West coast whale deaths could be due to food supply
provided by Oregon Sea Grant Extension
nusually high numbers of dead gray whales have been washing up on the shores of Mexico and the western U.S. this year, causing speculation that something, somewhere, may be killing off the giant marine mammals. But Oregon Sea Grant whale expert Dr. Bruce Mate believes the answer may lie in changes to the undersea ecosystem in the whales' summer feeding grounds off Alaska.
At least 65 gray whales were found dead this winter along the coast of Mexico's Baja Peninsula, where the animals migrate each winter to bear their young. Still more washed up along California shores in March, April and May as the whales migrated north to the Bering Sea.
The apparently higher-than-normal mortality rate has given rise to plenty of speculation. Some blame the deaths on pollution or sea-water changes caused by a huge salt-evaporation plant in Baja. Others suspect that the animals were killed by cyanide in a fluorescent dye used by drug smugglers to mark the sea during air drops of illegal narcotics.
Mate, a marine biologist and marine mammal specialist for Oregon Sea Grant at Oregon State University, says neither suggestion is likely. A more likely cause could be massive changes to undersea animal communities in the Bering Sea where the whales spend their summers feeding and rearing their young. Mate, who is based at OSU's Hatfield Marine Science Center, reviewed information on the gray whale fatalities during a recent meeting of the Mexican Society for Marine Mammalogy.
Figures show that the number of fatalities is the highest recorded in the 24 years that people have kept track of gray whale migrations. But Mate says the numbers may partly reflect the fact that, this year, people worked harder to count the whales. Just as many could have died in earlier years, he said, but gone unnoticed.
What was notable this year, according to Mate, is the number of adult whales found dead. He says more than half of the 65 dead gray whales counted in Mexico were adults, and quite a few of the rest were at least one year old. That's a marked contrast to most years, when observable gray whale corpses are typically very young a few weeks or a few months old.
"Typically, most of the animals you'd expect to see die during the reproductive season would be newborn calves," Mate said. "Only about 50 percent of calves live to one year of age. But this year, over half of the dead animals are adults and quite a significant number of the rest are yearlings. This is part of the population we're not used to identifying with such high mortality rates."
Despite widespread speculation about the whale deaths, Mate noted that this year's fatalities were spread out over a long stretch of coastline during a four-month period, so Mate and his colleagues do not think it was the result of a localized problem such as pollution or drug-runners' dye. But he admits it's hard to tell just what is going on, particularly on the hot, rugged shores of Baja.
"It's been virtually impossible to get good diagnostic information on the cause of death for these animals because the area is so remote," Mate said. "By the time a team can identify a mortality and get to it, the warmth of the sun has caused bloating and deterioration in the physiology of the carcass, so good diagnostics really can't be done."
Mate says one possibility for the gray whale mortalities might be that they are not getting enough to eat during the summer. The animals spend the summer months in the Bering Sea between Alaska and Siberia. They fatten up there and then fast during the migration south, and they don't eat during the winter or in the spring when they head north again.
"So gray whales might go without food anywhere from three to five months," Mate said, "and those that didn't fill up the tank, so to speak, in the Bering Sea may be returning on empty."
Gray whales feed on bottom-dwelling creatures in the Bering Sea. Researchers have noted huge changes there at all levels of the food chain, and some have theorized that those changes are part of an even larger disruption of ocean temperature and biomass patterns.
Mate says those changes might be affecting the amount of food the gray whales can find during the summer and they need a lot of food. Not only do they need to fatten up to get through the months when they don't eat, they also need fuel for the survive a 12,000-mile migration from Alaska to the Baja Peninsula the longest migration of any marine mammal.
|Contact: Bruce Mate, Oregon Sea Grant Extension Marine Mammal Specialist, 541-867-0202, bruce.matehmsc.orst.edu or Ben Sherman, Sea Grant National Media Relations, 202-662-7095, firstname.lastname@example.org|