Minnesota anglers get out early to fish along the north shore of Lake Superior. Contaminants in some Superior sport fish contain high levels of some persistent organic pollutants.
More than 40 years after the first human health advisories were issued for fish consumption because of Great Lakes toxic contaminants, concerns remain. In at least one of the five Great Lakes, Erie, levels of mercury in fish are actually increasing after years of decline, according to a report publicized this week. Toxaphene levels in Lake Superior fish also persist at levels that could damage human health, although the primary route of contamination has changed since 1970.
Direct industrial chemical dumping contributed to health concerns then. Today’s primary source is often the atmosphere. Chemical contaminants can travel thousands of miles through the atmosphere before falling out and polluting the aquatic food chain. Another source is thought to be non-native zebra and quagga mussels and round goby, which can make contaminants in sediments available in the food chain.
Although Banned, Some Toxic Chemical Levels Increase
Mercury levels in the keystone sport fish species in Lake Erie, walleye, increased between 1990 and 2007, according to a new report published in the journal Environmental Science and Technology. Mel Visser, author of the book Cold, Clear and Deadly, points out that toxaphene levels in Lake Superior fish have increased 50% since the U.S. banned the chemical in 1982, strongly suggesting global sources of contamination.
Most at risk from consuming contaminated sport fish are women of child-bearing age and their offspring. Mercury exposure in children has been associated with neurological harm. Visser notes that Canadian Inuit women of childbearing age consume 14 times the tolerable daily intake (TDI) of chlordane, toxaphene, PCBs and other pesticides and suffer infertility, stillbirths and birth defects. The chemicals involved have an affinity for cold northern lakes.
Visser has called for tough international action on global sources of these toxins, rather than focusing intensively on minor sources within the U.S. and Canada. In the late 1990s, Michigan and other Great Lakes states considered suspending some advisories because contaminant levels in sport fish had steadily fallen since their peak in the early 1970s. But environmental health advocates protested and the states largely backed off.
Photo: Jeff Gunderson, Minnesota Sea Grant