(CN) — New research shows that humans are taking a greater toll on killer whale populations than previously believed. Between pollution, overfishing and impacts with vessels — it doesn’t look good for the majestic, endangered creatures.
The most common human-related causes of death include infectious disease, malnutrition, congenital defects induced by chemical pollution, blunt force trauma from boat strikes and ingested fish hooks. These cases represent otherwise preventable deaths that can likely be reduced through further research and effort.
Researchers looked at blubber thickness and body length to determine an orca’s overall body condition. Not surprisingly, mature orcas tended to be healthiest prior to death and most often died from bacterial infections, emaciation and injuries caused by boat strikes, whereas calves were more likely to die from infectious disease, malnutrition or congenital defects and were typically healthy before their deaths. Understanding these interactions with humans is critical to ongoing management and conservation efforts for over 55,000 orcas worldwide.
Lead author Dr. Stephen Raverty from Canada’s Ministry of Agriculture and his team looked at pathology reports for 53 individuals stranded between 2001 and 2017, which revealed human activity to be a significant cause of death for orcas across every age group. The researchers published their findings Wednesday in the journal PLOS One.
“We know that some of the most important human-caused risks to orcas include those related to reducing salmon (overfishing, damming of rivers, historically poor hatchery practices, damage to rivers and ocean habitat), human-made contaminants like PCBs, and vessel disturbance and underwater noise. Our work shows that killer whales also can die from fisheries interactions and be struck and killed by boats,” said Raverty in an email.
Killer whales inhabit every ocean on Earth and exhibit a wide range of dietary preferences, behaviors, vocalizations, genetics and morphology, depending on where they’re found. Since these distinct populations face unique threats, a simple one-size-fits-all approach to conservation isn’t possible.
A standardized examination of stranded orcas is already routinely performed, but prior to this study that data had yet to be evaluated to understand mortality trends among these iconic creatures. The researchers brought in four board-certified veterinary pathologists with marine mammal experience to identify the causes of death among the stranded orcas studied – which they were able to do in 42 percent of the cases.
The eastern North Pacific Ocean contains three distinct orca lineages: resident (fish eaters), transient (marine mammal eaters) and offshore (fish-eating shark specialists). Among resident orcas, those from the northern and southern salmon-eating populations along the Pacific coast from southeast Alaska to Monterey Bay have been designated as threatened and endangered, respectively.
Raverty and his team found that fishing vessel strikes represent a significant cause of death in these populations, especially among the endangered southern resident populations which live near large human populations and frequent the waters around shipping lanes. Previously, boat strikes weren’t considered a major threat to orca populations, but new research has begun to alter that view.
“Killer whale movements are not always predictable,” said Raverty. “Whether they specialize in eating salmon or marine mammals, they basically are following food sources. Depending on the ecotype, these animals may migrate from the Vancouver Island, north to Alaska or from the Salish Sea south to California. As orcas transit more populated coastal communities, there is a risk of increase vessel traffic and potential ship strike. In areas such as the Fraser or Columbia River estuaries, urban and agriculture runoff can also introduce potential toxins or pathogens.”
This research shows that more can be done to understand the frequently fatal interactions between orcas and humans. While necropsy data is regularly recorded, that data still needs to be analyzed to draw valid conclusions and determine what can be done to mitigate the impact that human activities have on orca populations. The team’s research demonstrates a valuable return on investment for conducting these types of postmortem examinations.
“Integration of data into a centralized database with more effective communication between stranding response programs, regional and national government agencies and first nations communities will go a long way to foster collaborations between network responders, field biologists, research scientists and veterinarians,” Raverty said. “We are just getting a killer whale health database up and running that should help. Ultimately, information about killer whale health must be conveyed to the public and policy makers to ensure appropriate legislation is enacted and mitigation efforts initiated.”