A Vancouver Aquarium whale researcher is sounding the alarm over what he calls "puzzling" changes observed in the resident killer whale pods that live off the northern coast of B.C. and Alaska.

Dr. Lance Barrett-Lennard, a senior marine mammal scientist with the aquarium, says he fears changes in the ocean environment are prompting odd behaviour and an unusually high mortality rate, after spending the summer observing the whales aboard a research vessel.

Barrett-Lennard says one resident pod has lost seven matriarchs over the past two years, an unusually high death rate, and he's also noticed a lack of vocalizations from the normally chatty mammals.

“Resident killer whales are typically very vocal in the summer but, for the second year in a row, they have been remarkably quiet,” said Barrett-Lennard.

“So quiet that we often had difficulty finding them.”

The lack of vocalizations was just one of three unusual changes observed by the cetacean research team. Resident orcas were also seen the past two summers travelling in small groups, farther offshore to find food behaviour more typical in winter than summer.

At the same time, the Barrett-Lennard says the number of normally transient killer whales, also known as the Bigg's whale, has been increasing over the past 25 years. In the 1990's, resident orcas were sighted much more frequently than Bigg's orcas, but now the number of sightings is nearly equal.

“It’s unclear at this point if the loss of so many matriarchs or the increase in Bigg’s killer whales is having an impact on resident killer whale behaviour, but the changes we’ve seen over the last two years are striking and beg an explanation," says  Barrett-Lennard.

The difference two years makes

The Vancouver Aquarium provided two audio recordings made near resident killer whales off B.C.'s Central Coast.

The first, from 2011, is full of many calls as well as echolocation sounds and is typical of the sounds of resident killer whales prior to 2012.

The second, from 2013, is much quieter and consists mainly of the echolocation 'ticks' that the whales use to find fish. "Communication calls are absent apart from a quiet call at the 15-second mark," the Aquarium says.

With files from CBC News