In the Pacific Northwest (PNW), the southern resident killer whale (SR) population experienced an 18% decline from 1996-2001.  This population remains highly vulnerable to future declines because of its small size (~90 individuals), and was recently listed as an endangered population under the ESA.  The northern resident killer whale (NR) population is larger (> 200 individuals).  NR also experienced a decline in the late 1990’s but was less precipitous than the SR decline.  Simultaneous NR and SR population declines suggest that a shared pressure (e.g. reduced prey abundance) caused both declines but that an additional pressure (e.g. toxins) may be acting synergistically with loss of prey to more severely impact the SR. 

PNW killer whales are among the highest PCB-contaminated mammals in the world.  PCB effects are often heightened when food intake decreases, suggesting that mobilization of fat during times of low food intake releases more PCBs into circulation, increasing bioavailability for toxic effects. Since southern residents have significantly higher PCB loads than northern residents for both genders, this may explain the more precipitous southern resident decline under comparable nutritional pressures. It would also suggest that mitigation efforts should strive to simultaneously address both toxin loads and prey abundance.

Support for this work is being provided by the Center for Conservation Biology and the National Marine Fisheries Service. NMFS and SeaWorld have also kindly contributed samples for the above analyses.

We would also like to thank the Center for Whale Research for driving the boat during sample collections, assistance with orca identifications and logistical support, UW Friday Harbor labs for providing a boat for the study duration as well as laboratory support, and Soundwatch for their support in the field.