Biogeochemistry, stable-isotopic composition, and feeding habits of Arctic grayling (Thymallus arcticus) in the lower Fortymile River, Eastern Alaska

We report on the inorganic chemistry of Arctic grayling, an ecologically important resident freshwater sport and subsistence fish in the Fortymile Mining District of the Interior Highlands Ecoregion in eastern Alaska. Placer gold has been mined in the study area for more than a century and mineral exploration activity is currently being renewed there. We report functional biogeochemical baseline values and (or) ranges for 38 elemental concentrations in muscle (fillet) tissue, liver tissue, and stomach contents from 34 fish caught at 11 sampling sites within the watershed. Included in these data are geometric means for total Hg contents of 0.069 and 0.062 ppm in muscle and liver tissue, respectively. These levels are more than an order of magnitude below the Food and Drug Administration's permissible value for methylmercury in fish fillets. This study does not attempt to correlate fish age (weight) with tissue chemistry (we sampled only sexually mature fish); however, biogeochemical variation in association with fish age nevertheless occurs. For example, we found a positive correlation (r 2 =0.41) between the total Hg content in muscle tissue and fish weight. Because we are most interested in the relation between elemental concentrations in fish tissue and regional geochemistry (as influenced by the geochemistry of lithologic units), we normalized tissue elemental concentrations to fish weight. In general, we noted little variation in the normalized elemental concentrations in muscle tissue among samples at each of the 11 fish-sampling sites. No definitive link could be attributed between biogeochemical patterns and regional lithology. Differences among fish-sampling sites (as determined by an unpaired t test), however, were important for a few major nutrient elements (Ca, K, Mg, Na, P) and trace elements (Cr, Cs, Hg, Se). Stomach-content chemistry, also normalized to fish weight, varied widely (relative to liver or muscle tissue) and generally reflected sediment chemistry-a component of the ingested material. The N-isotopic compositions of fillet samples are homogeneous (δ 1 5 N=7.6-9.7 permil), reflecting a restricted, low trophic (primary predator) position for the grayling. C- and S-isotopic compositions (δ 1 3 C, δ 3 4 S values) of fillet samples range from -33.1 to -25.8 permil and from -8.4 to 8.2 permil, respectively, suggesting heterogeneity of food sources (both aquatic and terrestrial). Stomach-content material was examined for the occurrence and frequency of macroinvertebrates and their chemical composition in three fish. Results showed considerable diversity, with 9 to 15 invertebrate taxa of which both aquatic and terrestrial forms were represented.