The effect of Manila clam cultivation on an intertidal benthic community: the early cultivation phase

With increasing awareness of the use of the coastal zone, it is necessary to understand the potential environmental effects of aquaculture practices. This is especially important when non-native species, which may be competitively superior to native species, are cultivated. A 5-year experiment was established to study the environmental effects of the various stages of Manila clam. Tapes philippinarum Adams & Reeve, cultivation, from seeding, through on-growing, harvesting and post-harvesting. The aim was to monitor changes in biological and physical variables in the sediment which may be useful in formulating an environmental management strategy for the cultivation of this species. This paper describes the biological and physical changes that occur in the sediment during the early phase of clam cultivation. We compared the changes in netted plots (with and without clams) and unnetted control areas, 6 months before and after laying the clams. The clams were planted in April 1992 under netting at a density of 500 m-² (0.16 kg m-²), and in 6 months, had increased their weight to 3.2 kg m-² but decreased their number to 410 m-². A significant, but small increase in organic content (net only plots, 3.37%; control plots 2.42%) and in phaeopigment (netted plots, 8.6 mg m-²; control, 5.6 mg m-²) of the sediment in the netted plots relative to the control areas were seen. Short-term sedimentation rates on the netted plots were up to four times higher than in the control areas. The netting also encouraged the settlement of Enteromorpha sp. which, in turn, attracted Littorina littorea to feed on these plots. The infaunal community in the control areas was similar to that in samples 12 months earlier and continued to be dominated by the predatory polychaete Nephtys hombergii. Netted plots (with and without clams) had a greater abundance of deposit feeding polychaetes. particularly Ampharete acutifrons and Pygospio elegans, which were the dominant fauna in these plots. Within the clam treatments, the density of clams had a negative effect on the abundance of cirratulids, although mean abundance was generally greater than in the control areas. There were few physical changes to the experimental area after 6 months. The most important effect appears to be the increased sedimentation rate over plots with netting, which has led to an increase in productivity of those areas.