Legacy and current-use contaminants in pacific chinook salmon (oncorhynchus tshawytscha) stocks and their contribution to Resident killer whale (orcinus orca) contaminant burdens.

Abstract

The Endangered Southern Resident killer whales (SRKW, Orcinus orca) face significant threats including reduced abundance and quality of their primary prey (Chinook salmon, Oncorhynchus tshawytscha) and high levels of endocrine disrupting contaminants while experiencing a decrease in population growth over the past several decades. The sympatric Northern Resident killer whales (NRKW) also primarily consume Chinook but have lower contaminant burdens and have experienced consistent population growth for nearly five decades. This study characterized concentrations of 19 legacy and current-use contaminants in priority Chinook stocks/groups consumed by SRKWs and NRKWs, calculated Chinook contaminant body burden, estimated daily contaminant intake (EDI) from Chinook salmon in SRKW diet across seasons, and compared Chinook salmon related EDI between SRKW and NRKW during the months of May to October. Shelf Resident Chinook had higher mean muscle ∑contaminant19 concentrations and mean ∑contaminant19 body burdens than Columbia (4.9- and 2.6-fold, respectively) and Far North/Offshore Chinook (5.7- and 2.1-fold, respectively). This resulted in a disproportionately higher contaminant intake by SRKW from Shelf Resident Chinook throughout the year relative to Far North/Offshore Chinook. From May to October, an average SRKW consumed approximately 15.4% more Shelf Resident Chinook a day than an average NRKW resulting in a 5.3-fold higher EDI from Shelf Resident Chinook and an overall 1.62-fold higher EDI (1410 μg ∑contaminants19/day) from all three Chinook groups compared to NRKW EDI (869 μg ∑contaminants19/day). Results provide an updated characterization of contaminant exposure for NRKWs and SRKWs, insights into the current levels of priority contaminants in Chinook salmon, and an explanation as to why SRKW have higher contaminant loads than NRKW. Recovering at-risk wild Chinook salmon populations that have a more offshore rearing distribution and lower contaminant body burdens (ie, Columbia and Far North/Offshore Chinook stocks) should be a top priority to ensure the recovery of SRKW.