Mapping and quantifying the South African kelp resource

  • RJ Anderson Seaweed Unit, Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, South Africa
  • A Rand Department of Botany, University of Cape Town, Rondebosch 7701, South Africa
  • MD Rothman Seaweed Unit, Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, South Africa; Department of Botany, University of Cape Town, Rondebosch 7701, South Africa
  • A Share Seaweed Unit, Marine and Coastal Management, Department of Environmental Affairs and Tourism, Private Bag X2, Rogge Bay 8012, South Africa
  • JJ Bolton Department of Botany, University of Cape Town, Rondebosch 7701, South Africa

Abstract

Two species of kelp are exploited commercially in South Africa. Use of beach-cast Laminaria pallida is limited, although it comprises the bulk of the biomass on the northern West Coast. Ecklonia maxima dominates the southern West Coast and provides most of the material for the South African kelp industry. Harvests of E. maxima fronds for abalone feed have reached 3 000–5 000 tonnes fresh weight (t f wt) y–1, whereas beachcast material of both species (about 1 000t dry wt y–1) is collected mainly for alginate extraction. For the first time, a South African kelp inventory has been compiled using all available data for the 900km-long West Coast (Cape Agulhas to the Orange River), although a few gaps remain. Beds of E. maxima and L. pallida reaching the surface at low spring tides were mapped using one or several methods: infrared aerial photography, digital multispectral aerial imagery, Landsat satellite imagery, and physical mapping with a hand-held GPS. The data are on a GIS database. Landsat 5 TM imagery could identify the presence of kelp beds, but only infrared aerial imagery and multispectral imaging at low spring tides accurately quantified surface areas of kelp beds. Biomass of the main kelp (E. maxima) is variable in space (between 3kg f wt m–2 and 24kg f wt m–2) and time (changes of up to 50%), making estimates for management difficult. Using an average biomass value of 12kg m–2, the total biomass of surface-reaching kelp beds on the West Coast is estimated to exceed 593 000t f wt, but extensive subsurface beds remain unquantified. Results are discussed in relation to sustainable limits to harvesting, gaps in knowledge, and the improved management of kelp resources.

Keywords: Aerial infrared, Ecklonia maxima, GIS, kelp mapping, South Africa

African Journal of Marine Science 2007, 29(3): 369–378
Published
2008-02-07
Section
Articles

Journal Identifiers


eISSN: 1814-2338
print ISSN: 1814-232X