Invasion of sandflats in Langebaan Lagoon, South Africa, by the alien mussel Mytilus galloprovincialis: size, composition and decline of the populations

Since the 1980s the introduced Mediterranean mussel (Mytilus galloprovincialis) has successfully invaded the southern African coastline and has become the dominant species on rocky intertidal shores of the west coast. This study investigates a recent extension of its distribution into the sandy habitats of Langebaan Lagoon. Inspection of 22 potential sites (15 littoral and seven sublittoral) in the lagoon revealed three substantial mussel beds, two littoral and one sublittoral. All were located at sites in the mouth and lower reaches of the lagoon that experienced the full force of the incoming tide. The littoral mussel beds apparently started forming in the mid 1990s, and that in the sublittoral after the 1970s. In July 1999 the overall size of the littoral mussel beds was 0.99 ha, supporting an estimated 47 million mussels, but by March 2001 it had decreased by more than 88 to 0.10 ha and an estimated 3 million individuals. The primary cause of the dieback appeared to be sediment deposition, possibly aggravated by a scarcity of food (phytoplankton). The littoral mussel populations were comprised almost exclusively (99 ) of M. galloprovincialis. At the low-water mark of spring tide (LWMST) mean packing densities were 6–8 times lower than those at 0.5–0.6 m above LWMST, while median shell lengths were notably (c. 1.5 times) larger. A close (P <0.00005) relationship was recorded between packing density and median shell length. The sublittoral population differed from that sampled at LWMST, having no Aulacomya ater, a higher proportion of Choromytilus meridionalis (20 ) and substantial proportions of both large and small M. galloprovincialis. The competitiveness of M. galloprovincialis is discussed.Key words: population sizes, temporal changes, competitiveness, mortality, sedimentation