Mercury in Forage Fish from Mexico and Central America: Implications for Fish-Eating Birds.

Mercury (Hg) is a global contaminant of aquatic food chains. Aquatic birds, such as the osprey (Pandion haliaetus), with migratory populations breeding in Canada and the northern United States and wintering in the Central and South America, can be exposed to mercury on both the breeding and wintering ranges. We examined Hg levels in 14 fish taxa from 24 osprey wintering sites identified from satellite telemetry. Our main goal was to determine whether fish species that feature in the diet of overwintering and resident fish-eating birds reached toxicity thresholds for Hg. Mean Hg levels in fish whole carcasses ranged from a high of 0.18 µg g(-1) (wet weight) in Scomberomorus sierra to a low of 0.009 µg g(-1) in Catostomidae. Average Hg levels were within published toxicity threshold values in forage fish for only two sites in Mexico (Puerto Vallarta and San Blas Estuary), and all were marine species, such as mackerel (Scomberomorus sierra), sea catfish (Ariopus spp.), and sardinas species (Centropomus spp.). Except for one sample from Nicaragua, sea catfish from Puerto Morazan, none of the fish from sites in Central America had Hg levels which exceeded the thresholds. Nonmetric multidimensional scaling revealed geographical differences in Hg levels with significant pairwise differences between sites along the Pacific Ocean (Mexico) versus the Bay of Campeche, partly due to differences in species composition of sampled fish (and species distributions). Hg increased with trophic level, as assessed by nitrogen stable isotope ratios (δ(15)N but not δ(13)C), in freshwater and marine, but not estuarine, environments. Hg concentrations in forage fish do not account for the elevated Hg reported for many osprey populations on the breeding grounds, thus primary sources of contamination appear to be in the north.

What are the toxicological effects of mercury in Arctic biota?

This review critically evaluates the available mercury (Hg) data in Arctic marine biota and the Inuit population against toxicity threshold values. In particular marine top predators exhibit concentrations of mercury in their tissues and organs that are believed to exceed thresholds for biological effects. Species whose concentrations exceed threshold values include the polar bears (Ursus maritimus), beluga whale (Delphinapterus leucas), pilot whale (Globicephala melas), hooded seal (Cystophora cristata), a few seabird species, and landlocked Arctic char (Salvelinus alpinus). Toothed whales appear to be one of the most vulnerable groups, with high concentrations of mercury recorded in brain tissue with associated signs of neurochemical effects. Evidence of increasing concentrations in mercury in some biota in Arctic Canada and Greenland is therefore a concern with respect to ecosystem health.

Effects of dietary selenium on reproduction and body mass of captive lesser scaup.

The continental population of scaup has declined over the last three decades to levels that remain well below conservation goals. One proposed hypothesis for this decline is that increasing exposure to contaminants has led to decreased fitness in scaup, and selenium (Se) has been identified as a contaminant of concern. We conducted an experiment with captive scaup to simulate late spring migration exposure to environmentally relevant doses of dietary Se (control [<1 microg/g total Se], and 7.5 microg/g and 15 microg/g Se as selenomethionine) for 30 d, after which excess Se was removed from the diets prior to laying. Overall, we found no treatment effect on body mass, breeding probability, or clutch initiation dates. Egg Se concentrations of the 15 microg/g and 7.5 microg/g groups decreased rapidly after Se-supplemented diets were removed and within 12 and 8 d post treatment, respectively, were below the suggested egg teratogenicity threshold of 9 microg/g (dry wt). We found no significant intraclutch variation in egg Se deposition. Blood Se concentrations differed between groups in proportion to dose, with a half-life of 16 and 22 d for females when removed from the 15 and 7.5 microg/g treatments, respectively. Our findings suggest that exposure to dietary Se concentrations representative of spring-staging environments is insufficient to adversely affect body mass or reproduction in scaup that subsequently migrate to uncontaminated breeding areas (e.g., in prairie-parklands and boreal forest). Further studies of Se effects on scaup should focus on wintering birds in contaminated habitats.