By Charlie Otto Rasmussen, Great Lakes Indian Fish & Wildlife Commission

For many American Indians, a diet that includes traditional foods is a key ingredient in maintaining a healthy lifestyle. The presence of harmful toxins such as mercury in fish, however, has created challenges for tribal members seeking to serve nourishing and safe meals to their families.

In the Lake Superior region, a natural resource agency representing 11 Ojibwe tribes has developed a series of lake maps using GIS technology to help consumers assess the risk of eating walleye-a mealtime staple for centuries-from specific water bodies. The Great Lakes Indian Fish & Wildlife Commission (GLIFWC) is completing its 16th year of walleye mercury sampling in lakes where tribal spearfishermen harvest fish each spring. After testing walleye fillets from four different size groups, GLIFWC staff utilizes ArcView software to create color-coded maps that clearly display measured mercury levels in each lake being studied. In a glance, tribal fishermen can choose the appropriate lake in which to harvest fish.

"There's a heightened awareness of the dangers of mercury, especially to developing children," says Joe Dan Rose, GLIFWC fisheries biologist. "Many tribal spearers request these maps before the fishing season to select lakes and sizes of fish that are lower in mercury."

Burning coal, wood, and waste releases mercury into the atmosphere, and it commonly returns to earth in precipitation. Upon entering lakes and streams, certain bacteria chemically convert mercury into a more toxic compound known as methyl mercury. Methyl mercury travels up through the food chain as larger organisms eat smaller organisms, eventually accumulating at elevated levels in the flesh of top predator fish such as walleye.

For humans, methyl mercury is a neurotoxin affecting the brain and spinal cord. Women of childbearing age, children under age 15, and fetuses represent the most at risk group from methyl mercury poisoning. While fish such as walleye provide important dietary nutrients, too much methyl mercury is linked to developmental impairment in children. Overexposed adults may experience tremors and tingling sensations in the hands; blurred vision; speech problems; and, in rare cases, paralysis and death.

To address the different threat levels, GLIFWC staff generated twin mercury advisory maps for each member tribe using ArcView 3.x. Esri software was chosen because of its wide acceptance throughout the GIS community, convenience of access to training, and its ability to import and export a wide variety of different data formats. The maps are designed so that the top map is more restrictive-for pregnant women, women of childbearing age, and children under age 15. The bottom map is less restrictive-for women beyond childbearing age and men. Information, including color indexes and consumption guidelines, appears on the reverse side.

The criteria for walleye consumption are based on Wisconsin Department of Health guidelines, which recommend limiting intake of fish with 0.5 parts per million (ppm) mercury and avoiding all fish with 1.0 ppm or more of mercury.

Lab results from mercury-tested walleye were imported into ArcView as a dBASE table and joined to a water body coverage through a common field. Legend files (.avl) were created to interpret the data and color code each lake according to the length of the fish and mercury concentration.

Encompassing portions of Minnesota, Wisconsin, and Upper Michigan, the advisory maps highlight a region where Ojibwe Indians exercise fishing, hunting, and gathering rights reserved through treaties with the United States in the mid-1800s. In cooperation with state and federal agencies, GLIFWC monitors and manages natural resources on treaty-ceded lands.

Once testing of the 2004 walleye samples is complete, GLIFWC plans on issuing updated mercury advisory maps, using the latest version of ArcView, that include additional lakes in Michigan and Minnesota. Due in 2005, the new maps are expected to include revised fish consumption guidelines based on EPA and Food and Drug Administration standards.

For more information, contact John Coleman, GLIFWC Environmental Section leader (tel.: 608-263-2873, e-mail: colemanj@calshp.cals.wisc.edu), or Matt Hudson, GLIFWC Environmental biologist (tel.: 715-682-6619, e-mail: mhudson@glifwc.org). Mercury maps are available at www.glifwc.org on the Web.