MSc 2004

Dr Andrew Trites

Marine mammals and human health in the eastern Bering Sea: Using an ecosystem-based food web model to track PCBs.

The comprehensive changes that have occurred in the Bering Sea over the last 30 years have prompted a wide range of studies to better understand the ecosystem as a whole. One set of studies has used the Ecopath with Ecosim (EwE) modelling software to synthesise existing biological data and gain insight into how the ecosystem was before and after the system-wide changes.

This modelling framework provides a means for tracing contaminants through the ecosystem, and evaluating the role that persistent organic pollutants (POPs) may have played in the changing dynamics of the eastern Bering Sea. Using the EwE software, the likely pathways of PCB flow within the eastern Bering Sea were identified and health implications of contaminant exposure for Steller sea lions, other species of marine mammals, and humans were evaluated.

The base EwE model was refined from existing models and validated with traditional stock assessment data. Ecotracer (a component of the EwE software) tracked the bioaccumulation of contaminants moving through the system with biomass. The models estimated contaminant concentrations for species and functional groups that have not previously been measured. Results suggest that PCB concentrations for most species in the eastern Bering Sea have remained below threshold levels associated with negative reproduction and survival effects.

However, these concentrations may have subtle effects on adults and more serious effects on foetuses and nursing young, which could inhibit the recovery of Steller sea lions and other species that have declined in the eastern Bering Sea. Although the benefits of traditional foods appear to continue to outweigh the risks posed by contaminants for humans, PCB exposure and dietary intake for many Alaska Natives subsisting on marine mammals is above the USEPA Daily Reference Dose.

Results extend the existing eastern Bering Sea models and are important in terms of management alternatives for marine mammals and human health. They also synthesise evidence regarding the presence, extent, and movement of PCBs throughout the system. The refined eastern Bering Sea models are useful tools for exploring different scenarios and hypotheses, to inform resource managers, and to further our understanding of this ecosystem.


  • Coombs, A.P., 2004. Marine mammals and human health in the eastern Bering Sea: Using an ecosystem-based food web model to track PCBs. Master of Science thesis. University of British Columbia, Vancouver, BC. (Thesis)   91 pp. (PDF)
  • Trites, A. W., Bredesen, E. L. and Coombs, A. P., 2004. Whales, whaling and ecosystem change in the Antarctic  and eastern Bering Sea: Insights from ecosystem models. In Investigating the roles of cetaceans in marine  ecosystems. Monaco: CIESM Workshop Monographs pp. 85-92.
  • Coombs, A.P., Trites, A.W., and Pauly, D., 2003. Assessing impacts of contaminants on marine mammals and  human health in the eastern Bering Sea. Society for Marine Mammalogy, 15th Biennial Conference on the  Biology of Marine Mammals, December 14 – 19, 2003. Greensboro, North Carolina, USA. p. 35.
  • Krebs, C. J., E. Bredesen, A. P. Coombs, R. Daniel, R. de Graaf, A. Elz, A. Hall, L.Hooper, A. Janmaat, L.   Mehranvar, E. Rubidge, G. Slooten, P. Tamkee, and K. Welstead. 2001. Graphical presentation of data.   Bulletin of the Ecological Society of America. 82(4):247-248.
  • Coombs, A. P., A. W. Trites, and D. Pauly. 2001. Contaminant impacts on marine mammals and human health in the eastern Bering Sea: Using an ecosystem-based food web model for resource management. Society for Marine Mammalogy, 14th Biennial Conference on the Biology of Marine Mammals, November 28 – December 3, 2001. Vancouver, British Columbia, Canada. p. 47.