CHANGES IN THE POPULATION-STRUCTURE OF ASCOPHYLLUM-NODOSUM (L) LE-JOLIS DUE TO MECHANICAL HARVESTING

The use of a Norwegian suction cutter to harvest populations of the br own 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 d ensity of individual plants (clumps) within 0.25 M-2 quadrats, as well as the length of individual shoots within clumps were monitored befor e 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, re ducing the plant density from 92.6 to 73.6 individuals m-2. Preharvest size distribution of the tagged shoots was skewed to the right and be came more normal after the harvest. Tagged shoots in the harvested qua drats suffered a 42% mortality as compared to 11% of those in the cont rol quadrats. An understanding of the impact of the mechanical harvest ing on the harvested population is essential in the design of a manage ment strategy. Sources of variation in the impact of mechanical harves t include the tide level at time of harvest, length of time the machin e operated in one site, skill of the machine operator, and sharpness o f the machine cutting blades.