Distribution and status of marine invasive species in and bordering the West Coast National Park

Abstract

This study describes the present distribution and abundance of alien marine species within the West Coast National Park. The Mediterranean mussel Mytilus galloprovincialis was recorded in 79 % of sampling areas that provided suitable rocky-shore habitat. The highest biomass (25.4 ± 18.7 kg/M2) was recorded in the mid-shore on Marcus Island, and total intertidal biomass in the Langebaan/Saldanha system was estimated at 281.6 tons. Including famed and subtidal stocks, the area supports 4 362 tons M. galloprovincialis in summer and 959 tons in winter. No live individuals of the European shore crab Carcinus maenas were recorded, but a single dead carapace was found. The European periwinkle Littorina saxatilis had a limited distribution within Langebaan Lagoon and supported a total population of 2.4 million individuals. The alien anemone Sagartia ornata was recorded as locally abundant (up to 426 ± 81 individuals/m2) and was also restricted in distribution. Despite the extensive invasion of the Saldanha Bay-Langebaan Lagoon system by M. galloprovincialis, the threat to the West Coast National Park appears to be limited by low wave action and low nutrient supply to the rocky shores within the park. Although not present in detectable numbers, C. maenas poses a major threat to the park, should it become established there. The threats posed by L. saxatilis and S. ornata are presently unquantified but appear slight.