Are blue whales going silent off the California coast? Here's what new data shows
While humpback whale populations steadily recovered over subsequent years, blue whales — the biggest animals in the world — struggled amid changing ocean conditions. The authors uncovered that this change was due to foraging behavior. Such information is crucial for the restoration of whale populations, which have been decimated by human activity, said lead author John Ryan, a biological oceanographer at the Monterey Bay Aquarium Research Institute.
'We have to understand where and how they live,' Ryan said. 'And how our activities intersect with their lives.'
The scientists carefully listened to the ocean environment, about half a mile below the surface, just outside the Monterey Bay. They used an underwater device known as a hydrophone: 'It's picking out all kinds of sounds, within and outside the range of human hearing,' Ryan said.
The authors analyzed data collected from July 2015 to June 2021. They used machine learning to pick out humpback whale songs, which are complex and can span more than nine octaves, Ryan said. The scientists listened in on specific frequencies to pick out the songs of blue and pin whales.
An analysis of the different species revealed distinct patterns over the six-year period. The percentage of days that humpback whale songs were detected steadily rose from year to year. By contrast, the share of days that blue whales songs were captured rose, then dipped following the third year of the study.
The first year of the study marked a difficult time for whales. A marine heat wave depleted food resources, which meant harder hunting and scarcer prey. That year also coincided with 'the most severe toxic algal bloom we've ever seen on the planet,' Ryan explained. 'Poisoning of marine mammals by this toxic algal bloom occurred over the largest scale ever observed.'
A harmful algal bloom sickened hundreds of sea lions and dolphins in Southern California this year.
During the marine heat wave, krill and fish were in low abundance, forcing blue and humpback whales to spend more time foraging for food. This left less time for singing behavior.
By the third year of the study, krill — small, shrimplike crustaceans that blue whales depend on for calories — were relatively abundant.. But by the fifth year of the study, krill abundance dropped, while fish were highly abundant, benefitting only humpback whales.
'Humpback whales would have an advantage because they are more flexible in their diet,' Ryan said.
Based on chemical studies, the scientists suggest that blue whales traveled further to find food during this time, leaving less time for singing behavior.
The authors investigated other potential reasons for the changes in whale song, such as population abundance, but changes in foraging conditions were the most consistent explanation.
The study indicates that blue whales could be more vulnerable to changing ocean conditions than humpback whales. This could especially be a challenge in the future, as scientists project marine heat waves to become more frequent, longer, larger and more intense.
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USA Today
3 days ago
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