Orcas Leave Dilemma In Their Wake

Wednesday, July 20, 2005
SEATTLE, WA - The six transient orcas visiting Hood Canal since late January have presented scientists with a question: If orcas feed mainly on harbor seals, why are so many seals still in Hood Canal?

"Our observations and computer modeling suggest that they could have consumed as many as 900 seals," said Josh London, a doctoral student at the University of Washington who has studied harbor seal behavior since 1998. "We're kind of puzzled as to why we haven't seen a decline already."

Every fall, the Washington Department of Fish and Wildlife does an aerial survey of Hood Canal seals. Estimates have put the seal population at just over 1,000, but the method of counting could be faulty.

"When we do the aerial surveys or ground counts, you can count only the number of seals hauled out," London said. "Not all the population is going to be hauled out at the same time."

The arrival of the transient orca pod provided scientists with what seemed to be a perfect situation to determine how much the whales actually eat and their impact on the seal population.

But it didn't work out that way.

"There's a bit of a conundrum," said Paul Wade, a researcher with the National Marine Mammal Laboratory. "Either the whales don't need as much food or harbor seals are moving in quickly to replace the ones being eaten. That doesn't sound like typical harbor seal behavior to us."

A seal per day

Scientists were first alerted to the seal mystery in 2003, when a group of 11 orcas visited the canal. Based on computer models of how much orcas should eat, the pod consumed an estimated 900 seals in 59 days.

The 2005 group is estimated to have consumed the same amount during its stay. That's about one seal per whale per day.

But in both instances there has been no apparent decline in the seal population.

"Even though we're not seeing a decline ourselves, that doesn't mean there's not one out there," London said.

Both orca pods puzzled whale enthusiasts and scientists with their decision to visit the canal, as well as with the length of their stay.

In general, transient pods stay in a single location for only about two weeks. Both the 2003 and 2005 pods normally spend the summer in Alaska waters.

Scientists said the easy access to lazy food, since Hood Canal seals weren't accustomed to fleeing from hungry whales, is the main reason the whales stayed so long.

In 169 days this year, it appears the six whales have consumed as many seals as the 11-whale pod did during 59 days of 2003.

But how many seals the whales actually did eat is up for debate.

Feeding habits

Little is known about orca feeding habits. Scientists tracking the transient whales said they saw one to three kills in a three- to six-hour period. But they aren't sure if that behavior is confined to daylight hours or continues over a 24-hour period.

London said most big carnivores tend to feed at night or in the morning and evening hours, but tracking the orcas during that time is hard.

But because the seals' prey moves to shallower water at night and the seals move with it, it is reasonable to assume the orcas continue feeding, London said, and the computer model reflects that.

There is also the matter of the numbers.

Mother seals have only one pup per year.

"If they [the whales] really are consuming 900 seals as our model suggests, then it would be really hard to produce that many animals in a two-year period," London said.

Tracking seals

To help address the problem, London and another seal specialist, Steve Jefferies, have begun fastening instrument packs to seals' backs. The packs will record information about the time seals spend on shore, in the canal and surrounding waters. They will fall off in the fall when the seals shed their fur. The information will then be compared to information collected in 1998 and 2002. It is hoped the new data will reveal more about the seal population.

But for now, the whales seem content to stay in the canal. "It does just seem they don't have any particular reason to leave yet," Wade said.

A caution

It is not known for certain whether the whales are still in Hood Canal, but if they resurface, boaters are warned to give the pod a wide berth. Federal guidelines recommend boats stay 100 yards to the side of whales and not approach them from the front or back.

Researchers such as the National Marine Mammal Lab are issued permits to move closer to the whales.

Source: The Port Townsend Leader