While sophisticated tools are used to monitor behavioral changes of large marine vertebrates, determining whether these changes are meaningful for management and conservation is challenging. The Population Consequences of Disturbance model proposed a bioenergetics model to detect biologically meaningful population responses, where disturbance costs are linked to lost energy. The model assumes that changes in behavior, caused by disturbance, compromise maternal condition, reducing energy delivery to offspring, leading to reduced reproduction, increased offspring mortality, and eventually increased adult mortality. Given its coastal habits and past whaling history, gray whales' (Eschrichtius robustus) life history and ecology are better known than for many other baleen whales. However, their preference for coastal habitat increases their exposure to human disturbance. We created a female gray whale bioenergetics model to determine energy requirements for a two‐year reproductive cycle and determined the consequences of lost energy under three possible disturbance scenarios. An annual energetic loss of 4% during the year in which she is pregnant, would prevent a female from successfully producing/weaning a calf. For this reason, gray whale reproduction is particularly sensitive to disturbance during pregnancy. During the year in which she is lactating, she would wean her calf at a lower mass with a 37% energetic loss. A female would lack the energy to become pregnant during a year with a 30–35% energetic loss, and female mortality would likely occur at 40–42% annual energetic loss. Our model can be used for assessing disturbance costs or other effects associated with climate change and/or anthropogenic activities and can be applied to other species with similar life histories.

This study was conducted to determine the swimming velocities and breathing patterns of south-migrating gray whales (Eschrichtius robustus) and to estimate their minimum costs of transport during migration. Swimming velocities and breathing patterns were monitored with theodolite techniques from a coastal vantage point on Point Loma, San Diego County, CA. Estimates of energy expenditures were based on observed breathing rates and on extrapolations of tidal lung volume and oxygen uptake measurements made on unrestrained and on captive gray whales. The results of 74 whales monitored over 354 whale∙km yielded mean velocities of 2.0 m∙s−1 and mean breathing rates of 0.72 breaths∙min−1. The regression of breathing rates on swimming velocities (V) by the method of least squares is best described by the equation, breathing rate = 0.5 + 0.024 V3.0. The calculated minimum cost of transport for a 12-m, 15-t whale was 0.046 W∙s∙N−1∙m−1 at the observed mean swimming velocity of 2 m∙s−1 during the southward migration. The coefficient of total drag was estimated to be 0.06. Rates of lipid depletion approximate 6% of body weight per month if no supplementary food intake occurs for the 4- to 6-month period each year that gray whales are presumed to fast.

Baleen whales store energy gained on foraging grounds to support reproduction and other metabolic needs while fasting for long periods during migration. Whale body condition can be used to monitor foraging success, and thus better understand and anticipate individual-and population-level trends in reproduction and survival. We assessed the body condition of eastern North Pacific gray whales (Eschrichtius robustus) on their foraging grounds along the Oregon coast, USA, from June to October of three consecutive years (2016)(2017)(2018). We used drone photogrammetry and applied the body area index (BAI) to measure and compare whale body condition, which is a continuous, unitless metric similar to the body mass index in humans. A total of 289 drone flights were carried out over 106 photo-identified whales, which were grouped into demographic units by sex, maturity, and female reproductive status. Calves and pregnant females displayed the highest BAIs, followed by resting females, mature males, and, finally, lactating females, reflecting the significant energetic demands on reproductive females. In all three years, gray whale body condition improved with the progression of feeding seasons, demonstrating the accumulation of body energy reserves on the foraging grounds. Yet, body condition was significantly better in 2016 than in 2017 and 2018 when overall body depletion was observed, indicating a difference in prey availability and/or quality between years. We analyzed local upwelling patterns between 2013 and 2018 as an oceanographic proxy for prey and determined significantly greater upwelling between 2013 and 2015 than low upwelling years between 2016 and 2018. We hypothesize that these upwelling patterns created ecosystem shifts in primary productivity and zooplankton prey of gray whales, causing carry-over effects between foraging success and body condition in subsequent years. This study demonstrates the value of monitoring whale body condition to better understand temporal variation in foraging success, and potentially detect and describe the causes of anomalous changes in whale population health, such as the 2019 gray whale mortality event.

AEISTRACT Mean body lengths of gray whale calves were found to inaease linearly from 4.6 m at birth to 7 m by weaning at six mo. After weaning, rates of length inaease diminish, with calves reaching 8 m by one yr of age and 9 m by two yr. Evaluations of the weights of nine gray whales as functions of their measured lengths and girths reduce the emphasis placed on fast-induced seasonal variations in girth by Rice and Wolman (1971). From birth weights of just under one metric ton, calves double their weights by three mo of age and double again by weaning at six mo.

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