Grey whales migrate to solve a fundamental problem: the best place to eat and the best place to have babies are roughly 5,000 miles apart. Their Arctic feeding grounds are packed with food but far too cold for newborn calves, while the warm lagoons of Baja California offer safe, sheltered waters but almost nothing to eat. So every year, grey whales travel about 10,000 miles round-trip, and in some cases upwards of 14,000 miles, making it one of the longest annual migrations of any mammal on Earth.
Arctic Waters Have the Food
Grey whales are bottom feeders. They roll onto their sides in shallow Arctic seas, suck up mouthfuls of sediment, and filter out tiny crustaceans called amphipods, along with krill, cumaceans, and other invertebrates buried in or hovering near the seafloor. This style of feeding leaves visible mud plumes at the surface. The northern Bering and Chukchi seas are uniquely productive for this kind of dining because cold, nutrient-rich currents support dense colonies of these small organisms on the ocean floor.
Whales cluster around specific “hotspots” where prey is most concentrated. In the southern Chukchi Sea, for example, they reliably gather near a 50 to 60 meter deep trough where both amphipods and krill overlap. Farther north, near Point Barrow, Alaska, they shift offshore to follow krill pushed by wind patterns. These feeding grounds are available only during the warmer months when sea ice retreats, giving the whales a limited window, roughly May through October, to eat enough to sustain themselves for the rest of the year.
And they need to eat a lot. Grey whales spend months building up thick layers of blubber that serve as their primary energy reserve. Once they leave the Arctic, most adults eat little or nothing for the entire southbound journey, breeding season, and northbound return. They essentially fast for four to five months, burning through stored fat the whole time. Humpback whales, which follow a similar pattern, lose roughly 11,000 kilograms of blubber tissue over the course of a full migration cycle. Grey whales face comparable energy demands.
Baja Lagoons Protect Newborn Calves
The other half of the equation is reproduction. Grey whales give birth in a handful of shallow, warm lagoons along Mexico’s Baja California Peninsula, most notably Laguna San Ignacio and Laguna Ojo de Liebre. These lagoons are calmer, warmer, and more sheltered than the open ocean, which directly improves survival rates for both delivery and the first vulnerable weeks of a calf’s life. All three primary breeding lagoons are designated UNESCO World Heritage Sites, and Laguna San Ignacio remains largely undeveloped, free from container ship traffic, industrial noise, and urban pollution.
Newborn whale calves are born with very little insulating fat. In closely related species, newborn blubber thickness can be as thin as 0.7 centimeters, barely a quarter of an inch. That blubber thickens rapidly during nursing but leaves the calf dangerously exposed to cold water in the first days and weeks of life. The warm lagoon waters of Baja, which hover around 15 to 20°C (60 to 68°F), give calves time to nurse, grow, and build up enough blubber before facing the cold open Pacific on the northbound journey. In the near-freezing Arctic, a newborn with that thin fat layer would lose body heat faster than it could produce it.
The Migration Calendar
The journey follows a predictable schedule each year. Pregnant females are typically the first to leave the Arctic, heading south beginning in September and October. The bulk of the population follows, and most whales arrive in Baja California by late December. Calves are born in the lagoons from January through March.
The northbound return starts in February and stretches through late April. Mothers with new calves are the last to leave, hugging the coastline closely as they travel north. This staggered schedule means that at peak migration times, January through March, whale watchers along the California coast can see animals heading in both directions simultaneously.
Grey whales stick close to shore for most of the route, often swimming within a few miles of the coast. This coastal habit likely helps with navigation. Scientists have long hypothesized that whales may sense magnetic anomalies along the seafloor, using the earth’s magnetic field as a kind of internal compass. The continental shelf and its familiar bathymetry, the shape of the ocean floor, may also serve as landmarks. Staying near the coast additionally provides some shelter from storms and open-ocean predators like orcas, which are a real threat to calves.
Climate Change Is Reshaping the Route
The Arctic ecosystem that grey whales depend on is changing fast. As sea ice melts earlier in the year, less organic material sinks to the seafloor to feed the amphipod colonies that grey whales rely on. Instead, that productivity stays in the water column, feeding zooplankton higher up rather than the bottom-dwelling crustaceans the whales have evolved to eat. This weakening of what scientists call “pelagic-benthic coupling” is a fundamental shift in how Arctic food webs operate.
At the same time, warmer water flowing north through the Bering Strait is carrying krill and other pelagic prey into the Chukchi Sea, which has attracted humpback and fin whales into areas where they were previously rare. Grey whales now face new competition in waters they once had largely to themselves. Some grey whales have responded by shifting their feeding locations, moving farther offshore or farther north to find productive patches.
The population has shown signs of stress. NOAA declared an Unusual Mortality Event after a spike in strandings, and while those high stranding numbers have since subsided, calf production remains stubbornly low. In 2024, researchers estimated only about 221 mother-calf pairs, close to the record low set in 2022. Researchers believe many females have not yet regained the energy reserves needed to sustain pregnancy and nursing, a sign that the feeding grounds may not be providing what they once did. For a species whose entire life cycle depends on banking enough fat in summer to survive a 10,000-mile fast, even modest drops in food availability ripple across generations.