An average of 48 killer whales are estimated to die annually across the north Pacific Ocean, according to a new study by U.S. and Canadian scientists published in the journal Marine Mammal Science.

However, over the last two decades an average of only 10 killer whale carcasses per year were recovered, making each a rare opportunity to study the health of the species, the study found.

The examination of whale carcasses stranded on shore provides evidence of prey preference, growth rates, gestation, periods of reproduction and longevity, as well as the role of infectious diseases, toxins, sonar, detonations and vessel collisions.

Endangered southern resident killer whales have been shown to harbour some of the highest levels of polychlorinated biphenyls among all cetaceans, the sort of information that can be gleaned from the internal organs and blubber of dead, stranded whales.

Such information can provide an early warning on the health of the oceans and help to manage human activities to reduce the impacts on killer whales, which are considered a "sentinel species" at the top of the food chain, the study noted.

Of 371 reported strandings from 1925 to 2011, 32.1 per cent occurred in Alaska, 27.5 per cent in B.C., 20.5 per cent in Japan, 5.1 per cent in California, 4.0 per cent in Washington, 3.8 in Mexico, 3.5 per cent in Russia, 2.7 per cent in Oregon and 0.8 per cent in Hawaii. The strandings were evenly split between males and females, and there were no significant differences across age classes, except for Alaska, where about two-thirds were adults.

The greatest number of strandings occurred in May.

Of the 371 stranded killer whales, 328 died, including at least 122 in mass strandings.

The study involved the University of California Davis Wildlife Health Centre, B.C. Animal Health Centre, Northwest Fisheries Science Centre, Epitracker Analytics and Pacific Biological Station.

lpynn@vancouversun.com