Negative Effects of Commercial Mussel Dragging on Eelgrass Beds in Maine
PWRC Fact Sheet FS 2005-3054
Maquoit Bay, which forms the northwestern arm of Casco Bay, Maine (fig. 1), has an extensive eelgrass meadow of over 1,300 acres covering half the bay bottom. When eelgrass began piling up on the shoreline early in the summer of 1999, local residents knew something was wrong. Preliminary evidence pointed to commercial mussel dragging as the source of habitat disturbance — large bare areas within the eelgrass meadow were marked with distinctive, linear dredge scars on the bay bottom, and piles of mussel shell appeared to have been dumped overboard during mussel washing and sorting operations. Although natural resource managers, shoreline citizens, commercial harvesters, and scientists had all expressed concern over the years about the impacts of commercial mussel dragging on eelgrass habitat throughout Maine, no scientific study had ever measured the magnitude of damage from dragging. Significant damage to eelgrass beds by mussel dragging could have negative effects on the fishery, as eelgrass is known as important habitat for blue mussel larvae.
Impact of Dragging on Eelgrass
Commercial dragging for mussels occurred in Maquoit Bay throughout the 1990s, leaving four identifiable dragging scars ranging from 8 to 79 acres in size. The largest of these scars is shown in figure 2. Aerial photographs of Maquoit Bay taken in 2000 revealed that a total of 132 acres of eelgrass, or about 10 percent of the eelgrass in the bay, had been disturbed by dragging. Two sites that were dragged in 1999 had very little eelgrass cover in 2000. Dragging completely uprooted eelgrass plants, removing leaves, rhizomes, and roots. Underwater video measurements showed that dragging intensity was variable. In some areas, patches of mature plants remaining after dragging suggested relatively light impacts; however, an average of 86 percent of the recently dragged bottom was bare, indicating heavy dragging over most of the area. Two older dragging scars (one that had been dragged 2 to 7 years earlier and one that had been dragged more than 8 years earlier) showed continuous eelgrass cover in 2000, but drag marks were still evident and eelgrass abundance was still substantially reduced. Dragging did not alter the physical characteristics of the sediment.
Eelgrass Recovery Following Dragging
The pattern and rate of eelgrass bed recovery depended on initial dragging intensity. Aerial photographs showed patchy eelgrass regrowth in areas of relatively light dragging after 1 year, but very little revegetation in heavily dragged areas (fig. 3).
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U.S. Department of the Interior, U.S. Geological Survey, Patuxent Wildlife Research Center
Last Updated: October 28, 2005, URL: http://www.pwrc.usgs.gov/ Email the Webmaster, Email the Director