Whale Trails

A deep dive into where, why, and how whales migrate – and how to protect their blue corridors

16 February 2025

North Pacific humpback whale breaching at sunset near the Kupreanof Island in Alaska, USA. © naturepl.com / Tony Wu / WWF

From gray whales undertaking epic transoceanic journeys to minkes navigating shorter, seasonal shifts, whale migrations are deeply entwined with the arc of their lives and the health of the world's oceans.

From the Bering Strait south to the temperate and tropical Pacific and the Antarctic Peninsula, the world's oceans support a diversity of whale populations and their migratory routes. Whales travel these routes along the coast and across open oceans, moving in and out of international and national waters, some covering thousands of kilometres each year. Where people see vast, formless seascapes, whales chart a world alive with invisible cues: a whisper of colder water rising from the deep, a hidden ridge beneath the waves, or the chemical trace of plankton blooming in the distance.

"There are many hypotheses for why whales migrate," says Dr Alex Zerbini, senior research scientist at the Cooperative Institute for Climate, Ocean and Ecosystem Studies of the University of Washington, in the US, and a global expert on satellite tagging. "If I were to answer simply, I'd say they migrate to find optimal habitats for reproduction or foraging at different times of the year."

A satellite tag attached to an Antarctic minke whale in the Antarctic Peninsula, 2018. © WWF-Aus / Chris Johnson

Where, how, and why whales migrate are questions scientists like Zerbini are studying with increasingly sophisticated technology that allows them to follow whales on their oceanic journeys. And while migrations are habitual, they're learning that even within the same species, individuals behave differently depending on age, sex, or reproductive status. "We tagged a southern right whale off Argentina twice," says Zerbini. "The first time, she was alone and did one thing. She had a calf the second time, and her behaviour had completely changed."

Tracking giants

In the past, scientists studied whales as they appeared in different parts of the ocean, but their journey in between remained a mystery. “During the whaling period, they'd shoot marks onto whales and recover them when they were hunted. Later, we used photo identification, comparing flukes like fingerprints. But today, satellite telemetry [from whale tags] tells us how whales travel—where they stop, how fast they move, and what habitats they use along the way," Zerbini explains.

Dr Ari Friedlaender of UC Santa Cruz (USA) attaching a 'whale cam' tag to an Antarctic minke whale in the Antarctic Peninsula, 2018. © WWF-Aus / Chris Johnson

Tagging whales is hands-on work. "We go out in a little boat, get close to the whale, and attach the tag either with a pole or an air-rifle," he says. Researchers will likely use drones to tag shy and elusive species soon. The tags, equipped with transmitters, last several months and send signals every time the animal surfaces.

One of the most surprising discoveries for Zerbini came from humpback whales migrating from New Caledonia. "We thought they'd go straight to Antarctica, but we found them stopping at underwater seamounts—mountains rising from the ocean floor but not breaking the surface. Were they resting? Feeding? We don't know yet. But it's something we never would have learned without satellite tags," says Zerbini.

Alex Zerbini looking at whale satellite tracks displayed on a computer screen © Alex Zerbini personal archive.

He has loved whales since childhood and tagged more than he can recall. "My mom has one of those little kids' books where she asked me to draw something. I was three or four and drew whales and dolphins and sharks," he says. "It's always been my dream job." Over the years, he has worked alongside engineers, veterinarians, and fellow biologists to reduce the size and weight of the tags by up to 75%. These advances provide better data and minimise the impact on the animals.

Data from these tags is central to the work of scientists like Dr Ryan Reisinger from the University of Southampton in the UK, a world expert in marine predators’ ecology. "A lot of the work I do is to develop models that predict which areas are important for whales," he says. These models use machine learning and satellite data to look at where and how whales move in response to environmental factors like ocean temperature and food availability, to uncover patterns in their behaviour. This information is then used to create maps that can help plan marine protected areas or manage ship traffic to avoid harming whales.

Why whales migrate

It's widely accepted that migrating whales follow cues that may be biological, cultural (learnt behaviour from other whales), or linked to conditions at specific times or places. Baleen whales, in particular, evolved to take advantage of the full scale of the ocean. Blue whales, for instance, seem to "surf" a wave of ocean productivity, following food supplies to different areas. But how do whales know where to go if they're not chasing food?

Blooms of phytoplankton and algae in the North Atlantic waters visible from space on 14 May, 2015. © NASA image by Norman Kuring, NASA Ocean Color Group.

"To achieve these migrations, whales need to build a mental map of their world," says Reisinger. They likely rely on a combination of different types of information, like the Earth's magnetic and gravitational fields, ocean temperatures, and underwater topography. Also there's evidence suggesting humpbacks, for example, respond to chemical signals like dimethyl sulfide, released by plankton.

For humpback whales, migration spans hemispheres and seasons. They may travel more than 8,000 kilometres annually—one of the longest mammal migrations—as they move between their equatorial calving grounds and the ice-laden edges of freezing seas where they feed. "The Antarctic is incredibly productive during the summer," explains Reisinger. "There's a huge amount of food, but it's a terrible place for them to give birth." Warm waters closer to the equator, on the other hand, may be safer for calves and reduce the energetic toll of thermoregulation in newborns.

