ECOLOGICAL EFFECTS OF INTERTIDAL MANILA CLAM CULTIVATION - OBSERVATIONS AT THE END OF THE CULTIVATION PHASE

1. Marine aquaculture has come under close scrutiny by environmental p ressure groups, fisheries managers and scientists in recent years, bec ause of a shared concern over the physical and biological effects of f arming practices on the marine environment. 2. This paper describes th e environmental effects of intertidal Manila clam cultivation at the e nd of the cultivation phase immediately prior to harvesting the market able-sized clams, which were planted in ground plots 2.5 years earlier at a density of 500 m(-2). Although survival was poor, with a final d ensity of 26 m(-2) (0.78 kg m(-2)), this still represented a significa nt biological presence relative to other benthic organisms. 3. An expe rimental approach, using a 3 x 3 Latin Square design, was adopted. The treatments comprised net-covered plots of clams, net-covered plots wi thout clams and control plots without netting or clams. An additional set of controls, 50 m distant from the Latin Square, was established f or comparative purposes. 4. The presence of the netting, rather than t he clams, increased sedimentation rate which elevated the ground profi le by c. 10 cm and caused a small but significant increase in percenta ge fines and percentage organic content of the sediment. The netting a lso encouraged higher densities of some species of infaunal deposit-fe eding worms which became the dominant fauna. 5. During the first 6 mon ths of the cultivation process, the fauna was dominated by the opportu nistic spionid, Pygospio elegans. After one year, the stabilizing effe ct of the netting on the sediment led to the establishment of species such as Ampharete acutifrons and Tubificoides benedii, which displaced P. elegans as the community dominants, 6. The observed biological res ponses indicate that organic enrichment occurs within net-covered area s. However, the magnitude of community change is far less than that wh ich occurs in association with some other marine culture practices, wh ich create anoxic sediments and impoverished infaunal communities.