By Lizzy Ashley
Toxins of concern in the Salish Sea include persistent organic pollutants (like PCBs), hydrocarbons (from fuel), pharmaceutical compounds, and trace elements (including heavy metals).
Some elements (like copper, selenium, and calcium) are necessary for life at lower concentrations but can be toxic at higher levels. Other elements considered non-essential (like lead and mercury) also can be toxic to aquatic organisms at elevated concentrations. Monitoring trace element exposure in marine organisms is essential to assess potential risks to wildlife and humans.
Harbor seals, the most common marine mammal in the Salish Sea, are especially susceptible to trace element accumulation because of their long lifespans, small home ranges, and high position in the food chain. We analyzed trace element concentrations in livers of harbor seals that died and stranded in the San Juan Islands and compared findings to other harbor seal populations in the Salish Sea.
Data was collected through the San Juan Marine Mammal Stranding network, which is jointly run by The Whale Museum and the SeaDoc Society.
Adult harbor seals in San Juan County (SJC) had higher concentrations of copper and zinc than their South Puget Sound (SPS) counterparts, but these concentrations were not at levels suspected to affect marine mammal health. Concentrations of cadmium were also higher in SJC seals than in SPS seals but were much lower than the published “threshold” level of toxicity. Mercury concentrations were similar among seals from multiple sites in the Salish Sea and well-below harmful levels, except for in one 29-year-old seal from San Juan County. This stands to reason because as harbor seals age, they accumulate mercury in the liver. Despite this high liver concentration, there was no evidence of trace element toxicity in this seal, or for any of the harbor seals examined in San Juan County over the past twenty years. Interestingly, and for some additional perspective, mercury concentrations in harbor seals from the Salish Sea were significantly lower than those reported in a prior study of harbor seals in central and northern California.
While harbor seals are exposed to trace elements (naturally occurring, human-introduced, or both) in the Salish Sea, trace element toxicity is not a major threat to harbor seal health. This study examined a limited number of adult seals, so future monitoring efforts should focus on adult animals. While policies and innovations like ceramic brake pads free of copper, lead, mercury, and cadmium will reduce human inputs of certain metals, trace elements continue to enter the Salish Sea and should be monitored moving forward.
This study would not have been possible without the numerous San Juan County Marine Mammal Stranding Network volunteers who helped us collect carcasses and perform necropsies. We cannot thank these citizen scientists enough!