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About 1980 the tufted duck began to use a gravel-pit reserve, Häverner Marsh, as a molting site. As many as 350 adult ducks are to be seen June-August during post-breeding and the start of pre-breeding molt. The reasons for use by ducks are probably high densities of zebra mussels (Dreissena polymorpha) and lack of human disturbance, but hunting in this marsh has a negative effect. With rules passed in 1979 concerning nature preserves, access to the banks and sport fishing was prohibited and this was enforced since 1981. Since March 1986, main entryways into the nature preserves have been closed with barriers during periods when there has been no mining activity in the gravel pits. These steps resulted in an increase in molting ducks seen. The greatest disturbance comes from hunting, which is allowed by nature preserve ordinances April 1-November 15. There are four hunting districts in the Häverner Marsh and hunting pressure is great. Continued mining of quarries keeps tufted ducks away, a voluntary alternating-week hunting plan was not effective in helping ducks, and a complete ban on hunting is believed to be necessary where ducks use gravel pits.