COMPARISON OF THE STRUCTURE OF POPULATIONS OF ASCOPHYLLUM-NODOSUM (FUCALES, PHAEOPHYTA) AT SITES WITH DIFFERENT HARVESTING HISTORIES

Changes in the structure of the Ascophyllum nodosum population at Pubn ico, southwest Nova Scotia, Canada, at an experimental site subjected to mechanical harvest and at two control sites never subjected to mech anical harvest were monitored from 1991 to 1994. A bimodal population structure measured in terms of plant length was characteristic of all these sites before the experiment. The population structure of the exp erimental site became unimodal immediately after experimental harvest by machine and remained unimodal for the subsequent two years. However , a bimodal population structure began to appear in the third year. A bimodal population structure remained evident at the control site with bedrock as the substratum but was less evident at the other control s ite where the substratum is made up of boulders and cobbles. Movement of loose rocks with rockweeds still attached may have contributed to t he less distinct modal structure of this control site. Other sites wit h different harvesting histories monitored in the summer of 1992 showe d some interesting patterns. A unimodal population structure was evide nt in Argyle Sound and Pubnico Point South and at Charlesville, which had been harvested one and two years before, respectively. A bimodal p opulation structure was more evident at Frenchman's Point, which had b een harvested three years prior. The rate of change from a unimodal to a bimodal population structure may depend on the intensity of harvest . Extensive canopy removal in intensively harvested areas may be condu cive to an influx of recruits and to regeneration from the holdfast. H ence, plant length modal structure may be a useful measure of the rela tive state of recovery of a harvested population.