Thousands of gray whales are about to begin their annual southern migration to Mexico from their feeding grounds in the Arctic, where near-record low ice coverage last spring allowed pregnant females to get early access to food.

In the coming weeks, more than 20,000 gray whales will begin their annual two-month, 5,000-mile journey south — from the Arctic Circle, past San Diego, to warm lagoons off Baja California.

The pod has been feeding over the past four months in the Chukchi Sea and northern Bering Sea.

Near-record low ice coverage when they returned to the North Pacific last spring allowed pregnant females to get early access to vital feeding areas, increasing their chance for a full-term pregnancy, said Wayne Perryman, fisheries biologist at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla.

Pregnant females fast during the four-month round-trip migration, so it’s important for them to fatten up quickly upon their return in May, Perryman said.

Now that the whales have gained enough blubber, expectant females will lead the pod south to their birthing waters, he said.

Perryman and a team of researchers in December plan to conduct a census to assess the population as the whales pass by the Central California coast at a research station in Carmel.

“We’ll be up there counting gray whales, and then we’ll run these thermal censors so that we can count whales both day and night,” Perryman said.

Gray whale populations have been thriving because of recent protections, but so have their predators, Perryman said.

“Every year when the gray whale cows and calves start migrating north, there’s about 200 killer whales waiting to ambush them when they go into the Bering Sea,” he said. “So we’re starting to suspect that the top-down effects are having a significant impact on the growth of the gray whale population, because killer whales are taking a lot of calves.”

An estimated 1,100 calves have been counted on their northbound journey each of the past three years.

“We find that after three consecutive years of high calf counts it is not uncommon for us to have a count that’s lower than we expect,” Perryman said. “So while I think this will be a good year, I don’t think it will be a banner year.”

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