Effects of sewage effluent on the population structure of Brachidontes rostratus (Mytilidae) on a temperate intertidal rocky shore

A single polluted site at Boags Rocks, south-eastern Australia, and four control sites were used to evaluate the effect of secondary-treated sewage effluent on the population structure, recruitment, growth and mortality of the mussel Brachidontes rostratus. B. rostratus recruited in greater numbers to mussel clumps at the outfall site than at control sites, regardless of the origin of the mussel clumps. The effects of pollution on the soft tissue mass and shell growth of juveniles depended on the origin of the mussels; shell growth of mussels local to Boags Rocks was smaller than the average shell growth recorded in mussels at the control sites, and individuals transplanted into Boags Rocks had higher soft tissue mass than those at the control sites. Mortality was greater at the polluted site than the control sites, regardless of mussel origin. Changes in the population structure or B. rostratus as a result of the discharge of sewage effluent have implications for intertidal fauna that use the habitat created within mussel clumps.