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Marine mammal experts say there’s no apparent single cause behind a recent spate of dead whales on North Coast and Bay Area beaches — an assertion bolstered Tuesday by reported signs of an orca attack in remains of a young gray whale that washed up on the Sonoma Coast last weekend.

Evidence that the whale at Portuguese Beach succumbed to natural predation, like another whale found dead last month near Santa Cruz, should offer some reassurance to those concerned by the frequency with which dead or dying whales have been coming ashore in recent months, many too badly decomposed to reveal what killed them.

“It’s alarming because we’re seeing more than we typically see, but there’s nothing that unifies them,” said Sheila Semens, executive director of the Noyo Center for Marine Science in Fort Bragg.

National Marine Fisheries Services biologist Justin Viezbicke, coordinator of the federal West Coast Stranding Network, said the appearance of higher than usual numbers may be a function of a strong onshore current and wind conditions bringing ashore whales that otherwise would decompose in the ocean.

“At this point we’re not even concerned about it,” Viezbicke said Tuesday. “There’s so many whales that die every year that we just don’t see.”

At least nine whales have washed up on the Northern California coast since a pygmy sperm whale was found dying at Point Reyes National Seashore on Jan. 8.

They included a sperm whale, a humpback whale bearing signs of a ship strike, and an orca entangled in fishing gear found at MacKerricher State Park north of Fort Bragg, North Bay marine mammal experts said.

Most, however, were gray whales whose annual migration from their breeding area in Mexico to feeding grounds in Alaska takes them up the coast of California each year.

Three gray whales were beached just last week — one Tuesday at Half Moon Bay, one Thursday near Fort Bragg and a third Friday night or early Saturday on Portuguese Beach.

There also were reports of a very small gray whale on an inaccessible beach near Jenner, said Shawn Johnson, director of veterinary sciences at The Marine Mammal Center near Sausalito, though it was unclear Tuesday if experts had been able to verify the report.

Two staff members from the California Academy of Science traveled to the Sonoma Coast on Tuesday to examine the 28-foot carcass at Portuguese Beach.

Despite its advanced state of decomposition, they were able to find toothmarks, bruising and subcutaneous hemorrhaging consistent with an attack by a killer whale, said Maureen Flannery, collections manager for the academy’s Department of Ornithology and Mammology. It also was missing its lower jaw.

The whale was probably a calf of this year, and it would be “pretty typical” to be targeted by an orca, Flannery said.

Scientists said gray whale calves born over the winter are especially vulnerable as they travel north with their mothers in the spring, swimming out in open water for the first time.

A particularly healthy gray whale population also means there are simply more animals in the water confronting various threats, including those posed by humans, such as ship strikes and fishing gear, Semens said.

Semens said she suspects the scrawny tail-less female that came ashore between Caspar and Fort Bragg last week may have lost its tail to a snarl of fishing gear and thus been unable to hunt effectively.

“I think we’re seeing healthier whale populations in general, so we’re seeing more entanglements,” she said.

There is no evidence that the beached whales have been affected by warm water currents and a significant disruption in the food web that has killed and sickened hundreds of young sea lions along the California Coast this year.

The gray whale migration, in fact, coincided with a drop in water temperatures and a return of the upwelling that brought increased food availability offshore, Semens said.

But the gray whales are mostly just traveling through, not foraging, at this point, as they near the end of their migration, Johnson said.

“We haven’t seen any other species or animals that are malnourished at this point,” he said.

You can reach Staff Writer Mary Callahan at 521-5249 or mary.callahan@ pressdemocrat.com. On Twitter @MaryCallahanB.