[Press coverage] Persistent organic pollutants like PCBs, DDT, etc., reach remote areas of the earth via atmospheric and oceanic transport. This paper identifies another important mechanism for transport, animal migration. Ewald et al. show that the quantity of pollutants carried from the ocean to interior lakes of Alaska by migrating salmon can actually be significantly greater than that transported atmospherically.

Pacific salmon spawn in freshwater and then migrate downstream to the ocean where they spend the majority of their lifecycle. Prior to migration back upstream for spawning, they accumulate lipids both for the energy required for migration as well as for gonadal development. As they accumulate these lipids, they also accumulate lipophilic pollutants, contaminants such as PCBs and DDT.

Ewald et al. compared pollutant levels in two populations of a non-migratory fish, the arctic grayling in the Copper River delta of SE Alaska. One of the populations lived in a lake to which migratory salmon had no access. The other population lived in a salmon spawning lake. In this lake, grayling feed on salmon roe. They also could acquire pollutants through the food chain based upon degrading salmon carcasses, an important source of nutrients in this ecosystem.

The research revealed several interesting patterns. Most important, there were dramatic differences between the pollution levels in the grayling of lakes with vs. without salmon. The authors conclude that this "biotransport" of pollutants is far more significant than atmospheric transport, for three reasons.

• It is quantitatively a larger contributor to local contaminant loads for lakes within reach of salmon migration. Grayling in the spawning lake had concentrations of organic pollutants at more than twice the concentration as those in the salmon-free lake.
• Contaminants reaching the lakes via biotransport are biologically more available more quickly than those arriving via atmospheric transport. This is because migrating salmon, their roe and carcasses are fed upon directly by a host of predators. Atmospherically-transported pollutants must enter the food web via abiotic processes before they enter into a bioaccumulation process, and they enter it at much lower concentrations. Thus the biotransported pollutants are more readily available for bioaccumulation in the local food web.
• Biotransported pollutants are less vulnerable to environmental degradation because they are within lipid stores and protected from various oxidation processes like UV-radiation. This means that some pollutants that might never be sufficiently persistent to reach the remote lakes by atmospheric transport could arrive via biotransport.

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