Changes in the population structure of Ascophyllum nodosum (L.) Le Jolis due to mechanical harvesting

Abstract

The use of a Norwegian suction cutter to harvest populations of the brown alga Ascophyllum nodosum (L.) Le Jolis in southwestern Nova Scotia started in 1985. The impact of this type of mechanical harvest on the algal population structure was evaluated. Changes in the length and density of individual plants (clumps) within 0.25 m⁻² quadrats, as well as the length of individual shoots within clumps were monitored before and after an experimental harvest. The mechanical harvest cut mainly the longer plants, thus changed the initial bimodal size structure of the population to unimodal. There was a 20 to 36% plant mortality, reducing the plant density from 92.6 to 73.6 individuals m⁻². Pre-harvest size distribution of the tagged shoots was skewed to the right and became more normal after the harvest. Tagged shoots in the harvested quadrats suffered a 42% mortality as compared to 11% of those in the control quadrats. An understanding of the impact of the mechanical harvesting on the harvested population is essential in the design of a management strategy. Sources of variation in the impact of mechanical harvest include the tide level at time of harvest, length of time the machine operated in one site, skill of the machine operator, and sharpness of the machine cutting blades.