Necropsies of over 50 killer whales over the last decade show more mammals are dying as a direct result of human behaviours in the Pacific ocean than previously thought.
Researchers who studied the whales say identifying the causes of death is critical for the conservation of orca populations.
The results of the necropsies on 52 killer whales stranded on beaches in the northeast Pacific and Hawaii were published in a study in the journal PLOS ONE.
"Historically, we don't really have anything to compare in order to establish trends. So this was really a snapshot of mortalities over a 10-year span," said lead author Stephen Raverty, a veterinarian pathologist with B.C.'s Ministry of Agriculture.
He says they were able to determine the cause of death for 23 of the whales stranded between 2003 and 2014. The results show that human interaction may be more deadly to southern resident killer whales, in particular, than previously thought.
Out of a total of nine southern resident killer whales, four died because of traumatic incidents.
"We often think of these animals as being highly agile and able to avoid interactions with vessels or propellers and so on but that's not necessarily the case," explained Raverty who is also an adjunct professor at UBC's Institute of Oceans and Fisheries.
In some cases, the marks left on orcas' bodies allowed researchers to definitively confirm that they were killed after being struck by a ship. For example, with an orca named Luna , they recovered a 30-kilogram portion of damaged blubber.
"We could examine the margins and they actually conformed to the slope of a propeller."
Another whale had serrated, angular cuts where its dorsal fin had been removed, again consistent with a propeller blade.
Many other individual whales are suspected to have been killed by marine vessels based on necropsies which revealed internal bleeding.
"[This] suggests that the animal survived that initial impact and there was some subsequent bleeding and the animal goes into shock as a consequence of that."
He says whales that died because of trauma typically had healthy body conditions, whereas other individuals' bodies had deteriorated, suggesting a less immediate cause of death.
"That would suggest a more protracted process that might be related to suboptimal or lack of available prey. There may be a chronic disease process going on there. There may be tumours, parasitism."
Other necropsies identified causes of death from environmental factors that can also be linked to human behaviour such as a shortage of salmon, disturbances from ships and toxins.
Raverty says the results of the study show the benefits of necropsies to understand the health of killer whales.
He believes the information should be used to develop policies for the conservation of orca populations, one of the most straightforward solutions being to reduce the speed of marine vessels.