VANCOUVER -- There have been some puzzling changes in the behaviour of northern resident killer whales that live off the north-central coast of British Columbia and Alaska, says a marine mammal scientist from the Vancouver Aquarium.

Dr. Lance Barrett-Lennard, head of the aquarium's cetacean research team, said his team has noticed for the past two summers that the normally chatty mammals have been uncharacteristically quiet.

"They weren't vocalizing, and that was quite a striking change after years and years of being very familiar with how noisy they are and how easy to find acoustically," Barrett-Lennard said Thursday.

The cetacean team goes out in the summer to study the whales, finding the pods with the help of a hydrophone dropped over the side of the boat into the water.

The device, described by Barrett-Lennard as "basically a microphone in a salad bowl," can hear the whales within about a 25-kilometre radius and point the team in the right direction.

But finding the pods has become more difficult because they're not as loud.

"We were still blundering into them from time to time, finding them without the hydrophone and when we did, they were generally -- not always, but most of the time -- very quiet," Barrett-Lennard said.

Whales use sound to find each other, to locate prey and just to communicate, he said. The whales appear to be foraging actively and behaving normally in most every other way.

"We're seeing everything that we would expect to see visually but when we drop the hydrophone over the side, they're being very, very quiet," he said.

The whales have also been seen the past two summers travelling in smaller groups further offshore to find food -- behaviour more typical in winter than summer.

The team has also noticed an unusually high mortality rate among pod matriarchs, with seven or eight deaths among older females in the pod in the past two years. Normally, the team notices one or two deaths per year.

The deaths are likely coincidental and not linked, he said, but the effect of their departures could affect the pod.

"We know that the matriarchs are really important in these groups.... they're the glue that hold these groups together because all of the matriarch's sons and daughters stay with her for life."

The pod population is not declining despite the deaths, because there has been no unusual calf or cow mortality.

At the same time, Barrett-Lennard said the number of the rarer Bigg's killer whales has been increasing over the past 25 years, and the two species that used to avoid one another are now spotted in almost equal measure.

Barrett-Lennard said the changes are not alarming, but do need further study.