Negative Effects of Commercial Mussel Dragging on Eelgrass Beds in Maine

by Hilary A. Neckles, Frederick T. Short, Seth Barker, and Blaine S. Kopp

PWRC Fact Sheet FS 2005-3054

Introduction

       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.
       Therefore, the U.S. Geological Survey (USGS) provided funds from its State Partnership Program, which is focused on state natural resource management problems, to address issues concerning commercial mussel dragging in Maine eelgrass beds. In 2000, scientists from the USGS, the University of New Hampshire, and the Maine Department of Marine Resources embarked on a study to quantify the effects of mussel dragging on eelgrass in Maquoit Bay and to project the time required for the habitat to recover from dragging impacts.

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).
       Two independent methods were used to predict the rate of eelgrass recovery in heavily dragged areas. One method was based on measurements of the current eelgrass abundance at sites that were dragged at different times over the past decade. The other method involved development of a computer model of eelgrass revegetation, based on measurements of the rate of seedling appearance and plant growth in dragged areas. Both methods led to the same prediction — it takes an average of 11 years for eelgrass in dragged areas to revegetate to a level of 95 percent of the eelgrass cover found in undisturbed beds. It is important to note that the eelgrass meadow in Maquoit Bay was expanding during this study, indicating that conditions were highly favorable for eelgrass growth. The computer model predicted that under conditions less favorable for eelgrass growth, such as reduced water quality, recovery could take up to 22 years.

For More Information

• Neckles, H.A., F.T. Short, S. Barker, and B.S. Kopp. 2005. Disturbance of eelgrass Zostera marina by commercial mussel Mytilus edulis harvesting in Maine: dragging impacts and habitat recovery. Marine Ecology Progress Series 285:57-73.

Contacts

• Hilary A. Neckles, USGS Patuxent Wildlife Research Center, 196 Whitten Rd., Augusta, ME 04330, 207-622-8205 x 119, hilary_neckles@usgs.gov
• Frederick T. Short, Jackson Estuarine Laboratory, University of New Hampshire, 85 Adams Pt. Rd., Durham, NH 03824, 603-862-5134, fred.short@unh.edu
• Seth Barker, Maine Department of Marine Resources, PO Box 8, West Boothbay Harbor, ME 04575, 207-633-9507, seth.barker@maine.gov
• Blaine S. Kopp, USGS Patuxent Wildlife Research Center, 196 Whitten Rd., Augusta, ME 04330, 207-622-8201 x 114, bkopp@usgs.gov

or contact:
Director
Patuxent Wildlife Research Center (PWRC)
12100 Beech Forest Road
Laurel, Maryland 20708
(301-497-5500)

Visit the Patuxent Wildlife Research Center
Web page at http://www.pwrc.usgs.gov