ECOLOGY AND ENVIRONMENTAL-IMPACT OF MUSCULISTA-SENHOUSIA (MOLLUSCA, BIVALVIA, MYTILIDAE) IN TAMAKI ESTUARY, AUCKLAND, NEW-ZEALAND

The introduced, mytilid bivalve Musculista senhousia (Asian date musse l) occurs on the east coast of New Zealand in the Auckland region. Eig hteen sites were searched within the Tamaki Estuary: six had extensive mats of mussels and three contained small, isolated clumps. Core samp les were taken monthly during 1994/95 from two of the mat-forming popu lations. Densities reached 16 000 m(-2) at Bucklands Beach and 5000 m( -2) at Farm Cove. Both populations were dominated by a single cohort o f mussels. Mussels grew to about 20 mm in 12 months, after which growt h virtually ceased. Recruitment was sporadic into existing mats, but o ccurred adjacent to the monitored mat at Bucklands Beach in April 1995 . The area occupied by the initial mussel bed at this site decreased b y 60% over 1 year. Further core sampling revealed significantly fewer macrofaunal invertebrates under mussel mats compared to control sample s taken from areas of beach without mussels. Infaunal bivalves were mo st adversely affected by M. senhousia, showing an 8-fold decrease in a bundance within mats compared to cores in the control area. Our result s reveal that M. senhousia in the Auckland area has similar life histo ry features to those reported from populations outside New Zealand. We suggest that any adverse environmental effects caused by M. senhousia are likely to be local and short-lived.