Scientists Seek Clues To Salmon Shark Die-Off

January 3, 2008

Release from: Sarah Lemon
Mail Tribune (Oregon)

All sharp teeth and tough cartilage, the salmon shark is a species that doesn't elicit much sympathy.

But the sight of these small sharks dead and dying on Oregon beaches last summer sparked a flurry of phone calls to Oregon State University's Hatfield Marine Science Center in Newport. Soon, scientists could know what caused the unusual die-off.

"Most of these sharks when they die, they don't end up on the beach," says Bill Hanshumaker, the center's public marine education specialist.

Scientists first determined the sharks weren't juvenile great whites, as beach-goers first thought. Closely related to the infamous predator, salmon sharks are distinguished from their great white cousins by non-serrated teeth and a secondary keel, a small fin on the underside of its body. The species, however, share habitat off the coast of Oregon.

"They look just like a baby great white," Hanshumaker says.

About 20 sharks were reported dead or dying at spots up and down the coast beginning in early summer through the end of October, Hanshumaker says. A planned study of the carcasses, which will be sent off for testing this month, will be the first in Oregon to look at the die-off of salmon sharks, he adds.

The fact that the dead salmon sharks are all young and about the same size — 3 feet long — leads Hanshumaker to believe the deaths could indicate a larger problem plaguing the ocean. Determining the cause of the sharks' demise could be an important clue about the ecosystem's health, he says.

"You have to infer there's a failure in the food chain," he says.

Each summer, beginning in 2002, a low-oxygen zone has materialized off the Oregon Coast. The phenomenon — called a "dead zone" — has been characterized by rapid growth of microscopic plant life that dies, sinks to the ocean's floor and consumes oxygen. The death of oxygen-starved fish, crabs and other species follows.

Because of their status as "apex predators" at the top of the food chain, salmon sharks may only now be suffering the effects of the dead zone, Hanshumaker says. Warmer water closer to Oregon's shore this year could also play a role, he says.

"This summer brought 60-degree water nine miles from shore," the biologist says. "You normally have to go 40 to 50 miles out."

Cold water welling up from the ocean floor is rich in nutrients, attracting squid, which are a major food source for the salmon sharks, Hanshumaker says. The sharks dive to the bottom to feed and aren't usually found so near to shore, he adds.

Salmon sharks (Lamna ditropis) are large, powerful, warm-bodied and streamlined predators adapted for high-speed swimming. Reports from the U.S. Navy have clocked salmon sharks exceeding 50 knots, which would make the salmon shark one of the fastest fish in the ocean.

One theory of the sharks' death is bacterial meningitis, Hanshumaker says. But the scientist says he suspects such an infection would result from other factors "stressing" the sharks.