Gray Whale Condition and Calf Production
We estimate the number of gray whale calves born each year and assess the whale body condition during their northward migration in the spring. These surveys are essential to tracking the status of the eastern North Pacific gray whale population.
Monitoring gray whale condition and annual calf production are primary focus areas of our gray whale research at Southwest Fisheries Science Center. We estimate the number of gray whale calves born each year and assess the body condition of individual whales during their northward migration in the spring. These surveys are essential to tracking the status of the eastern North Paciﬁc gray whale population.
The eastern North Paciﬁc gray whale population spends a portion of the winter in the shallow, protected lagoons of northern Baja, Mexico, where females nurse and care for their new calves. The northward migration begins in mid-February and is segregated by age, sex, and reproductive condition, with newly pregnant females traveling north first. Mothers with new calves are the last to migrate, traveling along the West Coast between March and May.
Since 1994 we have counted mother-calf pairs by naked eye and with binoculars from the Piedras Blancas Light Station near San Simeon, California. This site is ideal because the whales generally pass within 500 meters of our observers and sometimes stop to nurse their young in the lee of the rocky point.
Counts of northbound mothers and calves have revealed surprising variability in calf production, ranging from 1643 calves in 2004 to 412 calves in 2023. This long-term dataset allows us to examine the relationships between changing environmental conditions (e.g., sea ice) and gray whale population dynamics.
Since 2015, we have also collected aerial photographs of mother-calf pairs to examine how body condition and health of individual whales relate to calf production and population abundance.
The amount of fat on gray whale mothers is an indicator of their physiological status and appears to be closely linked to their survival. Gray whales feed only opportunistically during most of their months-long migration, during which time mothers are also nursing their fast-growing calves. Females, therefore, require a large amount of blubber to fuel the journey to their Arctic feeding grounds. The amount of blubber the whales have depends in part on conditions in the Arctic during the summer prior to the migration. If the whales had a successful feeding season, successfully fattening, then they are best prepared to endure the migration and the energetic demands associated with gestation, birth, and nursing of their calves. If feeding conditions were unfavorable, our data indicate that fewer calves will be born or survive the migration - a pattern that is sometimes lagged over a period of years.
We use small drones, often called uncrewed aerial systems (UAS), to collect high-resolution aerial photographs of the mother-calf pairs as they migrate past our ﬁeld site in Central California. UAS offer a safe and non-invasive way to collect scientific data. In addition to a camera, the drone carries a laser altimeter, which measures the precise altitude from which each image is taken. The length and girth of a photographed whale are measured in the images, and then the altitude data is used to scale the image measurements to the true size of the whale to within a few centimeters. The girth to length ratio reflects the health of the whales and may be an indicator of ecosystem conditions on their Arctic feeding areas the previous summer. This research method (using photographs to measure objects) is called photogrammetry.
Perryman, W. L., M. A. Donahue, P. C. Perkins, and S. B. 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.
About Our Team
This project is directed by Dr. David Weller and led by research wildlife biologists Dr. Tomo Eguchi, Dr. Trevor Joyce, and Dr. Aimée Lang. These scientists also work closely with the International Whaling Commission and the International Union for Conservation of Nature (IUCN) on the conservation of whale populations Pacific-wide.