Whether these patterns map onto a supply of fish flesh or fresh foliage, the principle is the same: As waves of food sweep the world, waves of animals track them. At least, that’s what happens on land. No one had examined this phenomenon in the oceans before, until Abrahms and her colleague Ellen Aikens came up with a plan to study blue whales.

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The blue whales of the North Pacific spend their winters in their breeding grounds off California and Costa Rica. Come spring, they swim up the coast of North America toward the food-rich summer waters of the Pacific Northwest. They could make the journey in two months (and they do, on the reverse trip back south). Instead, they take twice that time, pausing to gorge themselves on blooms of krill that appear along the way. It’s a leisurely season-long tour of a continent-wide buffet line.

Scientists can get a good sense of this changing buffet by measuring the concentrations of chlorophyll in different patches of ocean. This green pigment reflects the amount of plankton, which in turn is eaten by krill. The more chlorophyll there is, the more food a blue whale might find.

By comparing chlorophyll counts to whale movements, Abrahms and her team expected to see that “they follow the timing of their prey, as it becomes available,” she says. But they were surprised to learn that the animals very rarely tracked contemporary waves of krill. Instead, their movements were strongly correlated with 10-year historical averages of chlorophyll. Put it this way: You could predict a blue whale’s movements with far more accuracy by looking at where their food has been than where their food currently is.

In retrospect, the idea that whales would surf historical green waves “makes a lot of sense, given how dynamic the ocean is,” says Abrahms. “There’s so much variability year-to-year that the whales can hedge their bets by going with the average timing that they’ve experienced in the past.” Indeed, the team also found that the whales favor areas with unusually low year-to-year variation in chlorophyll counts.

“It’s striking that they return to the best and most consistent areas on average, rather than tracking current conditions,” says Chloe Bracis from IFREMER. “This implies that they could be using memory to return to these locations.” It’s “a big step forward” to have that kind of data for a wild marine species, adds Sabrina Fossette from Swansea University, who would love to see similar studies in other marine mammals and sea turtles.

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Understanding whales’ migration decisions is especially important given how quickly the world is changing. Nature’s schedule is becoming more erratic, and once-reliable timings are fluctuating beyond their normal variations. “The characteristics of the Anthropocene are very different from the past,” says Abrahms. Once, a blue whale’s memory might have sent it off course in a weird year, but what if every year is weird? “The concern is that if the whales get the timing too wrong, they’d miss their food,” Abrahms says. “We’re starting to see a lot of that in other species, with migratory birds arriving in their breeding grounds after the best food has come and gone.”