Studies on the ecology of Saccostrea cucullata (Born, 1778) (Mollusca: Bivalvia) on the east coast of southern Africa

Abstract

Aspects of the population ecology of the rock oyster Saccostrea cucullata were studied in Transkei and southern Natal. Population size structure was determined from a survey of 12 sites in Transkei and five in Natal, while data on recruitment, growth and mortality were obtained from photographic monitoring of fixed quadrats at selected sites in Transkei. Most populations had a unimodal size distribution in which small individuals were poorly represented. Truncated size spectra were evident in both nature reserves and exploited areas, suggesting that size distribution was unrelated to human exploitation. Recruitment at all sites was poor at approximately 2 -4 individuals m-2 y-1. After three years, recovery of experimentally denuded areas was minimal both in nature reserves and eXfloited sites. Annual mortality varied considerably, being highest in exploited areas, (9 -11,5 individuals m-y-1), compared with 1,5 -1,8 m-2 y-1 in reserves. Newly settled animals reached a total length of 19 mm in their first year. Thereafter growth increments decreased rapidly to 11 mm and 5 mm in the second and third years respectively. It is estimated that the largest animals found (90 mm) were approximately 20 years old. S. cucullata exhibits characteristics typical of a species close to its geographical limit of distribution. Poor recruitment and slow growth rate decrease the resistance of the species to disturbance and reduce the potential of denuded populations for recovery. Aspekte van die bevolkingsekologie van die oester Saccostrea cucullata is in Transkei en suid-Natal bestudeer. Die bevolkingsgroottestruktuur is by 12 gebiede in Transkei en vyf in Natal bepaal, terwyl data oor aanwerwing, groei en mortaliteit deur middel van fotografiese studies van vaste kwadrate by geselekteerde gebide in Transkei verkry is. Die meeste van die bevolkings het 'n eenmodale' grootteverspreiding waarin klein indiwidue swak verteenwoordig was. Afgeknotte groottespektra is in beide natuurreservate en ontginde gebiede gevind, wat daarop dui dat daar nie 'n eenvoudige verwantskap tussen grootteverspreiding en ontginning is nie. Aanwerwing was swak by aile gebiede met net 2 -4 nuwe indiwidue