Population patterns of the alien mussel Mytilus galloprovincialis were investigated at two locations on the west coast of South Africa: Camps Bay and Groenrivier. Specifically, wave exposure and zonation were examined for their effect on mussel cover and total density, mussel size and recruit intensity. Two areas were compared at Camps Bay; the first was a prominent rock experiencing heavy wave action, and the second was a sheltered bay. Three areas were selected at Groenrivier, the third experiencing an intermediate gradient of wave action. A number of interesting patterns emerged, although not all were statistically significant. At both Camps Bay and Groenrivier mussel cover increased with increasing wave exposure and decreased with increasing tidal height. The exception to this zonational trend was exhibited in the very sheltered bay at Groenrivier, where mussel cover increased upshore, from 2% to 6%. Total mussel density and recruit intensity declined with a reduction in wave action at both locations. An unexpected zonational pattern occurred at both Camps Bay and Groenrivier - total densities tended to be greater upshore, a pattern which closely followed recruit densities. The maximum size achieved by mussels is greatest at more exposed areas, but the difference between sheltered and exposed regions at Camps Bay was not notable. Maximum shell size tended to decrease with increasing tidal height in all areas but the sheltered bay at Groenrivier, where little variation in maximum size occurred. Multivariate comparisons of population demography between all of the sites revealed that the sheltered site at Camps Bay was most similar to the semi-exposed site at Groenrivier, which points to similar environmental conditions in these areas. This pattern was repeated for total densities, a result which supports the previous suggestion. The sheltered bay at Groenrivier was most dissimilar in both demographic structure and density to all other sites. Bibliography: leave 89-99
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