusual Mortality Event, calf production has remained low—likely reflecting that some females have yet to regain the energetic resources needed to sustain pregnancy and lactation,” said Aimée Lang, lead author of the new NOAA Fisheries Technical Memorandum reporting the annual calf count.
Researchers have seen other declines in calf production since they started tracking the number of mother-calf pairs in 1994. One of the declines coincided with an earlier Unusual Mortality Event that ran from 1999 to 2000. Estimated calf production remained low in 2001 and then gradually rose in succeeding years.
Another decline from 2007 to 2010 was not tied to an Unusual Mortality Event but coincided with reduced food availability in the Arctic. The reduced food supply appeared to affect reproduction without depressing overall numbers of whales or resulting in increased observed strandings.
Scientists count the northbound pairs of female gray whales with calves from Piedras Blancas, site of a historic light station on the Central California Coast. “We have been tracking calf production in this population since 1994,” Lang said. “The data we’ve collected over the last 30 years have increased our understanding of the links between food availability, body condition, and reproduction in gray whales. They suggest that it may take time for the population to recover its reproductive capacity following events like this past Unusual Mortality Event.”
The science center will continue tracking gray whale numbers and reproduction to help understand changes in the population, including recovery from the recent die-off and its the relationship to climate driven changes in the environment.