ROCKWEED: Understanding the Impacts of Harvesting
(Published in the Quoddy Tides, August 11, 2000)
Most walks along the shores of Cobscook Bay involve some maneuvering around beds of rockweed draped over rocks and ledges or wayward clumps washed ashore in the mudflats and coves. Beneath these tangled masses of green, brown, and red vegetation often lie hidden worlds teeming with life. Sea urchins, green crabs, periwinkles, slipper shells, sponges, and sea stars are among those creatures who find food and moist places to hide in the seaweed as the tides retreat.
Interest in harvesting Cobscook's rockweed resource on a large-scale commercial basis has increased in recent years, particularly from Canadian processors. Local concern about the impacts of unregulated harvests has grown along with increasing demand. Very little is known about how harvesting impacts the long-term health of large beds of rockweed and the marine creatures and plants associated with them.
Recent research is beginning to improve our understanding. At the center of this research is Jill Fegley, a doctoral student at the University of Maine, who has been working with Professor Bob Vadas for the past five years to determine what ecological changes occur in large rockweed beds after harvesting. This research has been supported by the University of Maine Sea Grant program, the Department of Marine Resources, and the seaweed harvesting industry
"Our research is intended to answer two key questions: how quickly the rockweed beds recover after harvesting, and how harvesting impacts the marine plants and animals that make up the larger rockweed community," Fegley explains.
The research looked at the effects of two different cutting heights at each site, and compared the results with control sites - nearby areas that were not harvested. They have found that after one year of cutting at 15 inches from the holdfast, or base of the plant, there is little difference between the cut sites and the control sites. However, rockweed beds that were more severely cut, at 7 inches, show slower recovery after one year, and a decrease in the abundance and diversity of marine plants and animals that rely on the rockweed beds for food and refuge from predators.
"Some of the immediate impacts on other marine plants and animals were very obvious. For example, coralline algae - the reddish layer often seen on submerged rocks - died within a week after the rockweed was cut to 7 inches of length. You could see this clearly on the rocks because the normally red algae bleached completely white," Fegley reports. "One year after cutting, we still found a significant decrease in the abundance of more common marine animals like crabs, sponges, and smooth periwinkles."
"It's like in a forest - the knotted wrack serves as a canopy or cover for the plants and animals living beneath it," adds Fegley. "When that cover is removed or reduced, it allows more light and heat. These added stresses change the environmental conditions these plants and animals are adapted to."
Knotted wrack is particularly vulnerable to overharvesting because it does not reproduce easily. It is most effective at vegetative reproduction, where young shoots at the bottom of the plant can grow quickly when exposed to light. For this growth to occur, both the holdfast of the seaweed (where it attaches to the rock) and the parts of the stem and branches that contain healthy growth cells must remain on the plant after harvesting.
"Since this species of rockweed has such a difficult time reproducing, it is very important to leave the holdfast intact while harvesting, and to monitor for that," states Fegley.
The study also found that the age of the rockweed beds influences the ability of the rockweed to recover.
"In some old plants, we saw no regeneration two years after cutting, while younger plants were growing back with vigor after the same amount of time," reports Fegley. "This makes regulation difficult, because some beds can be cut and do fine, and other rockweed beds in the same area, under the same conditions, simply can't grow back. It's very site specific, and it takes experience to know which sites can sustain cutting."
Fegley is currently looking at what happens the second year after cutting, as well as the growth rates of knotted wrack after cutting and when no cutting takes place.
"With the information we have gathered so far, we have recommended to the state that if certain rockweed beds are going to be cut heavily, there should be a three year recovery period to allow the plants to grow back and provide sufficient cover for other marine species," Fegley states. "We also recommended that cutting height be limited to 16 inches - cutting lower on the rockweed stem reduces the plants ability to grow back, and leaves the rockweed beds much more vulnerable to overharvesting."
This differs from the proposed state regulations, which will be discussed at two public hearings later this month in West Boothbay Harbor and Ellsworth.
Fegley comments, "Our research is telling us what amount of cutting the knotted wrack can handle without damaging the resource - not just the rockweed itself, but the other species that rely on rockweed beds to survive. As scientists, we have to provide recommendations that protect the long term health of the resource for both economic and ecological reasons."
"There is still a lot that we don't know," she adds. "We don't know how repeated harvesting year after year impacts the resource, or what happens when you look at the impact of harvesting on a larger scale. The small size of our study areas allows the marine animals to move 'next door', but if larger areas are cut, where are they going to go?"
The potential for overharvesting and long-term damage to marine resources is high when we don't know how the resource, in this case, rockweed, fits in to the larger natural system. Increasing our knowledge about how fragile a resource is, or how well it can sustain commercial harvesting, is a key part of learning how to manage it wisely. The information gathered by Fegley and others is critical to efforts to manage Cobscook Bay resources in a way that can benefit the local economy in the long term while also maintaining the overall health of the bay.
This column was prepared by Cheryl Daigle and Jim Dow. Cobscook Soundings was a monthly column produced by the Maine Chapter of The Nature Conservancy. Its purpose was to share what is known about the workings of the Cobscook Bay marine environment, so that all who make decisions about the use or care of the bay have the best available information.
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