Bowhead whales in Nunavut, Canada, 2016. © VDOS Global / WWF-Canada

For bowhead whales, migration is a journey through a tapestry of ice and open water re-enacted over centuries. In winter, they shelter in the ice to mate and calve. As the ice retreats in summer, they migrate to higher-latitude feeding grounds, such as the Beaufort Sea and Davis Strait, where nutrient-rich waters support dense blooms of zooplankton, their primary food source. Bowhead whales’ migration patterns closely follow the edge of the sea ice, highlighting their adaptation to the extreme and dynamic Arctic environment. As they live to be 200 years old, some bowhead whales alive today swim through oceans that have changed enormously in their lifetimes.

Troubled waters

Humpback whale fluke with fishing boats in the background close in Northern Norway, 2014. © naturepl.com / Espen Bergersen / WWF

Changing oceans mean whale populations face mounting threats along their long migratory routes. Bycatch and entanglement in fishing gear claim the lives of an estimated 300,000 whales, dolphins, and porpoises every year. A recent University of Washington study assessed the global risk of whale-ship collisions for four species—blue, fin, humpback, and sperm whales—and suggests that ship strikes are the leading cause of death for these larger whale species. Sperm whales are also particularly vulnerable to plastic pollution, as they feed on squid and often mistake plastic bags for prey. Additionally, the growing noise pollution from cargo ships, tourist vessels, oil and gas exploration, and military activity disrupts whales' sensitive biosonar systems. These systems are essential for navigation, finding food and mates, communicating, and avoiding danger. “Recently, I heard a [simulated] comparison of ocean soundscapes from a few decades ago versus today," Reisinger says. "The difference was staggering."

Reisinger grew up in Kruger National Park, South Africa's oldest terrestrial protected area. It still surprises him sometimes that he’s researching whales around Antarctica. "When you're analysing maps of where whales forage and where fishing vessels are, it's all very abstract," he says. "But when you're actually in Antarctica, it's completely different. You see humpbacks, minke, and fin whales feeding and krill swarming—it's visceral. You feel the importance of these ecosystems."

Increasingly, though, that beauty comes with a warning. "The last few seasons, we've had warm, sunny days in Antarctica, which are pleasant for a human. But you can see the environment changing very rapidly," says Reisinger.

Changing oceans

Satellite data is helping scientists understand how changing ocean conditions affect migration patterns for many whale species. For example, 20 years of data from humpbacks tagged in Brazil reveal a gradual southward shift in feeding grounds, coinciding with ice retreat and krill movement.

And it's not just their movements that are affected. For southern right whales, environmental changes directly affect their reproduction. During El Niño years, survival and calving rates drop, notes Zerbini. Less food in their traditional feeding grounds may mean mothers lack the energy to sustain pregnancies. Southern right whales typically calve every three years, but recent data shows this has shifted to every four to five years.

"We've seen dramatic shifts in calving rates," explains Dr Els Vermeulen, research manager at South Africa’s University of Pretoria MRI Whale Unit. The Whale Unit she manages has a southern right whale database spanning over 50 years. Such long-term databases on large mammal species are a unique tool to study long processes such as climate change and its effect on wildlife.

Long-lived species like whales face an additional challenge: massive time scales. Some whales rely on areas that are stable over decades or even centuries, says Reisinger. But as ecosystems become more variable, their strategies may need to adapt faster than their cultural or biological cues allow. For example, whales might return to areas where food was once reliable, only to find it gone—a phenomenon called an "ecological trap."

As whale behaviours and migrations respond to climate stressors, entire ecosystems feel the ripple effects. "Whales are key players in the ocean health," says Zerbini. They move carbon and other nutrients around our global oceans. "If migration patterns change significantly over the long-term, it will have cascading impacts on marine ecosystems”, he notes. Their feeding habits also influence entire food webs. "Because they're so big and consume so much [more than a ton of krill a day for humpbacks, for example], they shape entire ecosystems. Working with them is humbling.”

Safer superhighways

A grey whale calf in Baja California, Mexico, 2016. The longest tracked whale journey was made by a western North Pacific gray whale called Varvara, which travelled between Sakhalin Island and Mexico, covering nearly 22,000km. © naturepl.com / Claudio Contreras / WWF

Protecting whales throughout their long migrations is challenging because they travel through up to 12 different territorial waters, each with its own laws, and spend significant time in international waters too. “The challenge is twofold. You have to predict where whales will be with high confidence and then communicate that dynamic information to multiple stakeholders,” says Reisinger. It can be hard to keep up. Still, there's optimism. "People feel the urgency now," he reflects.

Reisinger recently helped analyse 30 years of data from eight whale species to map global "blue corridors," that show where whale movements intersect with threats in areas where they feed, mate, give birth, and nurse their young – and along their migration superhighways. More than 50 research groups collaborated on the “Protecting Blue Corridors” report with WWF. Since publication, the report – and more importantly, the message about the need to protect whales’ migratory pathways – has featured in hundreds of media stories and reached millions of people worldwide.

A screen view of the Whale Safe tool by Benioff Ocean Science Laboratory and UCSB. © Mapbox / Open Street Map / Whale Safe

In places like California, having this kind of information is already saving whale lives. "There's a tool called Whale Safe," Reisinger shares. It combines tracking data with hydrophones to predict where whales are for the San Francisco Bay and Santa Barbara Channel. Ship speeds and lanes are adjusted to reduce the risk of fatal collisions – a huge threat around busy ports. The report will become even more useful as an online, interactive, and evolving tool - something that’s currently being developed.

And even as the oceans shift and humans draw new lines across our blue planet, whales still follow their own paths, and trace routes almost as vast as the ocean itself.