When Food Can Kill: Salmon and Killer Whale Populations

A study published in Environmental Toxicology and Chemistry explains the process by which threatened and endangered killer whales in the waters of northwestern North America become contaminated with persistent organic pollutants, particularly polychlorinated biphenyls (PCBs), that are present in chinook salmon, their primary food source.

Two years of this study were funded as part of the SeaDoc Society's competitive grants program.

Two main observations were made. First, nearly 100% of the contaminants in chinook salmon were acquired while they lived in the ocean. Second, southern killer whales are contaminated with higher concentrations of chemicals because 1) they eat more salmon than their northern counterparts and 2) southern salmon have higher levels of contaminants than northern salmon because the southern waters are more contaminated than the northern waters. In fact, PCB concentrations in southern salmon were almost four times those in northern salmon.

It is known that salmon can lose as much as 80% of their lipid (fat) stores as they journey back to their natal streams. Salmon stop eating during this time and draw energy from their lipid stores. Thus, they are less nutritious to whales than they would be otherwise. Furthermore, southern salmon were found to have lower lipid content than northern salmon. Whales therefore eat larger amounts of salmon and consequently are exposed to larger amounts of chemicals in the salmon. Southern whales, in particular, consume as much as 50% more salmon to compensate for the fact that their food is, per unit, less nutritious.

Salmon paradoxically help killer whales (food) and harm them (contamination). This study is important because it illustrates the increasing amount of damage to fragile ecosystems that occurs as industries continue to dump waste into rivers and oceans.

To read the entire review, Persistent Organic Pollutants in Chinook Salmon (Oncorhynchus tshawytscha): Implications for Resident Killer Whales of British Columbia and Adjacent Waters (Vol. 28(1):148–161), visit http://www.allenpress.com/pdf/ENTC_28.1_148.pdf