The Canadian government is calling attention to the special needs of offshore killer whales in a new document, “Recovery Strategy for the Offshore Killer Whale in Canada (PDF 3.8 mb).”
Offshores are a mysterious, little-understood group of orcas that roam the West Coast. They are related to the more familiar resident and transient killer whales, but they are genetically, physically and socially distinct. The name “offshore” sort of tells the story; they often remain miles off the coast, out of sight and out of mind for most researchers as well as the public.
Scientists cannot tell us if their population is increasing or decreasing, though it appears to be generally stable. It is not clear whether human activities are disrupting their behaviors. And without good data, these animals remain in a kind of limbo status, while the highly studied Southern Residents of Puget Sound remain solidly on the Endangered Species List with widespread concerns about their welfare.
While it is true that regulations protecting Southern Residents also protect offshores to a degree, more studies are needed to ensure the future of these unique orcas. As the new recovery strategy points out:
“Offshore killer whales face both anthropogenic and natural threats, limitations or vulnerabilities, including reductions in prey availability; contaminant exposure from prey; spills of substances harmful to the marine environment; acute and chronic acoustic disturbance; physical disturbance; interactions with commercial fisheries and aquaculture; direct killing; climate change; disease agents; fixed dietary preferences and natural decreases in prey supply; inbreeding depression; tooth wear; and mass stranding or natural entrapment.
“The small population size and typically large groupings of offshores makes the population particularly vulnerable to stochastic events.”
Offshores were first identified in Canadian waters in 1988. Since then, they have been confirmed in about 240 sightings in the U.S. and Canada, and their population has been estimated at roughly 300 animals. Although the full extent of their range remains a mystery, they seem to have moved to inland waters more frequently in recent years. The report notes:
“Although it is thought that their seemingly recent presence in inshore waters may reflect a shift associated with oceanographic conditions and/or distribution of prey, the data are also confounded by gradually increasing survey effort and public interest.”
Like the resident killer whales (Southern and Northern Residents), the offshores appear to be primarily fish eaters, with a specialization in eating sharks. They are known to prey on Pacific sleeper sharks, blue sharks, North Pacific spiny dogfish, chinook salmon and Pacific halibut — with sharks making up a significant portion of their diet.
Sharks are a good source of the fats needed for the high metabolism of orcas, but sharks live longer and tend to contain more contaminants. Consequently, offshores tend to have higher levels of PCBs and other contaminants than salmon-eating residents. Studies have revealed that PCB levels appear to be closer to those of transient orcas, which eat marine mammals. Offshores have significantly higher concentrations of DDT and PBDEs (toxic flame retardants) than either residents or transients. From the report:
“A high DDT to PCB ratio is found in offshores, characteristic of waters and sediments off the California Coast, where DDT comprises a more significant portion of contaminants and where prey may be exposed to elevated concentrations of contaminants relative to higher latitude waters; this shared characteristic ratio is thought to be an indication of offshore killer whales’ frequent occurrence off California.
“There are many sources of these persistent substances, often from urban and agriculture runoff, along the West Coast of North America. Runoff
from urban areas is especially troubling in California, where offshores are regularly sighted in the winter, often near large urban centers…”
“Of particular concern is offshore killer whales’ apparent targeting of the liver of at least one of their preferred prey, the Pacific sleeper shark. The liver is a lipid-rich meal, but is also a reservoir of heavy metals. All three shark species known to be consumed by offshores have a high mercury content, likely increasing the severity of heavy metal consumption and accumulation in offshore killer whales.
“Killer whales are thought to have evolved the ability to detoxify heavy metals such as mercury; however, it is unknown whether detoxification in offshore killer whales functions effectively enough to deal with their apparent diet preference for livers from intermediate-to-high trophic level prey, and exposure to an elevated contaminant environment.”
While shark populations along the West Coast appear to be stable at the moment, the number of sharks may have been greater historically, according to the report. In addition, basking sharks may have been an important prey source historically, and a steep decline in basking sharks may have affected the offshore orca population.
One of the greatest risks to the offshores is a spill of oil or other harmful substances. Killer whales have no sense of smell and make no apparent effort to avoid spills. The report notes:
“As described previously, the threat of oil spills and discharges holds risk for offshore killer whales, due to their grouping behavior. With multiple current proposals involving increased marine transport of petroleum products and other hazardous substances to and from British Columbia, an increase in large vessel traffic (e.g. tankers) in these waters heightens the risk of potential spills of substances harmful to the marine environment, and to offshores and their prey.”
Another significant risk is disease among offshore killer whales. Their high toxic loads can reduce their immune response, and their highly social nature increases the risk of disease exposure. According to the report:
“This highly social nature heightens the risk of rapid, pervasive infection and pathogen dispersal throughout the entire population… With an extensive geographic range adjacent to many large urban centers and intensive agricultural activity, offshore killer whales are exposed to numerous sources of emerging pathogens particularly near river and runoff outlets, where concentrations of infectious agents may be introduced into the marine environment.”
Offshore killer whales also are known to have extreme tooth wear, probably caused by their preference for eating sharks with their sandpaper-like skins. In some cases, teeth are worn to the gum line, which could open a route of exposure for infection.
Other risks include noise generated from human operations, including military sonar and seismic surveys, as well as chronic noise from shipping operations. Because of the close grouping among offshores, noise is likely to disrupt their feeding and social behavior.
The Canadian report articulates recovery strategies, primarily focused on learning more about the needs and threats to offshores — including studies on their population and cultural attributes, prey availability and toxic exposure, and response to various types of noise.
Comments will be taken on the new report until April 27. For information, go to Offshore Killer Whale Recovery Strategy.
In the U.S., offshore killer whales are protected under the Marine Mammal Protection Act, but they have not been provided any special stock status (PDF 493 kb) for additional protection or focused study.