Gray Whale Condition and Calf Production
We estimate the number of gray whale calves born each year and assess 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.
Calf Counts
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, which has been linked to sea ice cover and reduced prey availability in the Bering and Chukchi Seas as well as to broad-scale environmental indices of North Pacific climate. Total calf production of ENP gray whales has been low since 2019, which marked the start of an Unusual Mortality Event (UME) for gray whales on the west coast of North America. The most recent estimate of calf production was 85 calves in 2025, which is the lowest estimate of the time series. While ENP gray whales have experienced previous periods of low calf production, the duration and extent of the decline that began in 2019 exceed those of earlier periods and suggest that the population may be adjusting to previously unencountered shifts in the Arctic ecosystem. These results highlight the value of long-term monitoring in elucidating factors influencing the population dynamics of ENP gray whales.
Since 2015, we have also collected aerial photographs of adults and mother-calf pairs to examine how body condition and health of individual whales relate to population abundance and calf production and population abundance.
The amount of fat that gray whales carry is an indicator of their physiological status and appears to be closely linked to their survival. Gray whales feed only opportunistically during their months-long migration, a time during which time females are also frequently nursing fast-growing calves. Gray whales, therefore, require a large amount of blubber to fuel the return 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 building up their energetic reserves, then they are better prepared to undertake migration and the energetic demands associated with gestation, birth, and nursing of calves. If feeding conditions were less favorable, 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, also called uncrewed aerial systems (UAS), to collect high-resolution aerial photographs of the mother-calf pairs as they migrate past our field sites along the Central California coast. UAS offer a safe and non-invasive way to collect scientific data. The length, width, and body surface area of photographed whales are measured in each image. The altitude of the UAS and properties associated with the camera are used to scale the measurements within each image to estimate the true size of the whale to within a few centimeters. Ratios comparing width to length reflect the health of the whales and may be an indicator of ecosystem conditions on their Arctic feeding grounds during the previous summer. This research method (using photographs to measure objects) is called photogrammetry.
News Stories
News: Southwest Fisheries Science Center Celebrates 25 Years of Research on Gray Whales
Select Publications
Lang, Aimée R., Tomo Eguchi, and David W. Weller. 2025. Eastern North Pacific gray whale calf production 1994 - 2025. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-725. Doi: 10.25923/83br-pk61
Lang, Aimée, Tomo Eguchi, and David Weller. 2024. Eastern North Pacific gray whale calf production 1994-2024. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-707.
Joshua D. Stewart, Joyce, T.W., Durban, J.W., Calambokidis, J., Fauquier, D., Fearnbach, D., Grebmeier, J.M., Lynn, M., Manizza, M., Perryman, W.L., Tinker, M.T., & Weller, D.W., 2023. Boom-bust cycles in gray whales associated with dynamic and changing Arctic conditions. Science 382, 207-211. doi: 10.1126/science.adi1847.
Eguchi, Tomoharu, Aimée R. Lang, and David W. Weller. 2023. Eastern North Pacific gray whale calf production 1994-2023. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-685.
Eguchi, T., Lang, A.R., and Weller, D.W. 2022. Eastern North Pacific Gray Whale Calf Production 1994-2022. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-667.
Stewart, J. D. and Weller, D. W. 2021. Estimates of eastern North Pacific gray whale calf production 1994-2021. U.S. Department of Commerce, NOAA Technical Memorandum NMFS-SWFSC-653.
Perryman, W.L., Joyce, T.W, Weller, D.W. and Durban, J.W. 2021 Environmental factors influencing eastern North Pacific gray whale calf production 1994–2016. Marine Mammal Science 37: 448-62.
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.
More Information
Gray Whale Research at NOAA Fisheries
Gray Whale Population Abundance Project
Marine Mammal and Turtle Division
