Threats of toxins
Given their already low population sizes, the orcas are at risk from high levels of persistent organic pollutants which may compromise their immune and reproductive systems, leading to reduced calving and increased mortality rates. The southern resident population is particularly exposed to these human-made substances compared to the northern resident population due to higher concentrations of PCB (polychlorinated biphenyls), DDT (Dichloro-diphenyl-trichloroethane) and PBDE polybrominated diphenyl ethers) found in ocean sediment in the south.
A B.C. study by Dr. Peter Ross and others analyzing data on by sex, age and dietary preference discovered that contaminant levels actually decreased in reproductively active females, followed by an increase at the approximate age of 50. The reason is that females pass PCBs (and most likely other persistent pollutants such as PBDEs) on to calves through their high-fat milk. The calves receive the PCB concentrated milk at a time in their development when they are the most vulnerable.
"Marine mammals are wonderful creatures, and very good quality indicators of the health of an ecosystem," Ross said. "What they're telling us is that the ocean may be under significant climate stress, and that it is much more contaminated than previously thought."
Sources of Toxins
These toxic compounds are all human created. In North America we are using over 85,000 chemicals, many of which are found in common household products, computers or furniture. Recent studies have detected PBDEs in a wide range of supermarket foods as well as in orcas, other marine mammals, and salmon.
PBDEs, commonly known as flame-retardants, is a chemical that shares many characteristics with PCBs (polychlorinated biphenyls) which were banned three decades ago. PBDEs break down very slowly in the environment where they build up in living organisms, magnifying in concentration as they move up the food chain. In recent years there has been a doubling every two to five years of PBDE levels in wildlife, soil and human tissue samples, and these levels have reached particularly high concentrations in North America, where the use of this chemical has been highest. Laboratory tests on animals have shown that PBDEs have health effects similar to those of PCBs, a related class of flame retardants that was banned in the late 1970s because of its toxicity to humans.
For example, levels of toxic compounds in southern residents are three times higher than levels known to cause immunotoxicity in harbour seals. PCBs, PBDEs and other immunotoxins do not cause outright death, but experiments and captive feeding studies of seals have shown that contaminants can weaken immune systems, hamper reproduction, cause skin diseases and subtle changes in physiology.
