Aquatic organisms may have high mercury concentrations even in low source areas of the US far from cities because conditions are right for efficiently methylating the mercury raining down from the atmosphere. Most studies of mercury bioaccumulation have been done in lakes, reservoirs, and wetlands, whereas much less is known about mercury bioaccumulation in rivers and streams. Mercury in fish is primarily in the form of methylmercury. This is the form that is bioavailable to fish and organisms that eat fish, including wildlife, and humans. Nationwide, Wisconsin ranks second in the number of lakes and rivers with mercury advisories for fish consumption.
Together with concurrent sampling of other media:
- In 2002, skin-off fillets of predator fish were analyzed from 8 sites in the Western Lake Michigan Drainages (WMIC).
- In 2003, more detailed sampling was done at three sites in the WMIC representing one urban and two rural/non-cultivated watersheds (low and high wetland percentages). Sites were sampled once per year (June) for skin-off fillets of predator fish and twice per year (June and August) for whole/gutless/headless forage fish.
- In 2004-5, limited sampling for long-term trends in mercury concentrations began in the WMIC and includes sampling of predator and forage fish once per year at two sites using methods as in 2003-4.
Samples were collected by electrofishing and processed using clean technique. It was not possible to collect the same species of fish at all sites. The target for predator fish at all sites was 3-4 yr old (adult) fish; fish were aged using otoliths (ear bones). Fish stomach contents were analyzed to provide information on diet and stable isotopes were analyzed in fish tissue to provide information on longterm diet and food web relations. Each fish was analyzed individually.
Field sampling relied on personnel from many USGS offices and disciplines and cooperative assistance from the Menominee Indian Tribe of Wisconsin and Wisconsin Department of Natural Resources.
Detailed Sampling Sites - 2003
Oak Creek in South Milwaukee , WI
Predator fish: Green sunfish
Forage fish: Creek chub
Evergreen River near Langlade , WI
Pike River near Amberg , WI
Predator fish: Brown trout
Forage fish: Mottled sculpin, Blacknose dace
Preliminary results showed no predator fish with mercury concentrations that were near or above the USEPA standard of 0.3 mg/kg wet weight; however, one individual forage fish (blacknose dace) from a reference/non-cultivated site in 2003 had mercury at this concentration. In addition, mercury concentrations in some forage fish from the reference/non-cultivated sites were similar to mercury concentration in predator fish.
Data for additional chemical concentrations in other media are still being compiled and reviewed. Results of these analyses are being included in reports that will be completed in 2006-7.