MONTEREY — When swimming along California’s coast, blue whales, the largest known animals on earth, act like veteran tourists. Year after year, they choose to eat at the same spots that have given them a consistent fine dining experience.

Monterey Bay is one such dining hotspot that blue whales frequent on their migratory journey from Mexico and Costa Rica to British Columbia. But what’s guiding them back to these exact same sites? A recent study suggests that it’s their exceptional memory.

Instead of chasing unreliable bursts of food, blue whales have timed their movements based on consistent food availability. “They’re making decisions based on experiences they’ve gained over a long time scale,” said Briana Abrahms, lead author of the study and marine ecologist at the National Oceanic and Atmospheric Administration. Blue whales live up to 90 years, and are known to migrate seasonally. “They are consistently hitting foraging areas at the most predictable times of the year rather than necessarily at the peak time.”

Blue whales primarily feed on tiny crustaceans called krill. In Monterey Bay there are two time periods when krill numbers peak — May and August. In May, krill abundance is typically the highest, but this quantity fluctuates on a year-to-year basis. In August, on the other hand, krill numbers are relatively low, but there is a consistent supply that keep the whales coming back.

In the future, climate change will dictate whether these historic krill reserves continue to remain steady. What remains to be seen is whether creatures like blue whales, which rely heavily on past conditions to find food, will cope.

A decade of following blue whales

Before the 1990s, very little was known about the basic ecology of blue whales – where they migrated to, how long were they gone for, where they bred. A research team at Oregon State University started putting satellite tags on these whales and tracking their movement.

Most tags didn’t last longer than a year. So, the team had to tag individuals, old or new, every year.

They began to learn a lot about the whale’s migration patterns. But Abrahms wanted to understand what influenced their decisions about where to go and when to go.

On land, studies of elk and deer migration had indicated that these animals were tracking seasonal bursts of highly nutritive food. Scientists were able to use satellite images to estimate how much plant matter was available for the animals to eat. Chlorophyll, the photosynthetic pigment that gives plants their distinct green color, made this estimation possible.

But blue whales devour krill, which are zooplankton, and naturally don’t photosynthesize. “It is much harder to measure their (blue whales) feed,” Abrahms said. “We have to rely on proxies.”

Krill feed on phytoplankton, the marine equivalent to plant matter on land, and make for ideal proxies. By mapping chlorophyll in these marine algae, Abrahms measured phytoplankton abundance in the ocean. She used this measure to indirectly estimate how much krill was available for returning blue whales.

Then, she matched over 10,000 blue whale GPS locations obtained between 1999 and 2008 with chlorophyll maps corresponding to those years.

It appeared that blue whales weren’t responding to real-time food availability. Instead of going to a food-rich location that may have popped up in a given year, whales seemed to be returning to sites that served them well historically.

“At first that surprised us,” Abrahm said. But when compared to their terrestrial counterparts, marine systems are a lot less predictable on a year-to-year basis. “In a really variable environment in the ocean, I think that the whales are just hedging their bets,” she said.

Climate change and the future of blue whales

Until the early 20th century, blue whales were abundant in nearly all oceans of the world. They were then hunted down at an extraordinary scale for their meat and blubber. Today, only 10,000 blue whales are estimated to survive in the wild, and about 2,500 of those spend time off the west coast of the Americas.

Whale watching boats go out in the summer to catch a glimpse of these enormous living legends, spanning one third the size of a football field. But something changed between 2014 and 2016. “The blue whale sightings were generally a little less reliable than they had been previously,” said Kate Spencer, owner of Fast Raft Ocean Safaris operating in Monterey Bay.

The warm water blob that sat off California’s coast helped divert wet storms and winter and spring winds. These winds are crucial for churning ocean waters and bringing nutrients from the deep sea to life near the surface. “With warm water just sitting there, we didn’t get a lot of krill production for several years,” Spencer said.

Extreme events are likely to become more frequent in the future with climate change. Food hotspots that were once steady may not necessarily continue to be the same. With blue whale migration stops timed by their knowledge of past conditions, finding food may become challenging. “If the timing is off, that would be as bad as going to the wrong location,” Abrahms said.

Improved technology to track whales

In the late winter of 2017, Spencer remembers spotting nearly 60 blue whales in Monterey Bay. The timing was unusual, Spencer recalled, given that these whales weren’t known to be seen at that time of the year. According to Abrahms, this may be a situation where whales hung out for an extended period of time, anticipating more food to arrive.

Jeremy Goldbogen at Stanford University’s Hopkins Marine Station in Pacific Grove thinks that the last couple of years may be anomalous. “There was so much krill,” he said.

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Watch: Orca whale pod frolics in kelp bed near Pebble Beach To understand what’s so special about Monterey Bay, his team started mapping krill patches and tracking movements of blue whales in 2017. “We’re trying to figure out what happened in the ocean that led to so much krill in these particular summers and less in other years,” he said.

Unlike previously used satellite tags, advanced tags are now tracking blue whales. Instead of a single GPS location, the new tags record at least 100 GPS locations each day. Any fine-scale movement patterns that might have been missed earlier, may now become apparent.

“It’s the predictability that the whales might be shooting for,” said Goldbogen, “but may be the ocean is changing in a way that is less predictable now; maybe there are super dense (krill) hotspots now that blue whales are able to find and hang out for most of the summer.”

“There is a lot more we need to learn about these animals,” he reckons.