SEATTLE - The population of Eastern North Pacific gray whales, which summer in Alaska, has dropped in the past four years from an estimated high of more than 26,000 to less than 18,000, federal researchers say.
Scientists suspect the decline is related to low food supplies in the whales' Arctic feeding grounds, and they expect the numbers will rise again next year.
Also, the drop is a fluctuation one could expect from a population that has rebounded so well that it is now at the limit of what the environment can sustain, said Paul Wade, a marine biologist for the National Marine Fisheries Service's National Marine Mammal Laboratory in Seattle.
Environmentalists consider the decline an indication that the whale's population is still threatened by pollution, climate change, dwindling food supplies and hunting.
"If these numbers are correct, it's a very dramatic, very sharp decline in a short time period," said Michael Markarian, executive vice president of the New York-based Fund for Animals, which is involved in a second federal lawsuit challenging the Makah Tribe's right to hunt whales a right based on centuries of tradition and guaranteed in their 1855 treaty.
"In our mind, it's one of the big success stories," Wade said of the gray whales' recovery over the past century.
The Eastern North Pacific gray, a 30-ton behemoth that migrates annually between winter breeding grounds off Mexico and summer feeding grounds off Alaska, was nearly hunted to extinction in the 19th century by Yankee whalers. There were only a few thousand left at the beginning of the 20th century.
But in the past two decades, the gray-whale population has often topped 20,000, with a 1997-98 estimate of 26,700.
The population is calculated by counting whales as they migrate past central California between December and February. The counts are adjusted to account for whales passing at night, at times when they can't be easily seen from shore or due to their distance from shore and other factors.
The recent estimate of 17,414 could be as low as 14,322 and as high as 21,174, said Dave Rugh, wildlife biologist and project leader of the gray-whale census for the Marine Mammal Lab.
However, it's a "pretty big shift" from the late 1990s figure, Rugh said.
A 1998 "warm-water event" around Alaska hurt the whales' food supply, he said. And more than 300 dead whales were reported along the migration route in 1999 and 2000, suggesting many more also had died. The whales do not eat during the migrations.
Wayne Perryman, a NMFS fisheries biologist at the Southwest Fisheries Science Center in La Jolla, Calif., said calf production also dropped, from 1,388 calves in 1998 to 427 and 280 in the following two years. He suspects heavy ice kept pregnant whales from their feeding areas before winter birthing off the Baja Peninsula.
The number of strandings has since dropped dramatically, leading researchers to believe the whales' population will soon go back up. Perryman also saw less ice last summer and higher calf numbers this spring.
Some whales may have missed being counted because low food supplies up north prompted them to stay in more southern waters, Wade said.
Markarian contends the population drop bolsters the argument for putting the whales back on the Endangered Species List. Grays were taken off the list in 1994, but the Fund for Animals last year petitioned to relist them.
Anti-whaling activists contend the delisting was premature.
"Seventeen-thousand is a very low figure indeed and quite worrisome," said Charlotte de Fontaubert, oceans-campaign coordinator for Greenpeace.