An evaluation of the mortality of the brown seaweed Ascophyllum nodosum (L.) Le Jol. produced by cutter rake harvests in southern New Brunswick, Canada

Ascophyllum nodosum (rockweed) landings in the Atlantic Maritime provinces of Canada totalled 36,500 wet tonnes in 2009. Due to the relative slow growth, stochastic recruitment, and the importance of habitat protection, strict harvest regulations are in place in the region to maintain the integrity of this resource. Special harvesting rakes have been designed to cut the plants and not to dislodge the clumps and their holdfasts from the substratum. However, for various reasons, close to 6% of the biomass harvested annually contains holdfast material. This proportion is closely monitored by the province as it is assumed to represent clump mortality. However, due to the complex structure of A. nodosum clumps, this relationship with mortality is not simple. A study was carried out to evaluate the real impact of this detachment on the A. nodosum population of southern New Brunswick in 2004. The structure of harvested A. nodosum clumps with associated holdfast material was analyzed and compared to non-harvested clumps from the same harvest area. Results showed that when a rake strips a clump, it only detaches 17.4% of the holdfast surface, leaving 36.8% of the plant biomass and 80.3% of the shoot density intact. An analysis of storm-cast material from the same study area showed a similar effect in the clump structure, although the incidence of holdfast in the detached biomass could be as high as 30%. Due to the high biomass detached each year by coastal storms in New Brunswick, their impact on the A. nodosum resource is 21 times higher than the annual harvest.