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 pressure groups, fisheries managers and scientists in recent years, because of a shared concern over the physical and biological effects of farming practices on the marine environment.

2. This paper describes the environmental effcets of intertidal Manila clam cultivation at the end of the cultivation phase immediately prior to harvesting the marketable-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 density of 26 m^-2 (0.78 kg m^-2), this still represented a significant biological presence relative to other benthic organisms.

3. An experimental approach, using a 3 x 3 Latin Square design, was adopted. The treatments comprised net-covered plots of clams, net-covered plots without clams and control plots without netting or clams. An additional set of controls, 50 m distant from the Latin Square, was established for comparative purposes. 

4. The presence of the netting, rather than the clams, increased sedimentation rate which elevated the ground profile by c. 10cm and caused a small but significant increase in percentage fines and percentage organic content of the sediment. The nutting also encouraged higher densities of some species of infaunal deposit-feeding worms which became the dominant fauna.

5. During the first 6 months of the cultivation process, the fauna was dominated by the opportunistic spionid, Pygospio elegans. After one year, the stabilizing effect 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 responses indicate that organic enrichment occurs within net-covered areas. However, the magnitude of community change is far less than that which occurs in association with some other marine culture practices, which create anoxic sediments and impoverished infaunal communities.