Intra‐ and inter‐annual variation in gray whale body condition on a foraging ground

Abstract

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.

Do Gray Whales Count Calories? Comparing Energetic Values of Gray Whale Prey Across Two Different Feeding Grounds in the Eastern North Pacific

“…Mysid species may be more continuously available throughout the foraging season in the PCFG range, but their patchy distribution may require a gray whale to search harder and/or longer before it finds a suitably dense prey patch to offset foraging costs, as compared to a potentially more stable distribution of A. macrocephala in the Arctic. Two previous studies have associated poor body condition of PCFG gray whales with unfavorable environmental conditions linked with poor prey availability and the costs associated with increased time spent searching (Newell and Cowles, 2006;Soledade Lemos et al, 2020). When mysid patches become less dense and/or abundant, PCFG gray whales may have to work harder to find enough prey to meet their energetic demands.…”

“…Collection of zooplankton samples was conducted near the coast of Newport (44 • 38 12 N, 124 • 03 08 W), Oregon, United States, between June and October from 2017 to 2019 as part of a larger study on gray whale ecology (Soledade Lemos et al, 2020). Collection occurred using a simple, low-cost light trap, which was a modified plastic water jug with an LED light placed inside to attract zooplankton (adapted from design in Chan et al, 2016).…”

“…Increased prey quantity, quality, and availability in the Arctic potentially outweigh the trade-off of increased migration costs. Yet, PCFG whales are also able to gain critical energetic mass throughout a foraging season (Soledade Lemos et al, 2020) and successfully recruit calves , all while migrating half as far. Hence, the mystery of the PCFG persists: Why would a gray whale not join the PCFG?…”

Bayesian approach for predicting photogrammetric uncertainty in morphometric measurements derived from drones

“…Sev-eral studies have improved the accuracy of the altitude recorded by using a laser altimeter, e.g. Light-Ware SF11/C LIDAR (Dawson et al 2017), or by measuring a known-sized object during each flight to correct for barometric altitude inaccuracies using linear equations (Burnett et al 2019, Lemos et al 2020.…”

“…Durban et al (2015) first demonstrated the utility of using UAS for acquiring morphometric measurements of killer whales Orcinus orca in a remote location with limited occupied aircraft support. Since then, UAS have been used from tropical to polar environments and applied to several cetacean species of vastly different sizes and body shapes, including blue whales Balaenoptera musculus, humpback whales Megaptera novaeangliae, southern right whales Eubalaena australis, North Atlantic right whales E. glacialis, gray whales Eschrichtius robustus, fin whales B. physalus, Antarctic minke whales B. bonaerensis, and Bryde's whales B. brydei (Christiansen et al 2016, 2018, 2020a,b, Durban et al 2016, Gough et al 2019, Lemos et al 2020). However, a broad range of UAS platforms, sensors, and altimeters were applied in these studies, and there is no unified way to predict photogrammetric uncertainty across this methodological spectrum.…”