Science Advisory Report 2010/046

Impact of at sea disposal on resident killer whale (Orcinus orca) critical habitat: Science in support of risk management

Summary

• Resident killer whales in the coastal waters off British Columbia and Washington are heavily contaminated with persistent organic pollutants (POPs), including banned polychlorinated biphenyls (PCBs).
• The northern and southern populations of resident killer whales are listed, respectively, as threatened and endangered under the Canadian Species at Risk Act (SARA), which protects species at risk from being killed or harmed (section 32; Appendix I) and protects any part of their Critical Habitat from destruction (section 58; Appendix I).
• A PCB food web modeling tool was developed to explore hypothetical scenarios related to disposal at sea of dredged sediment material, evaluate the contaminant risks associated with possible disposal practices on resident killer whales and their habitat, and facilitate the development of risk management practices and protocols.
• Results of PCB food web biomagnification modeling from sampled sediments confirm that BC’s killer whales are highly vulnerable to the effects of biomagnification by PCBs.
• Despite Canada’s move to ban PCBs over 30 years ago, these persistent contaminants are still found in sediment and still affect biota.
• Natural sedimentation buries PCB-contaminated sediments, but mixing by benthic animals can keep older, more contaminated sediment near the surface, slowing the reduction of PCB concentrations in the surface sediments and prolonging the exposure of food webs to PCBs.
• Because killer whales are long-lived animals, and pass on a portion of their contaminant burden to their offspring, a significant reduction in PCB concentrations in killer whales will take decades.
• Disposal of materials with less than ambient PCB concentrations at a receiving site is not expected to increase PCB delivery to killer whales and may help to bury ambient PCBs.
• Disposing of dredge materials into killer whale Critical Habitat in particular, and to a lesser extent their habitat in general, containing PCBs at concentrations that are higher than the ambient PCB concentrations, are predicted to increase the delivery of PCBs to killer whale food webs.
• Disposal of materials containing greater than ambient PCB levels would benefit from an alternative strategy, including disposal at a site with high natural sedimentation rate, as long as this new site is deemed to be depositional and dredge materials are not further dispersed. This would ultimately help to bury such PCBs, and could reduce overall habitat exposure of killer whales to PCBs.
• A case-specific approach to SARA-based permitting and advice for ocean disposal would best enable an evaluation of any possible effect of a particular disposal operation on killer whales and their habitat.
• To improve the risk-based characterization of ocean disposal at sea activities, congener-specific (high resolution) techniques to measure of PCBs are recommended. In addition, measured contaminants for disposal at sea files should be expanded to additional priority contaminants (e.g. the flame retardant PBDEs or other replacements for flame retardants), particularly as they pertain to SARA concerns and SARA-listed species.  
• This modeling suggests that a target sediment PCB range of 0.012 to 0.200 μg·kg-1 dry weight would optimally protect killer whales. However, these protective sediment PCB values are currently exceeded in many parts of coastal BC.
• An additional understanding of PCB pathways in coastal waters, including sources, sinks, sedimentation rates, and substrate types in both dredge and disposal sites will better inform risk-based decisions regarding the likely fate and consequences of disposal  activities within killer whale Critical Habitat.

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