Changing ocean conditions threaten the health of whales. A new study has identified a strong correlation between the health of gray whales and changing ocean conditions. Resident gray whales along the coast of Oregon were found to shrink in body size when poor ocean upwelling shifted the availability of their prey.
For three years, experts with the Marine Mammal Institute at Oregon State University (OSU) used drones to monitor 171 whales during the foraging season between June and October.
The team discovered that the health of the whales declined following a period of poor upwelling, which is an ocean process that brings colder, nutrient-rich water closer to the surface. The poor upwelling likely diminished the availability of zooplankton, the gray whales’ primary food source.
“What we see is this compelling relationship between the oceanographic processes that control the quality and quantity of available prey and whale health,” said Professor Leigh Torres. “This research gives us an inclination that changes in ocean conditions might be causing skinny whales.”
The findings may also provide insight into the unusual gray whale die-off event that occurred in 2019 on the Pacific Coast, explained Professor Torres.
Along the Pacific coast of North America between Mexico and Alaska, more than 200 gray whales turned up dead. The body condition of most of the stranded whales indicated they had died of starvation, but scientists were not sure why.
“With this research, we’re trying to understand more about the health of the whales and how it varies throughout the foraging season and from year to year,” said study lead author Leila Soledade Lemos. “Once we establish a baseline for whale body condition, we can start to see what is healthy and what is not and why.”
The research team used drone images and fecal samples to conduct “health check-ups” on the whales in 2016, 2017, and 2018. The images allowed the experts to calculate the Body Area Index (BAI) of the whales, which is similar to the Body Mass Index (BMI) in humans. The fecal samples helped the researchers determine a whale’s sex, hormones, and diet.
“The first year the whales looked really fat and healthy. But after 2016, the whales were really skinny. You could see their skeletons,” said Lemos. “For these whales, body condition is strongly related to food availability. It is also related to when they invest in reproduction.”
The researchers documented nine pairs of mothers and calves in 2016, but only one pair in the two years following. Calves and pregnant females had the highest BAI numbers, while lactating females had the lowest BAI and the worst body condition.
Overall, the health of the gray whales deteriorated after poor upwelling conditions began in 2016. “There was a one-year lag, or carry-over, between the lack of prey in 2016 and the whales’ body condition the next year,” said Professor Torres.
The researchers now have four years of data on Oregon’s summer resident whales and will continue monitoring them to investigate how their health is connected to changing ocean conditions. Professor Torres said the research highlights the value of monitoring whale health over time.
The study is published in the journal Ecosphere.