Evaluating Gray Whale Health and Condition
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Gray whale body condition is an indication of the whales’ success at seasonal foraging, which in turn is indicative of reproductive trends of individuals and of the population. Two different methods are used to assess gray whale body condition in Laguna San Ignacio: First, digital photographic images taken from a boat are used to evaluate the amount of body fat present and the condition of three body areas (the head, scapula, and flank). Each are assigned a numerical score and classified as “good”, “fair” or “poor” condition depending on the amount of body fat-tissue in these regions.
The second method utilizes UAV-drone photogrammetry with video cameras to photograph gray whales as they surface to breathe. Digital images of the backs of each whale are selected from the videos and were catalogued by age and sex of the whales as females with calves and single adults without calves. Body condition is then evaluated based on the measurements of the width and length ratios of each photographed whale compared to measurements of known healthy individuals. All drone photographs are sorted into reproductive class categories based on total body length (e.g., calves, juveniles, and adults).
These methods allowed the monitoring of the whales' condition during the gray whale Unusual Mortality Event (UME) from 2019-2023 that was declared by the U.S. National Oceanic and Atmospheric Administration (NOAA). During this period gray whale mortalities and strandings increased significantly throughout the whales' range, many "skinny" whales were observed in the winter aggregation and breeding areas, and the number of newborn calves declined precipitously.
Boat-Based Condition Analysis: Results for 2019 to 2026 winter seasons
Since 2023 single gray whales have also shown a continuous decrease in the percentage of individuals with a good body condition, going from 70% in 2023 to 45.3% in 2026. During the same time period the percentage of single whales that present a fair body condition has increased, from 21.2% in 2023 to 40.9% by 2026. In 2025 the percentage of single whales with poor body condition spiked up to 12.9%, and 2026 showed a slight increase with 13.8%.
Female-calf pairs have shown predominantly a good body condition during and after the 2019-2023 UME, with very few females presenting a poor body condition. In 2025, 87.5% of the females presented a good body condition, and 12.5% a fair body condition. This year (2026) for the first time 100% of the females presented a good body condition, however it is important to highlight that the number of female-calf pairs in this past couple of years have been historically low for San Ignacio Lagoon with only 8 pairs.
UNMANNED AERIAL VEHICLE (UAV) ASSESSMENT OF BODY CONDITION
Watch a video on assessing condition with drones
Beginning in 2017 Unmanned Aerial Vehicles (UAVs) or drones have been used to obtain high resolution digital photogrammetric images of gray whales in Laguna San Ignacio. These images are used to evaluate and track trends in gray whale body and reproductive condition. To assess body condition images of the whales on the surface with their body contours perfectly visible in width and length and their bodies completely straight.
The dorsal surfaces of a gray whale are used to measure body length and width every 5% of the whale’s length from the tip of the rostrum to the notch of the tail flukes. For lateral images, height (dorso-ventral distance) was measured along the same 5% measurements.
All measurements are scaled (converted from pixels to meters) using the known altitude of the UAV (measured using a laser range finder), the camera sensor size, focal length and image resolution. Each whale was classified into a specific reproductive class according to published lengths as a calf, juvenile, adult including lactating females. These scaled measurements were used to estimate the volume of each 5% section, and these summed to estimate body mass. This process was repeated with multiple photographs of the same individual whales obtained over the winter for comparison of volume/mass gain or loss over the duration of their stay in the lagoon.
Changes in body condition over the winter for each reproductive class are then plotted as “early” in the season (mostly January) against “late” in the season (mostly late-March to early-April). Average body condition was calculated from the sum of data for all whales sampled for each length class (i.e., calf, juvenile, adult, and lactating females). A positive or good body condition means that an individual was in relatively better condition than the average individual of the same body length, whereas a negative body condition indicated that the individual was in relatively poorer condition. Every reproductive class except calves showed a declining trend in condition (volume/mass) over the winters from 2017 to 2020. This would be expected as all gray whales, except calves, are not feeding during the winter reproductive season and their body mass would decrease as they metabolize their body fat. Calves on the other hand are obtaining nourishment from the fat rich milk they nurse from their mothers, and their condition increases as they grow and put on weight during the winter. An emaciation threshold was derived from similar measurements of stranded, dead, emaciated whales (see red broken lines on figure below).
These trends of declining body condition suggest that if breeding females were unable to obtain sufficient food during the summer of their pregnancy, their condition may continue to decline to a low level after the birth of their calves. A long-term consequence of this strategy is that post-weaning females would require more time to recover sufficient fat reserves to breed again and reduce calving rates. This could account for the low calving rates observed in the wintering breeding lagoons during the UME from 2019 to 2023. In addition, the majority of stranded dead gray whale discovered in Mexico at the beginning of the UME were adults. UAV drone assisted photogrammetry has proven to be a valuable source of information to evaluate the health of individuals whales and their population as they recover from this extreme mortality event and continue to meet climate change effects on their food resources.
Changes in environmental conditions are likely to affect whale body condition through a direct impact on seasonal production of whale prey resources sources throughout the gray whales’ range. Additional research on the correlation between environmental changes and gray whale prey production in relation to the body condition will aid our understanding of the effect of these changes on individual gray whales and on the gray whale population in the future. Additional analysis will focus on identifying known breeding age females with known reproductive histories, to determine when they are expected to have calves during a winter but are not seen with calves.
