Biogeochemical patterns in prey species reveal complex mercury exposure pathways from the environment to Aleutian Steller sea lions

Several wildlife species exhibit marked spatial variation in toxicologically relevant tissue concentrations of mercury across the Aleutian Islands of Alaska, most notably the endangered Steller sea lion (Eumetopias jubatus). To unravel potential environmental and trophic pathways driving mercury variation in this species of concern, we investigated spatiotemporal and ecological patterns in total mercury concentrations and stable isotope ratios of carbon and nitrogen from muscle tissues of twelve mid-trophic level prey species of the region (n = 1461). Dividing samples into island groups explained biogeochemical variation better than larger spatial resolutions, with Amchitka Pass and Buldir Pass acting as strong geographic break points. Species-specific relationships between muscle biogeochemistry and length were common, and benthic feeding and increased trophic position were associated with higher total mercury concentrations. Considered together, tissue biogeochemical markers in this region are dependent on location and the species upon which they forage in remarkably nuanced manners.

Methods

Fish were collected during commercial fishing operations by Ocean Peace, Inc.  A subset of bycaught species were stored frozen on the ship until transferred to the University of Alaska Fairbanks (UAF).
At UAF, fish were identified to species, photographed, weighed, and measured.  Muscle samples were collected from the left lateral aspect of the fish, centrally located along the length of the fish. 
Muscle samples were freeze-dried and homogenized into a flour. 
Total mercury concentrations were obtained using a Direct Mercury Analyzer (Milestone DMA-80).  Standard reference materials had 100% ±10% recoveries from theoretical concentrations during each day of testing.
Data were reported as  the mean (µg/g) of the technical replicates.  The percent coefficient of variation (%CV) was calculated for each set of technical replicates; desired %CV was <10% to meet laboratory standards.