Moules zébrées dans le Lac Érié

Photo: USGS

Selon le Ministère de l'Environnement du Canada, la concentration de mercure dans la chair du doré jaune pêchée dans le Lac Érié est à la hausse, et les coupables sont la moule zébrée et les gobies, toutes deux des espèces invasives venues probablement dans les eaux de ballast des navires.

Les moules zébrées se nourrissent des sédiments du fond du Lac Érié et se font manger à leur tour par les gobies. C'est ainsi que le mercure dans le fond de l'eau entre dans la chaîne alimentaire pour aboutir dans la chair des dorés jaunes mangés par les humains.

La pêche dans les Grands Lacs est une industrie qui vaut $7 milliards: la pêche commerciale a pêché 2 millions de dorés jaunes en 2007 et les pêcheurs sportifs du Michigan, de New York, de l'Ohio et de la Pennsylvanie en ont attrapé 2,5 millions. Un homme d'affaires qui loue des bateaux n'est pas surpris de la trouvaille, et pense que la perche connaît la même contamination: il dit que les estomacs des perches qu'il attrape sont remplis de moules zébrées.

Même en préparant les poissons avec soin, il n'y a pas de moyens d'enlever le mercure pour la consommation humaine: le contaminant est dans la chair que l'on mange.

Il faut se rappeler aussi que la rivière Richelieu est infestée par la moule zébrée: on pourrait s'inquiéter pour les pêcheurs de dorés qui mangent leurs prises! Et l'on sait déjà que partout dans la rivière Richelieu les meuniers noirs contiennent trop de mercure (http://www.mddep.gouv.qc.ca/eau/bassinversant/bassins/richelieu/faits.htm).
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"Mercury fishing rising

Anglers have another reason to bristle over the Lake Erie invasion of zebra mussels and round gobies.

The Eurasian natives have caused mercury concentrations in Erie walleyes to rise 50 percent since they invaded the lake 20 years ago, while levels in other Great Lakes walleyes have declined or remained stable. That's according to a study by the Canadian Ministry of the Environment, which sampled more than 5,800 walleyes and lake trout between the mid-1970s and 2007.

"During the 1980s, Lake Erie walleyes had the lowest mercury levels, compared to the other Canadian Great Lakes," said researcher Sityana Bhavsar, who led the 30-year study recently published in the American Chemical Society's Environmental Science & Technology journal. "Although Erie levels are now similar to, or lower than, the other lakes, what's disturbing is the increasing concentration trend."

Mercury also can impact fish health and reproduction, but that wasn't an area explored by Bhavsar's research. He blames the Lake Erie mercury increase on gobies and zebra mussels, which Erie has in greater numbers than other waters of the Great Lakes system. Both invaders are native to the Black and Caspian seas and are believed to have arrived in Erie in the ballast water of transoceanic ships around 1990.

"They've changed the food web," Bhavsar said, explaining that mussels filter feed on the lake bottom, taking in mercury along with water. Gobies are also bottom-dwellers and forage heavily on zebra mussels. "As mercury goes up the food chain, it magnifies," he said. "By the time it reaches walleyes, they get the full web effect."

Humans are the top predators in the food chain and walleyes are coveted pan fare in the Great Lakes' $7 billion fishing industry. Canada's commercial fishermen harvested about 2 million walleyes in 2007, while recreational anglers in Michigan, New York, Ohio and Pennsylvania harvested a combined 2 1/2 million walleyes that same year. Pennsylvania does not have a commercial walleye fishery, but its recreational industry, including charter captains, is a key part of the Erie economy.

Matt Hrycyk of Chasin Steel charters said he's not surprised by the Canadian findings on walleyes and would expect to learn that perch are similarly affected.

"When you catch perch, they're just loaded with zebra mussels," he said. "Look in shallow water and you'll see the lake's covered with them. The gobies are everywhere, too. It's a shame, but there's nothing to be done about it." But while he and others may be meticulous in how they clean and fillet walleyes, there's no way to cut or cook mercury out of a fish's body, Bhavsar said. Unlike some industrial toxins that accumulate in belly fat, mercury builds in muscle tissue, which is the part people eat."

Excerpts from article written by Deborah Weisberg published in the Pittsburgh Post-Gazette here: http://www.post-gazette.com/pg/10185/1070323-358.stm

It's a shame that not only have we polluted our rivers and lakes with toxic contaminants, we also introduce alien species in our ecosystems that make the pollution effects even worse.