Beach-cast biomass and commercial harvesting of a non-indigenous seaweed, Mazzaella japonica, on the east coast of Vancouver Island, British Columbia

Abstract

As the number of seaweed introductions continues to grow globally, the beach-cast collection of non-indigenous macroalgae has become increasingly common for both esthetic and commercial reasons. These harvests and their effects, however, are poorly characterized in the literature. Since 2007, the British Columbia Ministry of Agriculture has been issuing licenses permitting the commercial harvest of beach-cast Mazzaella japonica, an introduced red algae, on eastern Vancouver Island. A 900 tonne quota was approved for the 4.2 km harvest region in 2014/2015, and was increased to 1500 tonnes for the 2015/2016 season. The objective of this study was to provide the first documentation of: (1) beach-cast seaweed accumulation, (2) depositional dynamics, and (3) the proportion of biomass removed within the harvest region. At peak biomass, we recorded 1586 tonnes of beach-cast seaweeds within the harvest region and up to 1.33 tonnes per meter of shoreline in areas of greatest accumulation. This biomass was highly transient and largely disappeared from the region in late December of both harvest seasons, likely as a result of longshore drift. On average, the commercial harvest removed less than 16% of the available biomass each week. These findings contribute to the limited literature on beach-cast harvesting, helping to inform management practices at both a local and global scale. Furthermore, they provide an important record of M. japonica’s abundance and distribution in the harvest region, which has previously been limited to anecdotal information.