The three pods of whales that make up the southern resident population are an icon of the Seattle/Vancouver Island area and a popular tourist attraction around the San Juan and Gulf Islands. Their numbers dropped by 20 percent between 1996 and 2001.
They were declared endangered under Canada’s Species at Risk Act in 2004 and under the U.S. Endangered Species Act in 2005. The U.S. government acted four years after conservation groups filed suit seeking protection of the whales.
Eighty-three whales are now in the southern population, down from 99 in 1996, while the northern population, which lives largely in the Straight of Georgia, has more than 200. Seven of the southern whales, including some breeding females, died last year.
The cause of their decline is unknown, but U.S. federal biologists with the National Oceanic and Atmospheric Administration say reduced salmon supplies, pollutants and disturbance from ships and recreational boats are possible causes.
Ross said each one of those factors is "significant enough to reduce the population." Chinook populations have declined. Also, PCBs are known to suppress immune systems of marine mammals.
The amount of PCBs found in the southern killer whales is higher than the levels that damage immune systems in seals and probably contributed to a massive die-off of European harbor seals killed by a virus epidemic in the late 1980s. Seals, however, may be more prone to mass mortalities than killer whales because they collect on rocks in large groups.
Deaths of the southern whales cannot be blamed on a specific chemical or pathogen but it is likely that immune suppression plays a role, Ross said. Some of the whales have died from infections. In many cases, they die at sea and their carcasses are never found.
The northern killer whale population—which the Canadian government has designated as a threatened species—also is contaminated with PCBs that exceed the amount known to harm immune systems.
PCBs were banned in the late 1970s but they persist in ocean and river sediments. A projection by Canadian scientists shows concentrations won't fall below the amount that suppresses immune systems until 2063 for the southern residents and 2030 for the northern ones.
NOAA's recovery plan for the species, released last year, includes cleaning up old pollution and reducing new pollutants, enhancing salmon populations and evaluating whether to regulate vessel traffic in the region.
In October, Canada's Department of Fisheries and Oceans was sued by six environmental groups for deciding not to protect the whales' critical habitat.
This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.