Sample collection and processing

Concentrations of THg, TSe, and MeHg⁺ were measured in archived Steller sea lion liver, muscle, heart, and kidney. Samples were collected opportunistically from carcasses from the eDPS and wDPS in Alaska from 2002 to 2013, including many age classes (n = 98; Table 1). Archived tissue samples were provided by the Alaska Region Stranding Network and the Marine Mammal Laboratory, the National Marine Fisheries Service/National Oceanic and Atmospheric Administration, and the Alaska Department of Fish and Game and from subsistence harvests by the Alaska Sea Otter and Steller Sea Lion Commission. Not all targeted tissues were collected from each animal or analyzed for a particular analyte, so direct tissue comparisons were made for matched pairs (same individual) of samples as available. Muscle samples were collected opportunistically, often without enough detail provided to definitively assure specific anatomical identification of skeletal muscle, so they were categorized according to type (skeletal, cardiac). General anatomical location of sampling (pectoralis, biceps, latissimus or longissimus dorsi [described in the present study as l. dorsi], and cheek) was reported in many cases.

Tissue type, age class, and analytes for 98 Steller sea lions (Eumetopias jubatus) sampled in the present study

THg = total mercury; TSe = total selenium; MeHg⁺ = monomethylmercury.

Samples included in our study were collected from relatively fresh carcasses (hunter-killed and/or without obvious signs of dehydration or autolysis) and kept cold (coolers) in the field. In the laboratory, samples were packaged into polyethylene freezer-quality storage bags, expelling air to reduce dehydration; frozen; and stored long-term at −80 °C. After transfer to the University of Alaska Fairbanks, partially thawed tissues were subsampled, trimming exterior surfaces with a clean knife or stainless-steel scalpel to produce a subsample free from external contaminants or dehydration. Instruments and cutting boards were cleaned thoroughly and rinsed with ultrapure water between samples. Subsamples (5–20 g) were weighed, placed in polyethylene sample bags, freeze-dried for 48 h (Labconco FreeZone 4.5; Labconco), reweighed, and homogenized (Retsch cryomill). Water content, determined by subtracting dry mass from wet mass, was used to calculate analyte concentrations on a wet weight basis.