Seaweeds are algae, common along most rocky coastlines in temperate waters. As photosynthetic plants they are an important source of primary production to nearshore areas. Their biomass provides habitat to fish and invertebrates, as well as substantial quantities of particulate material used by the nearshore detrital food web (which feeds important bivalves like scallops). Seaweeds are also consumed directly by commercially important invertebrates like sea urchins.
Seaweeds produce a variety of chemicals of interest to humans. Historically, seaweeds have been harvested around the world for colloidal compounds useful in the food industry and for other specialized applications. Agar, alginate and carrageenan have all been extracted for these purposes. Seaweed has also been harvested over the centuries as food, fertilizer or for animal fodder. The most well known commercial seaweed crops in Atlantic Canada are rockweed (Ascophyllum), Irish moss (Chondrus) and dulse (Palmaria).
Rockweed is a large, well branched brown seaweed harvested by hand rake in small skiffs, usually by one person. By law, 12.7cm of the plant is left behind by the cutting blade of the rake, enough to ensure regrowth. Each skiff can handle many hundreds of kilograms of rockweed, which is dropped off at buying stations scattered along the coast.
Approximately 20,000 tonnes is harvested per year in Nova Scotia, mainly along the western and southern shores. The New Brunswick Bay of Fundy harvest brings in about half that amount. Most of the rockweed harvest is processed into fertilizers and animal feed.
Irish moss is a small 'turf like' red alga also harvested mainly by hand rake with single crew skiffs. In this instance, the hand rake does not have a cutting blade, but the tine spacing is set quite small (5mm by regulation) so larger plants are pulled off the rocks and cling to the rake, leaving smaller plants behind for regrowth.
Traditionally, large Irish moss harvests occurred along the south western Nova Scotian shoreline and western Prince Edward Island. However, the demands for the Irish moss extract, carrageenan, follows a world wide market involving other red algae which have reduced the dominance of Irish moss. The present Nova Scotian harvest is in the order of 1,500 tonnes per year and the PEI harvest has all but collapsed.
Dulse is also a red alga, larger and less stiff than Irish moss. It is harvested by hand picking on beaches dominated by boulders which get rolled and tossed by storms from time to time. Most of the dulse harvest takes place in Grand Manan. A small amount is collected in Nova Scotia as well. Harvest amounts are quite low. The plants are dried and salted, and sold as a snack.
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