Scientists Look to Explain Whale Calf Sightings in SF Bay
Climate change, population growth linked to shifts in feeding and migration patterns
3/17/12
Recent sightings of a gray whale and her infant calf swimming near Alcatraz and Sausalito in San Francisco Bay illuminated a likely repercussion of melting polar ice, scientists said.
Gray whales normally mate in the tropical lagoons of Baja California in the winter, migrate north to chilly polar waters to feed on shrimp-like prey that blanket ocean floors in the spring and summer, and return to Mexico the following winter to give birth in the lagoons, where the young are protected from sharks and orcas that hunt in the open ocean.
But the relatively small size of the calf that followed its wayward mother into the Bay — about that of a newborn, which is 15 feet — indicated that its mother calved as she migrated south along the coastline — just as others of her kind are heading north for the summer feeding grounds.
“We’re seeing more and more calves born before they get all the way down to Mexico,” said Wayne Perryman, leader of the National Oceanic and Atmospheric Administration’s cetacean health and life history program at the agency’s office in La Jolla.
Researchers have been cataloguing changes in the population’s feeding and migration patterns. Some changes are thought to be the result of climate change, while others are linked to the recovery of the species from overhunting.
Perryman said gray whales, including pregnant cows, are leaving polar waters later than was the case 15 to 20 years ago, and he is testing reasons for the shift.
As ice sheets retreat northward, the whales might be taking longer to reach their food before they turn south again. Or perhaps they cannot find enough food in time to keep to traditional schedules.
“Gray whales are feeding farther and farther north,” Perryman said. “Their primary feeding grounds have shifted.”
Delayed migrations and open-ocean calving are not the only changes detected in the population, which has recovered to a population of more than 20,000 since hunting of the whales was outlawed in the 1940s.
Some gray whales have begun feeding off the northern Californian coastline, pursuing different prey than is found in polar waters, according to James Harvey, interim director of the Moss Landing Marine Laboratories.
Meanwhile, the population appears to be relearning direct migration routes that were forgotten when many of the giant mammals were slaughtered by whalers.
The whales have learned to go west around the Channel Islands off Southern California, instead of hugging the coast, and new research led by Harvey shows that they have resumed direct routes between the Point Reyes and San Francisco peninsulas, bypassing the mouth of the Bay.
“The more shortcuts you can take, the shorter the time you’re migrating and the less energy you’re using,” Harvey said.
Harvey said the wayward mother in the bay, meanwhile, was probably hugging the coast to protect her infant from predators when she accidentally followed the shoreline into San Francisco Bay.
“If you’re a female with a calf,” Harvey said, “the best thing to do is swim really close to the coastline.”
