May 27, 2020
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Rockweed harvesting: a recipe for sustainability

The ecological and sociological impacts of harvesting rockweeds are at the heart of a growing controversy in Maine, as evidenced by the emergence of anti-harvest alliances. Two of the underlying concerns are: Can rockweed (Ascophyllum nodosum) be harvested in an ecologically sustainable manner, and does its value as habitat or food for other organisms in near-shore ecosystems warrant a moratorium on harvesting?

Here, we discuss ecological aspects relevant to these questions. Collectively, we have conducted over 85 years of research on rockweed reproduction and ecology.

Adult rockweeds are 2 to 6 feet long and consist of an elongated main shoot with short, lateral-shoots. This alga grows from its tip and has excellent powers of regeneration. When cut, plants produce lateral shoots and become short and bushy.

This seaweed produces an air bladder annually and lives for several decades. However, there is a strong likelihood that many “holdfasts” (attachment structures) and lower segments of shoots may be hundreds of years old. Thus, the protection of holdfasts and lower shoots is critical to long-term survival of plants and rockweed beds.

A variety of field experiments with artificially seeded zygotes — reproductive structures, like seeds — on settling plates and natural rock surfaces indicate that water motion readily dislodges them and that thinning of adult plants increases water flow and dislodgement of zygotes. Thus, the patch size and density of plants is likely to be a critical cog in zygote establishment and possible remediation efforts.

Rockweeds suffer considerable mortality from natural breakage, storms, ice scouring, sedimentation and swift currents. This mortality is natural, evidenced by the collection of plants for centuries as drift on beaches. Dislodged plants in drift provide nutrition for microbial and detrital food chains. Most natural mortality occurs during fall and winter. In addition, grazing by herbivorous snails and small crustaceans creates holes in the shoots making them more susceptible to breakage.

Rockweed beds are more dynamic than casual observation would seem to indicate. These beds provide habitat for a diverse assemblage of small organisms that use the upright structures as nurseries to hide from predators, as food, or as reproductive sites.

Despite the various pressures on this alga, rockweed beds in Maine remain abundant, at least at present levels of removal. For example, Acadian Sea Plants Ltd. of eastern Canada reports that it removes 17 percent per year, which is sustainable. Given the large amount of biomass lost naturally, a confirmed 20 percent harvest per year would not seem unreasonable.

Several factors should be considered as the commercial harvesting of this species gears up. First, the tiny zygotes do not recruit easily, making remediation to disturbed beds difficult and long-term. Adult plants serve as habitat for other species, are weakly attached to substrates in some environments, and may be several decades old.

Conversely, this marine alga is the most dominant organism on all but exposed rocky shores in Maine, forming monocultures on most sheltered shores. It has high powers of growth and regeneration and can be a sustainable resource if managed correctly.

The removal of biomass of rockweed should be done without thinning the density of plants and beds, as this will allow for natural reseeding of zygotes. This requires an emphasis on cutting the tops or tips of plants and leaving the holdfasts and lower shoots intact for regeneration. Protection of holdfasts is absolutely critical, because it takes six to 10 years to attain harvestable size. Also, the greatest production of biomass comes from the oldest plants, which puts a premium on protecting holdfasts.

Any effective management plan has to begin with an accurate assessment of the resource. Presently, biomass estimates are used, but independent determination of biomass harvest is difficult, and we are concerned about the ability to detect cheating. Instead, the use of cutting height would make it easier to determine an average cutting height compared to assessing the amount of biomass removed from a section of the shore.

Also, an independent observer could make such height determinations, whereas testing biomass removal in an independent manner would be logistically impractical.

Lastly, enforcement of a management plan is needed if the resource is to be sustained. Given our work in this area over the years, a cutting height of 16 to 20 inches would seem to meet the precautionary principle for sustainability and for providing habitat for associated species.

Along with Robert L. Vadas and Brian Beal, Susan H. Brawley, Nicolas A. Blouin, and Wesley A. Wright contributed to this piece. Vadas is professor emeritus in botany, oceanography and zoology at the University of Maine; Beal is professor of Marine Ecology at the University of Maine at Machias; Susan H. Brawley is a professor of Marine Science at the University of Maine and president-elect of the Phycological Society of America; Nicolas A. Blouin is a Ph.D. candidate in Marine Biology at the University of Maine; Wesley A. Wright is an assistant scientist in botany at the University of Maine.

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