Salmon as transport vectors for mercury
JUPITERIMAGES
yearly contribution of methylmerof mercury smolts remove from By miraculously returning to their cury to the area from sockeye the system in a given brood year inland spawning grounds from salmon (excluding the exported is inversely related to the number the ocean to lay eggs and die, fraction) adds up to just 71 of spawning adults. This paraadult sockeye salmon do more gramssan infinitesimally small doxical finding is a function of than reproduce: live fish supply a amount considering that up to 1.3 “density-dependent” dynamics in bear’s feast, and their decaying million adults escape to these salmon ecology, he says: once the bodies transfer nitrogen and spawning grounds every year. adult population’s numbers exphosphorus from marine to freshNot surprisingly, mercury water ecosystems. concentrations in local fish Spawning salmon also are at least 10 times lower carry methylmercury, which than regulatory standards, they bioaccumulate while at according to Tim Sands, an sea, where they gain up to area management biologist 95% of their adult weight. with the Alaska Department Ecologists used to see this of Fish and Game; he devector-borne transfer of scribes this salmon fishery mercury inland from the as exceedingly healthy. oceans as a one-way delivMark Scheuerell, a fisherery. In a recent ES&T paper, ies biologist with the U.S. Matthew Baker and colNational Oceanic and Atmoleagues investigate the reAdult salmon transport methylmercury inland when they spheric Administration in turn route (2009, DOI return to their home streams to spawn. Young salmon Seattle, says the study’s take10.1021/es901798f). Their home message is that biolofindings indicate that about carry about 12% of it back to the ocean. gists should consider the 12% of the methylmercury salmon’s whole life cycle when ceed resource constraints in the brought by adult sockeye to assessing ecosystem health. watershedsbecause there isn’t spawning grounds is returned to Scheuerell works in Puget Sound, enough spawning habitat for the ocean by their progeny, called which has more contaminants adults or rearing habitat for smolts. than Bristol Bay. smolts, for instancesthen the cor“We’re trying to draw attention Until he became aware of Bakresponding smolt numbers will to this export component,” says er’s research, says Scheuerell, he plateau and stabilize accordingly. Baker, a doctoral candidate in hadn’t viewed smolts as a potenAccording to Baker, this means aquatic and fishery sciences at tial source of contaminants to that vast numbers of spawning the University of Washington Semarine predators. “So, what we adults in a given year don’t transattle. “It’s a lesser amount, but might think about doing is looklate into proportionately higher it’s still important to consider.” ing at contaminants of interest in mercury exports later. Salmon eggs are virtually free of juvenile salmon leaving Puget However, smaller adult methylmercury, Baker says. InSound rivers,” he says. “That populationssdepressed by habitat stead, smolts pick it up from their could allow us to model how condegradation or fishery diets, mainly zooplankton and taminants transfer directly from exploitationsdo. This needs different invertebrate species. Resalmon to predators, as opposed greater attention, says Baker. “It’s lying on data corresponding to 15 to bottom-up transfers occurring more important to look at juve“brood years” (which relate offamong lower species.” nile exports in these depressed spring to parents from a single Baker agrees. “We’re explicitly brood years because the amount spawning season) gathered by the trying to draw attention to these of mercury they take out is proAlaska Department of Fish and basic biological mechanisms and portionately greater than what the Game, Baker and his colleagues how accounting for them influadults bring in.” sampled both incoming adults ences our understanding of bioBaker’s research was done and outgoing smolts for mercury logical vectors,” he says. “This, along the Wood River system, in levels. These were converted into rather than questions relating to southern Alaska’s pristine Bristol gross import and export volumes the amount of mercury transBay. Annual mercury deposition for each brood year under ported to this particular system, is from the atmosphere in this 3590investigation. our main focus.” square-kilometer region amounts A key finding from this study, —CHARLES SCHMIDT to 2.1 kilograms. By contrast, the Baker believes, is that the amount 10.1021/es903305p
2010 American Chemical Society
Published on Web 11/18/2009
January 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 11
