Mapping and biomass estimation for a harvested population of<i>Gelidium</i><i>sesquipedale</i>(Rhodophyta, Gelidiales) along the Atlantic coast of Morocco

Abstract

Abstract The alga Gelidium sesquipedale is harvested by divers along the Atlantic coast of Morocco and used for agar production. Management of this natural resource has become necessary due to increasing industrial demand. Detailed maps were prepared to determine the distribution and biomass of the species, including a bathymetric map of potential sites that was constructed using a global positioning system coupled to an echo sounder. Samples were then harvested at these sites by scuba diving, and geostatistical tools were used to determine the biomass. This method was applied for four years along 20 km of the largest Gelidium beds (El Jadida). In 1999, biomass was estimated at 13,700 to 16,800 tonnes wet weight, and mean biomass density was about 800 g m−2, with the maximum not exceeding 3200 g m−2. These values are lower than those found for Spanish or French populations. In 2003, the estimated biomass was 33% less. A comparative study indicated that overharvested G. sesquipedale had been replaced by Ha...